Molecular, Biochemical, and Bioimaging Markers of MEN Syndromes

Abstract

Multiple endocrine neoplasia (MEN) syndromes are rare hereditary disorders characterized by the development of multiple endocrine and non-endocrine tumours with variable penetrance and age-dependent expression. Although uncommon, these syndromes are highly relevant from both biological and clinical perspectives, as they exemplify the direct link between germline genetic alterations and tumorigenesis. Early tumour detection is critical in MEN syndromes because many associated neoplasms—such as medullary thyroid carcinoma (MTC), pancreatic neuroendocrine tumours (NETs), pheochromocytomas, and parathyroid disease—may remain clinically silent for prolonged periods while retaining malignant potential. Delayed diagnosis is associated with advanced disease and worse outcomes, whereas early identification enables curative or organ-preserving interventions. This clinical challenge has driven the development of integrated diagnostic strategies combining genetic testing, biochemical markers, and imaging. Among these, genetic testing plays a pivotal role, providing definitive diagnosis, enabling family screening, and guiding risk-adapted surveillance. The aim of this review is to provide a comprehensive synthesis of genetically driven diagnostics in MEN syndromes, outlining the current state of the art and future directions in precision medicine.

1. Introduction

Multiple endocrine neoplasia syndromes comprise a heterogeneous group of rare, inherited cancer predisposition disorders characterized by the development of multiple endocrine and non-endocrine tumours with variable penetrance and age-dependent expression. Traditionally classified into MEN type 1 and MEN type 2, and more recently expanded to include MEN-like entities, these syndromes share a defined genetic basis and require lifelong surveillance. Despite their rarity, MEN syndromes represent paradigmatic models of hereditary oncogenesis, in which early diagnosis and individualized follow-up can substantially alter disease course and clinical outcomes. Early and accurate tumour detection is of critical importance in MEN syndromes. Many associated neoplasms—including MTC, pancreatic NETs, pheochromocytoma, and parathyroid disease—may remain clinically silent for prolonged periods while retaining malignant potential. Delayed diagnosis is frequently associated with advanced disease, increased morbidity, and reduced survival, whereas timely identification of early-stage lesions enables curative or organ-preserving interventions. Consequently, optimized screening strategies are central to MEN management, encompassing both asymptomatic mutation carriers and patients with established multisystem disease. The biological complexity and temporal evolution of MEN-associated tumours provide a strong rationale for an integrated diagnostic approach combining molecular genetics, biochemical biomarkers, and advanced imaging. Germline genetic testing enables definitive diagnosis, risk stratification, and cascade screening within affected families. Biochemical markers offer dynamic, organ-specific insights into tumour activity and disease progression, often preceding structural changes. Complementarily, modern imaging modalities—including functional and molecular techniques—allow accurate tumour localisation, staging, and assessment of treatment response. The integration of these diagnostic domains enhances sensitivity, reduces diagnostic uncertainty, and supports personalized surveillance and therapeutic decision-making. Against this background, the present review critically examines the current and emerging role of integrated diagnostics in MEN syndromes, focusing on how the coordinated use of genetic, biochemical, and imaging tools can improve early detection, refine risk assessment, and guide individualized patient management. By synthesizing evidence across disciplines, this review aims to provide clinicians, laboratorians, and imaging specialists with a comprehensive framework for contemporary diagnosis and longitudinal care in MEN syndromes.

2. MEN Syndromes: Genetic Background and Clinical Presentations

Multiple endocrine neoplasia syndromes are rare hereditary cancer predisposition disorders characterized by the development of multiple endocrine tumours and an autosomal dominant inheritance. First described in 1903, the term MEN was introduced in 1968 [1] and currently includes MEN1, MEN2, MEN4, and MEN5. Specifically, MEN2 comprises the clinical variants MEN2A, MEN2B (also referred to as MEN3), and familial medullary thyroid carcinoma (FMTC). In the 1990s, the MEN1 and RET genes were identified as causative of MEN1 and MEN2, respectively [2,3]. Only in 2006 was CDKN1B linked to MEN4 [4], and in 2021, MEN5 was proposed to define a syndrome associated with germline pathogenic or likely pathogenic variants (PVs) in the MAX gene [5]. Clinically, MEN1 can present with pituitary adenomas, parathyroid tumours, and gastroenteropancreatic NETs, while MEN2 is characterized by MTC, with pheochromocytoma occurring predominantly in MEN2A and MEN2B; the latter may also present with marfanoid habitus and mucosal neuromas. MEN4 and MEN5 exhibit phenotypes overlapping with MEN1, with MEN5 typically associated with pheochromocytomas and paragangliomas. The clinical expression of MENs is highly variable and requires an integrated approach comprising biochemical, imaging, and genetic evaluations. Genetics and clinical phenotypes of different MEN syndromes are summarized in Table 1 and discussed below.

Table 1.

Genetics and clinical phenotypes of different MEN syndromes.

Type	Inheritance	Gene	Most Frequent PVs a	Key Components	Other Associated Tumours
MEN1	Autosomal dominant	MEN1	Frameshift, followed by missense, nonsense, and splice site PVs	-Parathyroid adenomas/hyperplasia (90%), onset between 20 and 25 years, with hypercalcemia by age 50 -Anterior pituitary adenomas (30–40%): prolactinomas (20%), somatotropinomas (10%), corticotropinomas (5%), non-functioning tumours (5%) -Gastroenteropancreatic NETs (30–70%): gastrinomas (40%), non-functioning pancreatic NETs (20–80%), insulinomas (10%), glucagonomas (<1%), vasoactive intestinal polypeptidomas (<1%)	Adrenal adenomas (27–36%) Bronchial/thymic (8%) Gastric carcinoids (7–35%) Angiofibromas (80%) Collagenomas (70%) Lipomas (30%) Meningiomas (8%)
MEN2A	Autosomal dominant	RET	p.Gly533Cys (exon8); PVs in codons 609, 611, 618, 620 (exon 10); PVs in codons 630 and 634 b (exon 11); PVs in codon 790 (exon 13); p.Val804Leu, p.Val804Met (exon 14)	-Medullary Thyroid Carcinoma (90%) -Pheochromocytoma (50%) -Parathyroid adenoma/hyperplasia (25%)	Cutaneous lichen amyloidosis c Hirschsprung disease d Papillary thyroid carcinoma e
MEN2B			p.Ala883Phe b (exon 15), p.Met918Thr f (exon 16)	-Medullary thyroid carcinoma (early/aggressive) (>90%) -Pheochromocytoma (40–50%) -Parathyroid adenoma/hyperplasia (rare)	Marfanoid habitus Intestinal ganglioneuromatosis Mucosal neuromas
FMTC			p.Gly533Cys (exon8); PVs in codons 609, 611, 618, 620 (exon 10)	-Medullary Thyroid Carcinoma	Pheochromocytoma (<1%) Parathyroid adenoma/hyperplasia (<1%)
MEN4	Autosomal dominant	CDKN1B	mainly frameshift and nonsense g	-Parathyroid adenoma/hyperplasia (56%) -Anterior pituitary adenomas (23%) -Pancreatic or duodenal NETs (15%; most frequently nonfunctioning tumours, followed by gastrinomas	Carcinoid tumours (4–7%) Adrenocortical tumours (4–7%) Papillary thyroid carcinoma (8%) potentially other tumours (e.g., reproductive organ tumours, such as cervical neuroendocrine carcinoma, meningioma, and renal angiomyolipoma)
MEN5	Possibly predominant paternal	MAX	no common variants identified to date (insufficient data)	-Pheochromocytoma (>85%) -Paraganglioma (7.9%)	Parathyroid adenoma (3.1%) Pituitary adenoma (8.5%) Ganglioneuroma (4.7%) Neuroblastoma (5.5%); Ganglioneuroblastoma (0.8%)

Legend. PVs: pathogenic/likely pathogenic variants; NETs: neuroendocrine tumours; ^a as reported by the ClinVar [6]; ^b associated with high risk of medullary thyroid cancer; ^c mainly associated with PVs in codon 634 and with p.Val804Met; ^d mainly associated with PVs in codons 609, 611, 618, 620; ^e mainly associated with PVs in codon 790 and with p.Val804Met, ^f associated with the highest risk of medullary thyroid cancer; ^g based on a limited number of reported cases.

2.1. MEN Type 1 (MEN1)

The MEN1 is a tumour suppressor gene located on chromosome 11q13 [3], containing 10 exons, and encoding a 610-amino-acid protein called menin [7] (Figure 1).

Figure 1.

Schematic representation of the MEN1 gene and its protein product, menin. The MEN1 gene, located on chromosome 11q13, comprises 10 exons (shown as blue boxes at the top). It encodes menin, a 610-amino-acid protein (green ellipse), depicted from the N-terminus (NH₂) to the C-terminus (COOH). Three nuclear localisation sequences (NLS1, NLS2, and NLS3) are located in the C-terminal region of the protein and mediate its nuclear translocation.

Menin is a nuclear scaffold protein that regulates gene transcription by coordinating chromatin remodelling and interacting with multiple transcription factors. Menin inactivation leads to loss of control over cell proliferation, contributing to the pathogenesis of MEN1 [8]. The typical tumour spectrum of MEN1 includes parathyroid tumours, gastroenteropancreatic NETs, and pituitary adenomas. As shown in Table 1, more than 20 endocrine and non-endocrine neoplasms may develop in patients with MEN1, leading to high variability in clinical phenotype [9]. The diagnosis of MEN1 is established in patients with ≥2 typical MEN1 tumours (parathyroid adenomas, gastroenteropancreatic NETs, pituitary adenomas) or with one typical tumour plus a first-degree relative affected by MEN1. Molecular diagnosis requires identification of a heterozygous PV in the MEN1 gene; a variant of uncertain significance (VUS) does not allow for a diagnosis of MEN1. Approximately 1–4% of MEN1 PVs are intragenic deletions/duplications undetectable by Sanger sequencing or next-generation sequencing (NGS) [10], requiring methods such as multiplex ligation-dependent probe amplification or gene-targeted microarrays for single-exon copy-number changes. Notably, NGS can detect post-zygotic constitutional mosaicism (i.e., a PV arising during embryogenesis and confined to a subset of somatic cells), even at low allelic frequencies [11].

2.2. MEN Type 2 (MEN2A, MEN2B, FMTC)

The RET (Rearranged during Transfection) proto-oncogene, identified in 1985 [12], is located at 10q11.21 [13] and comprises 20 exons [14]. RET encodes a transmembrane receptor tyrosine kinase that transduces signals for cell proliferation, migration, and differentiation, and consists of three main domains: an extracellular ligand-binding domain, in which there are four cadherin-like domains and a cysteine-rich domain (CRD), a hydrophobic transmembrane domain, and an intracellular domain, which contains the tyrosine kinase domain (TKD) (Figure 2).

Figure 2.

Schematic representation of the RET proto-oncogene and the corresponding RET protein, with the codons most frequently mutated in MEN2A and MEN2B. The RET proto-oncogene, located on chromosome 10q11.21, comprises 20 exons (blue boxes, top). It encodes a transmembrane receptor tyrosine kinase composed of an extracellular ligand-binding region containing four cadherin-like domains (CAD, yellow) and a cysteine-rich domain (CRD, green), a hydrophobic transmembrane domain (TMD, red), and an intracellular region containing the tyrosine kinase domain (TKD, orange); the two C-terminal isoforms, RET9 and RET51, are indicated. Codons most frequently affected by germline pathogenic variants are mapped to their corresponding exons: MEN2A-associated variants (red) cluster in exons 8 (codon 533), 10 (codons 609, 611, 618, 620), 11 (codons 630, 634), 13 (codon 790), and 14 (codon 804), whereas MEN2B-associated variants (dark blue) are located in exons 15 (codon 883) and 16 (codon 918). Purple asterisks denote codons associated with Hirschsprung disease, and the green asterisk denotes the codon associated with cutaneous lichen amyloidosis.

RET plays a crucial role in renal, enteric nervous system, and neuroendocrine development [15]. Germline PVs in the RET gene are associated with distinct phenotypes depending on their functional effect. Indeed, loss-of-function PVs across all exons of the RET gene may be responsible for Hirschsprung disease [16] or congenital anomalies of the kidney [17]. In contrast, gain-of-function PVs cluster within specific exons (8, 10, 11, 13–16) and are responsible for MEN2, including all its clinical variants, with MTC representing their shared feature. Specifically, exons 10 and 11 encode the CRD in the extracellular domain, which contains 16 cysteine residues, including six (p.Cys609, p.Cys611, p.Cys618, p.Cys620 in exon 10, and p.Cys630, p.Cys634 in exon 11), which are primary hotspots for MEN2A PVs. The most common PVs responsible for MEN2A are located at codon 634 and are associated with MTC, pheochromocytoma, and, more rarely, primary hyperparathyroidism (pHPT). Cutaneous lichen amyloidosis may occasionally be present in carriers of PVs in codon 634. The p.Ala883Phe in exon 15 and, more frequently, the p.Met918Thr in exon 16 are associated with MEN2B. Interestingly, PVs in codon 790 (exon 13) and the p.Val804Met (exon 14) can be associated with papillary thyroid carcinoma in addition to MTC (Table 1). The American Thyroid Association Guidelines Task Force classifies PVs in the RET gene according to their risk of aggressive MTC to support phenotype prediction and guide the timing of prophylactic thyroidectomy and biochemical screening [18]. The clinical diagnosis of MEN2 is established based on characteristic tumour patterns or syndromic features (Table 1): MEN2A by the occurrence of two or more endocrine tumours (MTC, pheochromocytoma, or parathyroid hyperplasia/adenoma), FMTC by the presence of multiple cases of MTC in a family without other endocrine tumours, and MEN2B by the diagnosis of early-onset MTC in a patient with mucosal neuromas, medullated corneal nerve fibres, distinctive facies, and marfanoid habitus. Molecular diagnosis is confirmed by identifying a heterozygous gain-of-function PV in the RET gene.

2.3. Emerging and MEN-like Syndromes

Among other MEN syndromes with a clinical phenotype overlapping MEN1, there are MEN4 and, as lately suggested, MEN5, due to PVs in the CDKN1B and MAX genes, respectively. As shown in Table 1, MEN4 clinically overlaps with MEN1 and is primarily characterized by parathyroid tumours (described in 56% of patients), followed by pituitary and gastroenteropancreatic NETs [19]. Compared with MEN1, endocrine tumours in MEN4 occur less frequently and at a later age of onset, consistent with a lower penetrance of CDKN1B PVs. However, to date, no clinical criteria reliably distinguish MEN4 from MEN1; diagnosis requires identification of a heterozygous PV in the CDKN1B. Insufficient data on the MEN4 phenotype and follow-up complicate proper clinical management. Among other hereditary cancer syndromes overlapping with MEN1, there are the familial isolated pituitary adenoma (FIPA), due to PVs in the AIP gene, and mainly characterized by the earlier onset of pituitary adenomas than in MEN1, and the familial isolated primary hyperparathyroidism (FIHP), mainly due to PVs in the CASR and CDC73 genes, which presents with parathyroid hyperplasia or adenomas. Recently, germline PVs in the MAX gene were first described in 2011 [20] in association with hereditary pheochromocytomas and paragangliomas, and have been implicated in an emerging MEN syndrome, called MEN5. This entity also includes pituitary, parathyroid, and non-endocrine neoplasms such as neural crest-derived tumours (e.g., ganglioneuromas and neuroblastomas) [21]. Only a few families have been described so far, with a likely paternal mode of inheritance, according to a possible parent-of-origin effect (i.e., PVs cause disease only when paternally inherited) [21,22].

2.4. Clinical Manifestation

The clinical manifestations of MEN syndromes are heterogeneous and often overlap with common endocrine disorders, potentially delaying diagnosis. The characteristic tumour spectrum, typical clinical presentations, temporal patterns of disease development, and common diagnostic pitfalls associated with MEN syndromes are summarized below. In particular, the major clinical manifestations and characteristic diagnostic “masks” of the different MEN syndromes are displayed in Table 2.

Table 2.

Clinical manifestations and characteristic “clinical masks” of MEN syndromes.

Syndrome (Gene)	Principal Tumours (Penetrance)	Typical Clinical Presentation	Conditions It May Mimic (“Clinical Masks”)
MEN1 (MEN1)	Parathyroid adenoma/hyperplasia (90%) Anterior pituitary adenoma (30–40%) Gastroenteropancreatic NET (30–70%)	Hypercalcaemia, nephrolithiasis, bone disease Amenorrhoea/galactorrhoea, acromegaly, Cushing Recurrent peptic ulcers, hypoglycaemia, secretory diarrhea	Apparently sporadic pHPT; isolated prolactinoma; Zollinger–Ellison syndrome; insulinoma-related hypoglycaemia
MEN2 (RET)	Medullary thyroid carcinoma (>90%) Phaeochromocytoma (40–50%) pHPT (MEN2A ≈ 25%)	Thyroid nodule, cervical nodes, calcitonin-related flushing/diarrhea Episodic hypertension, palpitations, headache, sweating MEN2B: marfanoid habitus, mucosal neuromas	Solitary thyroid nodule; resistant or paroxysmal hypertension; MEN2A lichen amyloidosis mistaken for dermatitis; MEN2B mucosal neuromas in infancy
MEN4 (CDKN1B)	Parathyroid adenoma/hyperplasia (56%) Anterior pituitary adenoma (23%) Pancreatic/duodenal NET (15%)	As MEN1 but milder, later onset and lower penetrance	MEN1 phenotype with no detectable MEN1 variant; sporadic pHPT or pituitary adenoma at an older age
MEN5 (MAX)	Pheochromocytoma (>85%) Paraganglioma (7.9%) Pituitary/parathyroid; neural-crest tumours	Catecholamine excess; mass effect; ganglioneuroma/neuroblastoma in younger patients	Apparently isolated or familial pheochromocytoma/paraganglioma

Legend: NET, neuroendocrine tumour; pHPT, primary hyperparathyroidism; Penetrance values correspond to those reported in Table 1. MEN2 encompasses MEN2A, MEN2B and FMTC; variant-specific features are indicated.

The natural history of MEN syndromes is characterized by a predictable sequence of tumour development that provides the basis for age-specific surveillance strategies (Table 3).

Table 3.

Typical sequence of clinical events and tumour development in MEN syndromes.

Syndrome	Usual Presenting Lesion	Subsequent Tumour Development	Surveillance Entry Point
MEN1	pHPT, usually first (onset 20–25 y; hypercalcaemia by ~50 y)	Pituitary adenomas and gastroenteropancreatic NETs accrue over decades; more than 20 endocrine and non-endocrine lesions are possible	Calcium–PTH from age ~10
MEN2A	MTC (preceded by C-cell hyperplasia), usually the earliest	Pheochromocytoma later; pHPT in ~25%, often concurrent with MTC or on follow-up (median 30–40 y)	Calcitonin and genotype; thyroidectomy timing by RET risk
MEN2B	Mucosal neuromas and marfanoid features in infancy; aggressive MTC, often within the first year of life	Pheochromocytoma in adolescence or adulthood; pHPT is rare	Prophylactic thyroidectomy in infancy
FMTC	MTC only, typically later and more indolent	Pheochromocytoma and pHPT each <1%	Calcitonin and genotype
MEN4	pHPT first, mirroring MEN1 but at an older age	Pituitary (23%) and pancreatic/duodenal NETs (15%) follow; lower penetrance than MEN1	As MEN1, genotype-defined
MEN5	Pheochromocytoma/paraganglioma, usually the dominant feature	Pituitary, parathyroid, and neural-crest tumours (ganglioneuroma, neuroblastoma); data sparse	Metanephrines and PPGL imaging

Legend: pHPT, primary hyperparathyroidism; MTC, medullary thyroid carcinoma; NET, neuroendocrine tumour; PPGL, pheochromocytoma/paraganglioma. Ages of onset and surveillance intervals reproduce those stated in the manuscript.

Finally, clinical presentations vary according to age, physiological status, and tumour burden, and may mimic more common conditions. Key diagnostic clues and potential pitfalls are summarized in Table 4.

Table 4.

Clinical presentations and diagnostic pitfalls of MEN syndromes across life stages and special clinical contexts.

Clinical Context	MEN1	MEN2	MEN4/MEN5	Red Flags Suggesting MEN	Major Diagnostic Pitfalls
Children and adolescents	Early pHPT; later pituitary adenomas and GEP-NETs.	Early MTC (especially MEN2B); pheochromocytoma surveillance required.	Rare; MEN-like or hereditary PPGL phenotypes.	Young-onset endocrine tumour, multiglandular pHPT, family history.	Assuming thyroid nodules, hypercalcaemia, or hypertension are sporadic.
Pregnancy	Hypercalcaemia may increase maternal–fetal complications.	Pheochromocytoma must be excluded before intervention.	Occasional MAX-related PPGL.	Severe hypertension, palpitations, hypercalcaemia.	Symptoms attributed to normal pregnancy.
Adult presentation	pHPT, pituitary adenomas, pancreatic/duodenal NETs.	MTC, pheochromocytoma, MEN2A-related pHPT.	MEN4: pHPT/pituitary disease; MEN5: PPGL-dominant phenotype.	Multiple endocrine tumours or positive family history.	Treating each tumour as an isolated sporadic condition.
Atypical presentations (‘disease masks’)	Nephrolithiasis, osteoporosis, peptic ulcer disease, hypoglycaemia.	Thyroid nodule, diarrhea, flushing, anxiety, hypertension.	Mimic MEN1 or hereditary PPGL syndromes.	Unusual combinations of endocrine and non-endocrine manifestations.	Failure to recognize syndromic associations.
Diagnostic triggers	Young-onset or recurrent pHPT; pHPT plus pituitary adenoma/NET.	Any MTC; bilateral pheochromocytoma; MEN2B phenotype.	MEN-like phenotype with negative MEN1/RET testing.	Need for germline testing and family screening.	Delayed referral for genetic evaluation.
Consequences of delayed diagnosis	Metastatic NETs, recurrent pHPT.	Advanced MTC, hypertensive crisis from pheochromocytoma.	Missed surveillance opportunities.	Missed cascade testing in relatives.	Preventable morbidity and mortality.

In conclusion, detecting MEN syndromes remains challenging because the initial manifestations are often subtle, age-dependent, and frequently resemble common sporadic endocrine disorders. Patients may present with a single tumour, mild biochemical abnormalities, or non-specific symptoms long before the full syndromic phenotype becomes apparent, and family history may be absent because of de novo variants or unrecognized disease in relatives. Consequently, recognition requires a high index of suspicion, careful assessment for multiple endocrine and non-endocrine manifestations, and timely use of germline genetic testing, which is essential both for confirming the diagnosis and for enabling surveillance of at-risk family members.

3. Biochemical Markers

Early identification and monitoring of biochemical abnormalities are essential for patients carrying MEN1 or MEN2-associated mutations. In recent years, the biochemical work-up has been reduced through conscious decision-making, taking into account patient- and age-related penetrance. For MEN1 in particular, symptom-based clinical assessment with high-accuracy biochemical tests is preferred, with additional biochemical testing when needed [23,24,25,26]. Analyses should be performed in (ISO15189-accredited) laboratories using validated methods. Endocrine biomarkers are measured using immunoassays or liquid chromatography coupled to mass spectrometry (LC-MS/MS), the latter of which is generally better standardized and less susceptible to analytical interferences.

3.1. Parathyroid Tumours (MEN 1 and 2)

Primary hyperparathyroidism in MEN is almost always multiglandular, unlike sporadic cases of pHPT. The biochemical work-up for MEN-related parathyroid tumours is similar to that for sporadic disease. Screening starts at ages 10–11 for MEN1 and those with high-risk RET mutations, and at age 16 for those with moderate-risk RET mutations [18]. Biochemical testing focuses first on serum calcium and second on parathyroid hormone (PTH). Total serum calcium is typically measured photometrically; about half is albumin-bound, while only ionized calcium is biologically active. When albumin concentrations are within the reference interval, these measurements suffice. Albumin-corrected calcium calculations may be used when albumin concentrations are abnormal, though corrections can be inaccurate as well, for instance, when pH is altered [27,28]. Direct measurement of ionized calcium using electrochemical methods is preferred, but less available and more expensive. Once hypercalcemia is confirmed and supplement—or medication-related causes are excluded, PTH is measured to assess parathyroid involvement. PTH is usually measured with immunoassays targeting the C- and N-terminal epitopes of the peptide. The immunoassays suffer from poor standardization, hence differences in PTH measurements when using assays from different manufacturers. Differences in antibody specificity also cause discrepancies between second-generation assays (detecting PTH_1–84 and peptide fragments, often overestimating levels) and third-generation assays (detecting PTH_1–84 only) [29,30]. Analytical interference by (heterophilic) antibodies may also cause false high or low results. Recent development of LC-MS/MS methods may help to recalibrate PTH immunoassays and reduce inter-assay variation [31]. Elevated calcium concentrations with elevated or inappropriately normal PTH concentrations suggest a parathyroid tumour. Mild PTH elevation may indicate familial hypocalciuric hypercalcemia, though this is rare in the context of MEN screening. After confirming PTH-mediated hypercalcemia, imaging studies should be performed preceding parathyroidectomy.

3.2. Medullary Thyroid Carcinoma and Chromaffin Tumours (MEN 2)

3.2.1. Medullary Thyroid Carcinoma

As nearly all patients with MEN2 mutations develop hereditary MTC, prophylactic thyroidectomy is performed in children at an early age, preferably in the first year of life, before the age of 5, or when serum calcitonin concentrations rise, all depending on the risk profile of the mutation [18]. Thyroidectomy, in children and adults, is followed by regular (every 3 months to annually) serum calcitonin (the 32-amino acid-containing peptide hormone derived from thyroid C cells) measurements using an immunoassay. Patients with palpable thyroid nodules or lymphadenopathy typically show clearly elevated calcitonin concentrations (>100 pg/mL), after which FNA biopsy with calcitonin staining or measurement in fine-needle washouts is used to confirm MTC. Smaller tumours or C-cell hyperplasia may show normal or slightly elevated concentrations. Calcitonin immunoassays suffer from poor standardization [32]. C-cell hyperplasia occurs in chronic autoimmune (Hashimoto) thyroiditis and can cause a mild increase in basal calcitonin levels. This is seen mainly in the early goitrous phase; in the later atrophic stage, calcitonin secretory reserve declines as C cells are progressively lost [33,34]. Such low-grade hypercalcitoninaemia is benign but relevant to MEN2 surveillance, where coexisting thyroiditis may account for a borderline value that would otherwise raise concern for early neoplastic C-cell change. Therefore, results must be interpreted with assay-specific and often sex- and age-specific cutoffs. Also, patients with MEN2 should be monitored using the same immunoassay over time. Mild calcitonin elevations (<20 pg/mL) are often false positives, caused by medications (e.g., PPIs), kidney disease, or analytical interference (e.g., heterophilic antibodies, macroprolactin) [35,36]. Additional biomarkers, such as serum procalcitonin, may improve diagnostic accuracy. Provocative tests such as pentagastrin or calcium stimulation are generally no longer recommended.

3.2.2. Pheochromocytomas

Hereditary pheochromocytomas, unlike sporadic cases, are usually bilateral and occur at a younger age. Annual screening should begin at the same time as screening for pHPT [18]. Because excess secretion of (nor)adrenaline can trigger a hypertensive crisis during surgery, all MEN2 patients must undergo biochemical screening for pheochromocytoma before therapeutic thyroidectomy. Screening is also recommended for women with MEN2 who are planning a pregnancy. Biochemical testing focuses on the stable O-methylated catecholamine metabolites: free metanephrine and free normetanephrine in plasma or 24 h urine samples [37]. Rare dopamine-secreting tumours require additional measurement of the dopamine metabolite in plasma, 3-methoxytyramine. Measurements are performed using HPLC with electrochemical detection (ECD) or LC-MS/MS. Because O-methylated metabolites reflect continuous leakage of catecholamines from storage vesicles, they are less affected by acute sympathoadrenal activation. However, proper preanalytical conditions remain essential to prevent false-positive tests: blood sampling should be performed fasting, at rest, in the supine position, and samples must be cooled immediately to prevent degradation of metanephrines [38,39]. Acute and chronic illness, specifically myocardial infarction, intracerebral hemorrhage, and stroke, can elevate catecholamines as well as metanephrines. Chronic kidney disease may increase urinary metanephrines, while plasma free metanephrines are minimally affected. Normetanephrine concentrations are age-dependent and should be interpreted using age-adjusted reference intervals. Certain medications may cause analytical interference depending on the method used; for example, acetaminophen, amoxicillin, and rifampicin can falsely elevate normetanephrine when measured by HPLC-ECD, but not by LC-MS/MS. Plasma metanephrines generally offer higher sensitivity than urinary measurements, especially in asymptomatic screening. Positive results should be followed by imaging studies and further clinical evaluation.

3.3. Anterior Pituitary and Gastroenteropancreatic Neuroendocrine Tumours (MEN1)

Anterior pituitary tumours and gastroenteropancreatic NETs may develop in MEN1 (for an overview, see Table 2 and Table 5). From age 10, prolactin and IGF-1 (and optionally gastrin) may be measured every 1–3 years based on clinical judgement, as these are the most prevalent tumours besides non-hormone-secreting tumours. In adults, annual prolactin, IGF-1 and gastrin testing is recommended, with additional markers only when clinically indicated.

Table 5.

Overview of biomarkers for anterior pituitary tumors and gastroenteropancreatic (MEN1).

	Biomarker	(Pre/Post)Analytical Considerations
Anterior pituitary tumours
Prolactinoma [23,40]	Prolactin	Stress at venipuncture Anti-dopaminergic drug use Standardization of assay Assay interferences, specifically Macroprolactin
Somatotropinoma [23,41]	Insulin-like growth factor 1 (IGF-1) (GH suppression test)	Standardization of assay Immunoassay prone to interferences Use age and sex-specific reference intervals/SD values The choice of normative dataset contributes to differences in IGF-1 SDS and interpretation.
Corticotropinoma [42]	Cushing screening: 24 h urine free cortisol, late-night saliva cortisol, 1 mg dexamethasone suppression test (DST)	Hydrocortisone or other glucocorticoid use 2× 24 h urine collection: collection errors Late-night saliva: blood contamination DST: CYP3A4 inducer use, estrogen (i.e., contraceptives) Standardization of (immuno) assay Immunoassay prone to interferences
Gastroenteropancreatic neuroendocrine tumours
Gastrinoma [23,43]	Gastrin Typically, concentrations > 10 ULN (in the absence of PPI and with increased basal gastric acid secretion (pH < 2))	The patient should be fasted When using PPI, be cautious with temporary withdrawal and follow guidelines for advice. Poor stability of gastrin; immediately cool plasma samples or process quickly and store frozen Standardization of assay: immunoassays measure different gastrine peptides Immunoassay prone to interferences, cross-reactivity, and CCK
Insulinoma [44]	Insulin Only screen if clinical symptoms manifest Complete work-up for hypoglycemia Inappropriate high insulin (and C-peptide) concentration in relation to plasma concentrations at the same time	The patient should be fasted or undergo a provocative fasting test Avoid hemolytic samples Standardization of assay Immunoassay prone to interferences Cross-reactivity with insulin analogues (assay-dependent)
Glucagonoma [43]	Glucagon Only screen if clinical symptoms manifest Typically concentrations 10–20× ULN	The patient should be fasted Poor stability of glucagon in plasma (cool immediately!) Standardization of assay Immunoassay prone to interferences
VIP-oma [45]	Vasoactive intestinal polypeptide (VIP) Only screen if clinical symptoms manifest Typically concentrations 2–10× ULN	Take a blood sample from a symptomatic patient (e.g., periodic secretion) Instable peptide, needs a specific blood collection tube with protease inhibitors, cool immediately, and store at −70 °C until processing Standardization of assay Immunoassay prone to interferences Assay is only run in specialized laboratories
Somatostatinoma [46]	Somatostatin Only screen if clinical symptoms manifest Concentrations > 3× ULN	Standardization of assay Immunoassay prone to interferences Assay mainly run in research laboratories Cut-offs not well validated

Legend. ULN: upper limit of normal.

4. Imaging and Molecular Imaging

Imaging plays a central role in the management of MEN syndromes, supporting both early tumour detection and longitudinal disease monitoring. Given the multisystem involvement and variable biological behaviour of MEN-associated neoplasms, a multimodal imaging strategy is essential, integrating anatomical techniques with functional and molecular imaging to maximize sensitivity and diagnostic confidence. Structural modalities provide high-resolution assessment of organ morphology, while functional imaging enables lesion characterization, whole-body staging, and evaluation of biologically active disease. In clinical practice, imaging in MEN syndromes serves two complementary purposes. Surveillance imaging is performed in asymptomatic mutation carriers or stable patients to detect subclinical disease at a curable stage, guided by genotype-specific risk and age-related penetrance. In contrast, problem-solving imaging is employed to clarify equivocal biochemical findings, localize hormonally active tumours, or define disease extent in symptomatic or progressive cases. The appropriate balance between these strategies is critical to optimize outcomes while minimizing unnecessary investigations.

4.1. Strength of Evidence and Clinical Role of Imaging Modalities

The expanding range of anatomical and molecular imaging techniques available for MEN syndromes has substantially improved tumour detection and disease characterization. However, the level of evidence supporting individual imaging modalities varies considerably according to tumour type, syndrome, and clinical setting. For this reason, imaging findings should be interpreted within an evidence-based framework that distinguishes established modalities incorporated into clinical guidelines from emerging approaches supported primarily by retrospective studies, small patient cohorts, or preliminary clinical experience. In this review, imaging modalities are therefore classified into three broad categories:

Established imaging modalities comprise techniques supported by international guidelines and substantial clinical experience, including cervical ultrasonography for thyroid and parathyroid assessment, MRI for pituitary and pancreatic lesions, CT and MRI for pheochromocytoma localization, [^99mTc]Tc-MIBI scintigraphy for parathyroid imaging, [⁶⁸Ga]Ga-DOTATATE PET/CT for somatostatin receptor-positive NETs, and [¹⁸F]F-DOPA PET/CT for MTC and pheochromocytoma in selected clinical scenarios.

Advanced but increasingly established molecular imaging techniques include [¹⁸F]fluorocholine (FCH) PET/CT for pHPT, particularly in complex cases and reoperative settings. Diagnostic performance is consistently high, especially for surgical planning. Lesion-based sensitivity appears lower in MEN1-associated disease than in sporadic pHPT because of the predominance of small hyperplastic glands and multiglandular involvement.

Emerging or investigational imaging approaches include fibroblast activation protein inhibitor (FAPI)-based PET imaging and other novel radiotracers. These techniques have demonstrated promising preliminary results in selected neuroendocrine neoplasms, including MTC, but currently remain supported by limited clinical evidence and have not yet been incorporated into routine surveillance algorithms or international guideline recommendations. Accordingly, the diagnostic pathways proposed in this review prioritize established and guideline-supported imaging modalities, while recognizing the potential future role of emerging molecular imaging approaches as evidence continues to evolve.

4.2. Parathyroid Imaging in Multiple Endocrine Neoplasia (MEN)

Primary hyperparathyroidism represents the most prevalent endocrine manifestation in MEN1, affecting up to 90% of patients during their lifetime [23]. A comparable clinical phenotype can also arise from mutations in the CDKN1B gene, resulting in the much rarer MEN4 syndrome [47]. In contrast to sporadic pHPT, which is most often caused by a single parathyroid adenoma, parathyroid involvement in MEN1 and MEN4 is typically multiglandular, characterized by hyperplasia of all parathyroid glands [47,48,49,50,51,52,53,54,55,56,57,58,59]. In contrast to MEN1, pHPT in MEN2A occurs in only 20–30% of patients carrying RET mutations, typically presents at a median age of 30–40 years and may present as either a single adenoma or multiglandular disease [60,61]. The highest incidence is observed among carriers of codon 634 mutations in exon 11 [60,62]. Parathyroid disease is rarely the presenting manifestation of MEN2A and is most commonly diagnosed concurrently with MTC or during surveillance of known mutation carriers [60,61]. The clinical course of MEN-related pHPT is usually indolent, with mild hypercalcemia accompanied by mildly elevated or inappropriately normal PTH concentrations [61]. When symptoms occur, they most frequently reflect renal involvement, with hypercalciuria and nephrolithiasis reported in approximately 15–20% of cases [60]. Surgical indications generally mirror those applied in sporadic pHPT, with resection directed toward abnormal glands identified during preoperative evaluation [61,62]. Importantly, biochemical screening for pheochromocytoma must always be completed before any parathyroid surgery in MEN2A patients [60,61]. This fundamental difference poses distinct challenges for both preoperative imaging and surgical strategy. The role of preoperative parathyroid imaging in MEN1 remains a matter of debate. Current consensus guidelines state that there is no uniform agreement regarding the need for, or the optimal type of, preoperative imaging in adults with MEN1-associated pHPT [23]. In children and adolescents, imaging decisions are typically individualized. Nevertheless, most expert centres routinely perform at least a cervical US examination to evaluate for coexisting thyroid pathology and to assess cervical lymph nodes. Traditional parathyroid localisation techniques include neck US and ^99mTc-methoxyisobutylisonitrile ([^99mTc]Tc-MIBI) scintigraphy with single-photon emission computed tomography/computed tomography (SPECT/CT). Ultrasound (US) is widely available and avoids ionizing radiation, but its diagnostic performance is highly operator-dependent and limited in the detection of ectopic glands. Dual-phase [^99mTc]Tc-MIBI scintigraphy relies on differential radiotracer washout between thyroid and parathyroid tissue, while dual-tracer subtraction techniques allow suppression of thyroid gland activity and can facilitate the detection of ectopic parathyroid tissue beyond the reach of US. Magnetic resonance imaging (MRI) plays only a complementary role within the multimodal imaging work-up of patients with MEN1. While MRI provides clear added value for the evaluation of associated endocrine neoplasms—most notably in the pituitary gland and pancreas—its performance for parathyroid lesion detection remains limited. Parathyroid adenomas typically appear as oval, lymph node–like structures that are hyperintense on T2-weighted images and hypo- to isointense on T1-weighted images [48,49,50]. Time-resolved contrast-enhanced MRI techniques have been explored for parathyroid adenoma detection; however, their application in MEN1 patients has not been systematically reported [51]. Moreover, as these techniques generally require prior knowledge of lesion location and MEN1-associated parathyroid lesions are often very small and multiglandular, their potential utility in this specific patient population appears limited. Over the past 10–15 years, [¹⁸F]-fluorocholine (FCH) positron emission tomography combined with computed tomography or magnetic resonance imaging (PET/CT/MRI) has emerged as a highly effective parathyroid imaging modality [50,52,53,54,55,56,57,58,59]. Even in challenging clinical settings, its performance has been remarkable (Table 6 and Table 7 and Figure 3).

Table 6.

Comparison of imaging modalities for parathyroid localization.

Modality	Advantages	Limitations	Role in MEN1
Cervical ultrasound	Widely available; no ionizing radiation; evaluates thyroid pathology and lymph nodes	Highly operator-dependent; limited detection of ectopic glands	Routinely performed by most expert centres
[99mTc]Tc-MIBI scintigraphy with SPECT/CT	Established technique; detects ectopic tissue beyond ultrasound reach	False positives in thymic NETs, multinodular goitre, thyroiditis, inflammatory lymph nodes; higher radiation than FCH PET/CT(MRI)	Traditional localisation technique; less favoured when FCH PET/CT is available
MRI	No ionizing radiation; excellent for pituitary and pancreatic lesions in MEN1	Limited parathyroid detection, as MEN1-related lesions are often small and multiglandular	Complementary role; primarily for non-parathyroid MEN1-associated neoplasms
[18F]-fluorocholine PET/CT(MRI)	Superior diagnostic performance; lower radiation than [99mTc]Tc-MIBI and 4D-CT; excellent in reoperative setting; cost-effective as a first-line option	Lower sensitivity for hyperplastic glands than adenomas; false positives in thymic NETs, goitre, thyroiditis, and inflammatory lymph nodes	First-line modality; essential for reoperative setting

Legend. FCH, [¹⁸F]-fluorocholine; MEN, multiple endocrine neoplasia; MIBI, methoxyisobutylisonitrile; NET, neuroendocrine tumour; PET/CT (MRI), positron emission tomography/computed tomography (magnetic resonance imaging); SPECT/CT, single-photon emission computed tomography/computed tomography.

Table 7.

Recommended parathyroid imaging in MEN syndromes before and after surgery.

Setting	Parathyroid Imaging	Key Considerations
MEN1/MEN4, before initial surgery	Cervical ultrasound; [99mTc]Tc-MIBI SPECT/CT where FCH PET/CT(MR) is unavailable; MRI as a complementary problem-solver	Multiglandular involvement is typical; sensitivity is lower for hyperplastic glands than for adenomas
MEN2A, before initial surgery	Cervical ultrasound, usually performed alongside thyroid assessment, [99mTc]Tc-MIBI SPECT/CT, where FCH PET/CT is unavailable; MRI as a complementary problem-solver	Biochemical screening for pheochromocytoma must precede any parathyroid surgery
After surgery (persistent or recurrent disease)	FCH PET/CT	Accurate preoperative localization is essential before reoperation

Legend. FCH, [¹⁸F]fluorocholine; MEN, multiple endocrine neoplasia; MIBI, methoxyisobutylisonitrile; PET/CT (MR), positron emission tomography/computed tomography (magnetic resonance); SPECT/CT, single-photon emission computed tomography/computed tomography.

Figure 3.

FCH PET/MR in a patient with MEN1 syndrome. 24-year-old patient with MEN1 syndrome. The coronal FCH PET MIP image (A) shows a large pituitary adenoma (arrow) and three parathyroid adenomas (arrowheads). The pituitary adenoma is also seen on the axial T2w-weighted fat-suppressed MR image ((B); arrow) and on the axial FCH PET/MR image ((C); arrow). On the axial T2w-weighted fat-suppressed MR images (D,F) and FCH PET/MR images (E,G) of the neck, an orthotopic left-sided upper pole parathyroid adenoma ((D,E); arrowheads) is seen, as well as an ectopic right-sided intrathyroidal parathyroid adenoma ((F,G); arrowheads).

In summary, the role of imaging in MEN-related PHPT is to guide surgery rather than establish a diagnosis. FCH PET/CT provides the highest localization performance in MEN1, especially for recurrent disease, but sensitivity is lower than in sporadic hyperparathyroidism because of multiglandular hyperplasia. Evidence for MEN4 remains scarce, whereas imaging in MEN2A is generally complementary to biochemical surveillance and clinical management.

4.3. Medullary Thyroid Carcinoma Imaging in Multiple Endocrine Neoplasia (MEN)

Medullary thyroid carcinoma (MTC) is the hallmark tumour of MEN2 and the leading cause of disease-related mortality in affected patients. Imaging plays a central role in the initial evaluation, staging, surgical planning, and follow-up of MTC, complementing genetic risk stratification and biochemical monitoring with calcitonin and carcinoembryonic antigen (CEA).

4.3.1. Ultrasound

Neck ultrasonography is the first-line imaging modality for suspected or confirmed MTC and is essential for evaluating the thyroid gland and cervical lymph nodes [18,63]. However, ultrasonographic features of MTC overlap substantially with those of other thyroid malignancies, and currently available risk-stratification systems have been primarily developed for papillary thyroid carcinoma rather than MTC [64,65]. Cervical lymph node metastases are common, particularly in the central and lateral neck compartments, making careful nodal assessment crucial for surgical planning [66]. The diagnostic performance of ultrasound is highly operator-dependent and generally lower for central than lateral compartment lymph nodes [63,67,68]. Nevertheless, because of its wide availability, lack of ionizing radiation, and high sensitivity for cervical disease, ultrasound remains the cornerstone of preoperative imaging in MTC.

However, with the exception of cystic changes typically observed in papillary thyroid cancer metastasis, ultrasound characteristics of DTC and MTC metastasis overlap (Figure 4). In patients with suspicious cervical lymph nodes, ultrasound-guided fine-needle aspiration cytology (FNAC) can further improve diagnostic accuracy. Measurement of in the needle washout fluid is particularly useful for confirming metastatic MTC, especially when cytological findings are inconclusive or limited by scant cellularity [69,70]. Although thyroglobulin (Tg) measurement in FNAC washouts is primarily used to identify lymph node metastases from differentiated thyroid carcinoma, combined assessment of Tg and CT may be valuable in selected patients with an uncertain histological diagnosis or a history of multiple thyroid malignancies, helping to distinguish metastatic MTC from differentiated thyroid cancer involvement. Therefore, the integration of cytology with biochemical analysis of FNAC washouts represents an important adjunct to ultrasound-based nodal evaluation and may contribute to more accurate preoperative staging and surgical planning [36].

Figure 4.

(Left panel): A 44-year-old male patient with papillary thyroid cancer. Pre-surgery ultrasonographic images showed a metastatic lymph node at level VI with “thyroidization”, microcalcifications (white arrow) and cystic areas (white star). (Right panel): A 55-year-old female patient with medullary thyroid cancer. Pre-surgery ultrasonographic images showed a metastatic lymph node at level 2 with microcalcifications (white arrow) and macrocalcifications (white arrow heads).

4.3.2. Computed Tomography

Contrast-enhanced CT (ceCT) complements ultrasound by providing a more comprehensive assessment of locoregional disease extent, vascular anatomy, and mediastinal lymph node involvement [70]. Although its sensitivity for detecting the primary thyroid lesion is limited [70,71], ceCT is valuable for surgical planning and for evaluating distant metastases, particularly in the lungs and liver.

4.3.3. Magnetic Resonance Imaging

MRI is primarily reserved for selected clinical situations, particularly when locally invasive disease is suspected. Owing to its superior soft-tissue contrast, MRI is more accurate than CT for assessing tracheal, laryngeal, and cartilage invasion and is therefore the preferred modality when local extension is a concern [72,73]. MRI also plays an important role in the evaluation of liver metastases and is considered the imaging modality of choice for detecting bone metastases when combined with diffusion-weighted imaging [70,74].

4.3.4. Molecular Imaging

According to current clinical guidelines, after surgery, clinical examination and ultrasonographic evaluation of the thyroid and neck lymph nodes are required in patients with serum calcitonin levels < 150 pg/mL. Moreover, US, CT, MRI, and bone scintigraphy (BS) were traditionally recommended for patients with postoperative calcitonin > 150 pg/mL and/or a short calcitonin/CEA doubling time (<6 months) [75]. In molecular imaging, several different radiopharmaceuticals are used (Table 8) [76]. PET radiopharmaceuticals are currently preferred due to the inherent higher spatial resolution and sensitivity of PET/CT imaging. ATA guidelines did not recommend PET imaging for patients with recurrent MTC; however, the European Society of Medical Oncology (ESMO) included PET imaging to identify locoregional and/or distant metastases [77]. National Comprehensive Cancer Network (NCCN) guidelines recommend [⁶⁸Ga]Ga-DOTATATE PET/CT in patients with postoperative calcitonin levels > 150 pg/mL [78]. The European Association of Nuclear Medicine (EANM) guidelines recommended [¹⁸F]F-DOPA as the radiotracer of choice for patients with MTC and elevated calcitonin [79]. However, synthesis of [¹⁸F]F-DOPA needs a cyclotron, which is not readily available, and, therefore, the more accessible [⁶⁸Ga]Ga-DOTATATE is frequently used in MTCs. A theragnostic treatment option that [⁶⁸Ga]Ga-DOTATATE can provide is another advantage of this radiotracer. Although its overall sensitivity is low, [¹⁸F]FDG PET/CT may play a role in patients with higher CEA and short doubling time, especially in aggressive forms of MTC (Table 8).

Table 8.

PET and SPECT radiotracers for medullary thyroid carcinomas.

Target	PET Radiotracers	SPECT Radiotracers
Uptake mechanism
Glucose metabolism GLUT transporter	[18F]FDG
L-type amino acid transporter	[18F]F-DOPA
Tumour receptor
Somatostatin receptor	[68Ga]Ga-DOTATATE [68Ga]Ga-DOTANOC [64Cu]Cu-DOTATATE	[111In]In-pentetreotide, [99mTc]Tc-depreotide
Noradrenalin receptor	Meta-[18F]Fluorobenzylguanidine	Na [123I]I/Na [123I]I-MIBG
Cancer-associated fibroblasts	[68Ga]Ga-FAPI-04,46,2286 [68Ga]Ga-DOTA.(SA.FAPI)2 and DOTAGA.(SA.FAPI)2	[99mTc]Tc-HYNIC-FAPI-04 [99mTc]Tc-FAPI-04
Cholecystokinin-2 receptor (CCK2R)	[68Ga]Ga-DOTA-CCK-66 [68Ga]Ga-DOTA-MGS5 (minigastrin analogue)	[111In]In-CP04 (CCK2R-) ligand
Anti-carcinoembryonic antigen (CEA) antibody	[68Ga]Ga-IMP288 (ImmunoPET)
Osteoblastic reaction	[18F]NaF [68Ga]Ga-DOTA-zoledronate	[99mTc]Tc-MDP, HDP

FDG: Fluorodeoxyglucose, PET: positron emission tomography, SPECT: single-photon emission computed tomography, MIBG: Meta-iodobenzylguanidine [23,24].

[¹⁸F]F-FDG PET/CT

[¹⁸F]FDG PET/CT is the most widely available PET radiotracer and reflects increased glucose metabolism associated with aggressive, poorly differentiated MTC [79]. [¹⁸F]FDG uptake correlates with GLUT overexpression and increased hexokinase activity, although small, indolent, necrotic, calcified, or sclerotic lesions may be missed [79]. Reported patient-based sensitivity and specificity range from 17 to 93% and 68–92%, respectively [80,81]. In recurrent MTC, the pooled DR is approximately 59%, increasing in patients with calcitonin ≥ 1000 ng/L, elevated CEA, or rapid calcitonin/CEA doubling times (Figure 5A) [81,82]. Beyond diagnosis, [¹⁸F]FDG PET/CT provides prognostic information: uptake is significantly associated with progressive disease [83], while lower baseline SUVmean predicts longer progression-free survival in patients treated with vandetanib [84].

Figure 5.

Metastatic MTC on [¹⁸F]FDG, [⁶⁸Ga]Ga-DOTATATE, [¹⁸F]F-DOPA and [⁶⁸Ga]Ga-FAPI. (A) Recurrent MTC intense [¹⁸F]FDG uptake in supraclavicular, mediastinal and abdominal lymph node metastases; (B) bone metastases from MTC showing moderate [⁶⁸Ga]Ga-DOTATATE uptake; (C) postoperative MTC recurrence: [¹⁸F]F-DOPA positive lymph nodes (right upper neck and central compartment, arrows); (D) recurrent MTC: [⁶⁸Ga]Ga-FAPI shows bone and liver metastases.

[⁶⁸Ga]-Somatostatin Analogues (SSAs)

Because somatostatin receptor expression is variable in MTC, [⁶⁸Ga]Ga-SSA PET/CT generally demonstrates lower uptake than in other NETs [79]. False positives may occur in inflammatory or benign conditions, whereas small lesions, liver metastases, or dedifferentiated tumours may yield false-negative findings [79]. The pooled DR in recurrent MTC is approximately 63.5%, increasing with higher calcitonin levels [85]. A major advantage is the excellent detection of bone metastases due to low skeletal background activity, outperforming bone scintigraphy and MRI in small studies [86] (Figure 5B). [⁶⁸Ga]Ga-SSA PET/CT also affects management decisions, identifying occult nodal or osseous disease and guiding surgery, radiotherapy, or peptide receptor radionuclide therapy (PRRT) in up to one-third of patients [87]. PRRT with [¹⁷⁷Lu]Lu-DOTA-peptides has shown limited but potentially meaningful benefit in selected patients with somatostatin receptor-positive advanced MTC refractory to standard therapies. Reported outcomes are modest, with disease stabilization more common than objective response, although symptomatic improvement and prolonged progression-free survival may occur [88,89].

[¹⁸F]F-DOPA PET/CT

Due to its high sensitivity, [¹⁸F]F-DOPA PET/CT is recommended by EANM guidelines as the preferred PET tracer for recurrent MTC [79]. Reported sensitivities range from 45% to 93%, with pooled per-patient and per-lesion DRs of 66% and 71%, respectively [81] (Figure 5C). Detection significantly improves in patients with calcitonin ≥ 1000 ng/L or rapid calcitonin doubling times [81]. Several studies demonstrated superiority over conventional imaging and substantial impact on management, particularly through improved detection of nodal and bone metastases and guidance for surgery [90,91].

Fibroblast Activation Protein (FAP) Inhibitors

Fibroblast activation protein (FAP), expressed by cancer-associated fibroblasts within the tumour microenvironment, is associated with aggressive tumour behaviour and poor prognosis in MTC [92,93,94]. Radiolabeled fibroblast activation protein inhibitors (FAPI) have recently emerged as promising PET tracers with potential theranostic applications. Early studies demonstrated superior detection of metastatic disease compared with both [⁶⁸Ga]Ga-DOTANOC and [¹⁸F]FDG PET/CT, particularly for liver, lung, bone, and pleural metastases (Figure 5D) [95,96,97]. In a prospective phase II trial, [⁶⁸Ga]Ga-CTR-FAPI-30 PET/CT-guided surgery improved lesion detection, altered surgical management, and achieved favourable biochemical cure rates, supporting the growing clinical relevance of FAPI imaging in MTC [98].

Comparison of PET Radiotracers in MTC

Comparative studies remain limited, but available evidence generally supports superior performance of [¹⁸F]F-DOPA PET/CT over [¹⁸F]FDG and [⁶⁸Ga]Ga-SSA PET/CT in recurrent MTC [99,100,101,102]. [¹⁸F]F-DOPA appears particularly effective for liver and nodal metastases, whereas [⁶⁸Ga]Ga-DOTATATE may better detect skeletal lesions [101]. Emerging evidence suggests that FAPI PET may further improve diagnostic accuracy compared with both [¹⁸F]FDG and SSA imaging [95,96,97,98], although isolated reports indicate variable performance in pediatric metastatic MTC [103]. Radiation exposure is broadly comparable among tracers, with effective doses of approximately 0.020–0.025 mSv/MBq for [¹⁸F]FDG, 0.020 mSv/MBq for [¹⁸F]F-DOPA, 0.025 mSv/MBq for [⁶⁸Ga]Ga-SSA, and 0.0123 mSv/MBq for [⁶⁸Ga]Ga-FAPI [79,104]. Notably, the sensitivity and detection-rate ranges cited above derive largely from small, retrospective, single-centre series employing heterogeneous calcitonin thresholds and reference standards, and should be read as indicative rather than definitive.

4.4. Pheochromocytoma and Paraganglioma Imaging

Pheochromocytomas (PCCs) and paragangliomas (PGLs), collectively known as PPGLs, are rare NETs originating from chromaffin cells of the adrenal medulla or extra-adrenal sympathetic/parasympathetic ganglia [105]. PPGLs have an estimated incidence of 2–8 cases per million annually, with a prevalence of up to one in 2000–6500 in autopsy series, often underdiagnosed due to nonspecific symptoms like hypertension, headaches, and palpitations. Approximately 80–85% arise in the adrenal medulla (PCCs), while 15–20% are extra-adrenal PGLs, subdivided into sympathetic (thoracic/abdominal/pelvic) or parasympathetic (head-and-neck, HNPGLs). These tumours exhibit significant genetic heterogeneity, with over 40% harbouring germline mutations, influencing their clinical behaviour, metastatic potential, and imaging phenotypes [106]. Up to 50% are hereditary, linked to mutations in 19 susceptibility genes, clustering into pseudohypoxic (Cluster 1: SDHx, VHL, FH, MDH2, EPAS1) and kinase signalling (Cluster 2: RET, NF1, MAX, TMEM127) pathways [107]. Metastatic disease occurs in 15–50% of cases, particularly in Cluster 1 tumours, necessitating multimodal imaging for staging [108]. Imaging goals include tumour localisation, multifocality detection (up to 30% in hereditary cases), malignancy assessment, and avoidance of biopsy due to the risk of catecholamine crisis. Anatomical imaging modalities such as CT and MRI remain foundational for initial detection and localisation, providing detailed morphological characterization [109]. Molecular imaging, including PET and SPECT, provides functional insights by targeting specific transporters and receptors overexpressed in PPGLs [110]. Genotype-driven imaging selection has emerged as a paradigm shift, tailoring tracer choice to underlying genetic clusters for optimized sensitivity and therapeutic planning [111]. Computed tomography, particularly multidetector CT (MDCT), is the workhorse for PPGL imaging due to its speed, availability, and multiplanar reconstruction capabilities. Non-contrast CT reveals well-circumscribed masses with mean attenuation > 20 Hounsfield units (HU), reflecting high proteinaceous content and catecholamine granules; adenomas typically show <10 HU due to lipid content. Post-contrast arterial phase demonstrates avid, heterogeneous enhancement (>130 HU in PCCs) from hypervascularity supplied by branches of the aorta or inferior mesenteric artery; venous phase shows persistent enhancement with delayed washout [105,112]. PCCs often exceed 3 cm at diagnosis, with punctate calcifications (10–20%), cystic degeneration, or hemorrhage causing heterogeneity. Adrenal PCCs may invade adjacent structures, while sympathetic PGLs cluster along the organ of Zuckerkandl or bladder wall, showing a “hockey stick” appearance on coronal views. HNPGLs, like carotid body tumours, splay the carotid bifurcation (“lyre sign”), making them recognizable [113]. The major limitations related to CT include radiation exposure (10–20 mSv), suboptimal soft-tissue contrast for small HNPGLs, and inability to assess bone marrow metastases, which are overcome by the other imaging modalities. MRI surpasses CT in soft-tissue resolution and radiation-free imaging, making it first-line for children, pregnant patients, and HNPGL surveillance. T1-weighted images show homogeneous hypointensity relative to muscle; T2-weighted sequences reveal classic hyperintensity (“light-bulb bright” sign in 66% of PCCs) from myxoid stroma and low cellularity [105]. The “salt-and-pepper” pattern arises from flow voids (pepper) in hypervascular tumours and high T1 hemorrhages (salt), seen in 50% of large lesions. Dynamic gadolinium-enhanced sequences mirror CT hypervascularity: early arterial peak at 20–30 s, plateau phase. Chemical-shift imaging (in-phase/out-of-phase) demonstrates no signal dropout in PPGLs versus adenomas, with an adrenal-to-spleen ratio > 80% specificity. Diffusion-weighted imaging (DWI) shows restricted diffusion in cellular tumours, higher in necrotic ones. 3T MRI with fat-suppression enhances small HNPGL detection, while whole-body MRI detects bone metastases with 90% sensitivity [114]. The principal drawbacks of MRI include longer scan times, motion artefacts, and contraindications (e.g., pacemakers) [105]. PPGL imaging leverages tumour biology: chromaffin cells express norepinephrine transporter (NET) for catecholamine reuptake, L-type amino acid transporter (LAT1), aromatic L-amino acid decarboxylase (AADC), glucose transporters (GLUT1/3), and somatostatin receptors (SSTR2/5). Cluster 1 mutations (SDHx) cause pseudohypoxia via Hypoxia-Inducible Factor (HIF) stabilization, upregulating SSTR2/GLUT, downregulating NET. Cluster 2 activates kinase pathways (pseudonormalized oxygen sensing), preserving NET/LAT [115,116]. Metastases avidly uptake [¹⁸F]FDG due to TCA cycle defects, thus demanding whole-body imaging to detect multifocality (20–40% in SDHB) (Table 9 and Table 10).

Table 9.

Summary of the principal tracers used for molecular imaging of PPGLs.

Tracer	Mechanism	Strengths	Weaknesses	Preferred Genotypes
Na [123I]I-MIBG	NET uptake/storage	Therapy selection, PCC	Low SDHx sens. (50%)	RET/NF1 Cluster 2
[18F]FDG	GLUT/hexokinase	Metastatic/SDHB	Indolent tumours	SDHx Cluster 1
[18F]F-DOPA	LAT/AADC	PCC/HNPGL 96%	Carbidopa needed	Cluster 2, VHL
[68Ga]Ga-DOTA-SSA	SSTR	Metastatic 95%, PRRT	Costly cyclotron	SDHx, Cluster 1

Legend. MIBG, metaiodobenzylguanidine; NET, neuroendocrine tumour; PCC, pheochromocytomas; SDHx, succinate dehydrogenase enzyme complex; RET, Rearranged during Transfection; NF1, Neurofibromatosis Type 1; FDG, Fluorodeoxyglucose; GLUT, Glucose Transporter; SDHB, Succinate Dehydrogenase Complex Iron Sulfur Subunit B; F-DOPA, fluoro-dihydroxyphenyl-L-alanine; LAT, L-type amino acid transporter; AADC, aromatic L-amino acid decarboxylase; HNPGL, head-and-neck paraganglioma; VHL, von Hippel-Lindau; DOTA-SSA, DOTA somatostatine analogue; SSTR, somatostatine receptor; PRRT, peptide receptor radionuclide therapy.

Table 10.

Genetic classification and imaging priority.

Cluster	Genes	Imaging Priority	Metastases Risk	Rationale
Hypoxic	SDHx, VHL, FH	[68Ga]-DOTA-SSA [18F]FDG	High (30–70%)	SSTR ↑, NET ↓, glycolysis ↑
Kinase	RET, NF1, MAX	[18F]F-DOPA/Na [123I]I-MIBG	Low-moderate	NET/LAT preserved
Rare	TMEM127, EPAS1	[18F]-FDOPA primary	Variable	Mixed phenotype

Legend. MIBG, metaiodobenzylguanidine; NET, neuroendocrine tumour; SDHx, succinate dehydrogenase enzyme complex; NF1, Neurofibromatosis Type 1; FDG, Fluorodeoxyglucose; FDOPA, fluoro-dihydroxyphenyl-L-alanine; LAT, L-type amino acid transporter; VHL, von Hippel-Lindau; SSA, somatostatine analogue; SSTR, somatostatine receptor; FH, Fumarate Hydratase; RET, Rearranged during Transfection; MAX, MYC-associated factor X; TMEM127, Transmembrane Protein 127; EPAS1, Endothelial PAS Domain Protein 1.

Meta-iodobenzylguanidine (MIBG) mimics norepinephrine, taken up via NET and stored in neurosecretory granules. Na [¹²³I]I-MIBG SPECT/CT sensitivity reaches 83–100% for PCCs but drops to 50–70% in metastatic/SDHx PGLs due to NET downregulation [117]. Physiologic uptake occurs in salivary glands, heart, liver, and spleen; blockers (e.g., phenoxybenzamine) reduce background. Its major advantage consists of guiding [¹³¹I]-MIBG therapy (response in 30–50% metastatic cases), but detains as limitations, including low spatial resolution, a 24–48 h delay for images and approximately 20% false negatives [111,118]. [¹⁸F]-FDG (or 2-deoxy-2-[fluorine-18]fluoro-D-glucose) PET exploits the Warburg effect [119] in aggressive PPGLs, with SUVmax > 10 correlating to SDHB mutations (sensitivity 90% vs. 50% in non-SDH) [120]. It excels in succinate dehydrogenase (SDHx)-related metastases and bone/soft-tissue lesions, detecting occult disease missed by NET tracers. Overall, the sensitivity ranges from 75% to 100%, with MEN2 having suboptimal rates (40%) [105]. Dual-tracer protocols ([¹⁸F]FDG + others) achieve >95% concordance but have to deal with pitfalls and lower sensitivity in indolent cluster 2 tumours. Dihydroxyphenylalanine is a precursor of catecholamines; thus, its radiolabelled tracer [¹⁸F]F-DOPA is taken up through neutral LATs and is also expressed on cell membranes of PPGLs (Figure 6, Figure 7 and Figure 8).

Figure 6.

[¹⁸F]F-DOPA PET/CT of a patient with paraganglioma. [¹⁸F]F-DOPA PET/CT of a patient with paraganglioma of the right temporo-jugular region ((A,B); red arrows). Corresponding T1-weighted (C) and gadolinium-enhanced MRI (D) sequences highlight the lesion.

Figure 7.

Comparative images of a patient with bilateral pheochromocytoma and unexpected paraganglioma of the mediastinum and left carotid region. (A,B) [¹⁸F]F-DOPA PET/CT confirms the bilateral pheochromocytoma (red arrow heads) plus the paraganglioma of the left carotid region (red arrow). (C,D) [⁶⁸Ga]-DOTATOC PET/CT of the same patient documenting the same lesions as mentioned above, plus the mediastinal para-aortic paraganglioma (red circle), not visible on [¹⁸F]F-DOPA PET/CT.

Figure 8.

Contrast-enhanced CT of the patients with bilateral pheochromocytoma and unexpected paraganglioma of the mediastinum and left carotid region (shown in Figure 7). Contrast-enhanced CT of the patients with bilateral pheochromocytoma and unexpected paraganglioma of the mediastinum and left carotid region ((A), axial thorax; (B), coronal neck; (C), axial abdomen), confirming the presence of an enlarged left adrenal gland (diam. 34 mm), characterized by rapid, inhomogeneous contrast wash-in and retention of the contrast medium in late scans ((C), red arrow heads). The contralateral adrenal gland shows a similar 27 mm enlargement (red arrowheads). The additional paraortic ((A), red circle) and left laterocervical ((B), red arrow) lesions are only retrospectively identified.

[¹⁸F]F-DOPA is decarboxylated by AADC into [¹⁸F]-dopamine via LAT1, offering 96% sensitivity for PCCs, 90% for HNPGLs, and outperforming MIBG in cluster 2. Carbidopa premedication blocks peripheral decarboxylase, enhancing the tumour-to-background ratio. Similarly to [¹⁸F]F-DOPA, [¹⁸F]F-FDA (fluorodopamine) targets NET/AADC but with higher PCC specificity [105,111]. Gallium-68-labelled DOTA peptides (DOTA-TOC/TATE, DOTATATE) bind SSTR2, overexpressed in 80–90% of cluster 1 PPGLs. Sensitivity exceeds 90% for metastatic/SDHx disease with SUVmax > 20, making the modality a first-line recommendation in the EANM guidelines [111]. [⁶⁸Ga]Ga-DOTATATE PET/CT illustrates intense uptake in multifocal SDHB-related PGLs, guiding peptide receptor radionuclide therapy. Theranostic pairing with [¹⁷⁷Lu]Lu-DOTATATE yields PR in 30–60% of refractory cases (Table 6). SPECT alternatives ([^99mTc]Tc-HYNIC-TOC) provide cost-effective options [121]. Cost-effectiveness favours [⁶⁸Ga]Ga-DOTA-SSA ($2500 vs. $5000 FDG in high-risk) [122]. Notably, these cost-effectiveness data derive from a limited number of modelling studies conducted in specific healthcare settings and, accordingly, absolute costs and incremental ratios are not directly transferable between systems. Additionally, reported SUV thresholds are scanner- and protocol-dependent and are not universally applicable; they should be interpreted alongside qualitative assessment rather than as fixed diagnostic cut-offs and the conclusions should be regarded as setting-dependent.

In conclusion, the genetic classification delineates the hierarchy of tracers to be employed, as summarized below [123]:

• SDHx (esp. SDHB) → [⁶⁸Ga]Ga-DOTA-SSA first (97% sens.) → [¹⁸F]FDG; Na [¹²³I]I-MIBG < 50%.

• VHL: [¹⁸F]F-DOPA (100% PCC, 90% PGL); alternate [⁶⁸Ga]Ga-DOTA-SSA.

• RET/NF1: Na [¹²³I]I-MIBG or [¹⁸F]F-DOPA (95%).

• Unknown/Sporadic: [¹⁸F]F-DOPA (PCC) or [⁶⁸Ga]Ga-DOTA-SSA (PGL) → reflex to genetic testing.

5. Integrated Diagnostic Algorithms in MEN Syndromes

5.1. Integrated Diagnostic Pathway for MEN1

MEN1 is characterized by the development of pHPT, gastroenteropancreatic NETs (GEP-NETs), and pituitary adenomas. Following confirmation of a pathogenic MEN1 variant, surveillance should be initiated in childhood and continued throughout life. For parathyroid disease, regular biochemical screening using serum calcium and parathyroid hormone measurements represents the cornerstone of surveillance. Imaging is not generally recommended as a screening tool but becomes relevant after biochemical confirmation of pHPT, particularly when surgical intervention is planned. Cervical ultrasonography is usually performed as the initial imaging modality, while [¹⁸F]fluorocholine (FCH) PET/CT has emerged as the preferred second-line technique for lesion localization, especially in recurrent or persistent disease and in patients requiring reoperative surgery. For pancreatic and duodenal NETs, surveillance combines periodic biochemical evaluation with cross-sectional imaging. Gastrin, insulin, glucagon, vasoactive intestinal peptide, and other hormone-specific biomarkers should be measured according to tumour type and clinical presentation. Pancreatic MRI represents the preferred anatomical imaging modality because of its high sensitivity and lack of ionizing radiation. Functional imaging using somatostatin receptor PET/CT, particularly [⁶⁸Ga]Ga-DOTATATE PET/CT, should be considered when biochemical abnormalities are detected, when lesions are identified on anatomical imaging, or when tumour staging is required. Pituitary surveillance relies primarily on periodic biochemical assessment, including prolactin and insulin-like growth factor 1 (IGF-1), supplemented by additional hormonal testing according to clinical suspicion. Pituitary MRI remains the imaging modality of choice and should be performed when biochemical abnormalities emerge or symptoms suggest pituitary involvement. Overall, the diagnostic pathway in MEN1 follows a sequential strategy in which genetic diagnosis initiates lifelong biochemical surveillance, with imaging reserved for lesion localization, staging, treatment planning, and assessment of disease progression (Figure 9).

Figure 9.

Integrated diagnostic pathway for MEN 1 syndrome.

5.2. Integrated Diagnostic Pathway for MEN2

In MEN2 syndromes, germline RET testing represents the cornerstone of diagnosis and risk stratification. Current surveillance strategies are largely determined by the specific RET variant and its associated risk category, allowing individualized timing of biochemical monitoring, imaging investigations, and prophylactic interventions. Medullary thyroid carcinoma is the defining feature of MEN2 and develops in nearly all affected individuals. Surveillance begins with serial calcitonin measurements, which serve as the most sensitive marker of C-cell disease. Neck ultrasonography is performed to evaluate thyroid morphology and cervical lymph nodes. When calcitonin concentrations become elevated or suspicious lymph nodes are identified, ultrasound-guided fine-needle aspiration with cytological evaluation and calcitonin measurement in needle washouts may provide additional diagnostic confirmation. In patients with advanced disease, rapidly increasing calcitonin concentrations or suspected metastatic spread, cross-sectional imaging and molecular imaging techniques, including [¹⁸F]F-DOPA PET/CT and selected somatostatin receptor-based PET tracers, contribute to staging and treatment planning. Pheochromocytoma surveillance relies primarily on annual measurement of plasma free metanephrines or urinary fractionated metanephrines. Positive biochemical findings should prompt adrenal imaging using computed tomography or MRI. Functional imaging, particularly [¹⁸F]F-DOPA PET/CT, may be useful for tumour localization, assessment of multifocal disease, and detection of extra-adrenal lesions. Importantly, biochemical exclusion of pheochromocytoma remains mandatory before any thyroid or parathyroid surgery. Primary hyperparathyroidism occurs less frequently in MEN2A than in MEN1 and is generally identified through periodic calcium and parathyroid hormone measurements. Following biochemical confirmation, cervical ultrasonography is usually the initial localization technique, while FCH PET/CT may provide additional information in selected cases, particularly in patients undergoing reoperative surgery or when ectopic glands are suspected. Thus, the MEN2 diagnostic framework combines genotype-based risk stratification with organ-specific biochemical surveillance and targeted imaging, enabling timely intervention while minimizing unnecessary investigations (Figure 10).

Figure 10.

Integrated diagnostic pathway for MEN 2 syndrome.

5.3. Integrated Diagnostic Approach to MEN4 and Emerging MEN-like Syndromes

The rarity of MEN4 and the recently described MEN5 currently limits the availability of evidence-based surveillance protocols. Nevertheless, the same principles of integrated diagnostics apply. In patients presenting with MEN-like clinical features but lacking pathogenic variants in MEN1 or RET, extended hereditary endocrine tumour gene panels should be considered. Identification of pathogenic variants in CDKN1B supports the diagnosis of MEN4, whereas pathogenic variants in MAX have been associated with the emerging MEN5 phenotype. Given the incomplete understanding of disease penetrance and natural history in these syndromes, surveillance should remain phenotype-driven. Biochemical monitoring should focus on the endocrine organs known to be affected, while imaging should be tailored to specific biochemical abnormalities or clinical manifestations. MRI, ultrasound, and molecular imaging modalities should be selected according to tumour type and suspected disease location, following principles similar to those applied in MEN1 and MEN2. Future multicentre registries and prospective studies will be required to refine surveillance algorithms for these rare entities and to establish evidence-based recommendations for risk-adapted screening.

6. Conclusions and Perspectives

The increasing availability of comprehensive genetic testing, high-sensitivity biochemical assays, and advanced molecular imaging is transforming the management of MEN syndromes from phenotype-driven diagnosis to genotype-guided precision surveillance. The present review highlights the central and evolving role of genetic testing as the cornerstone of integrated diagnostics in MEN syndromes. In contemporary clinical practice, molecular characterization is no longer a confirmatory tool but rather the primary entry point for diagnosis, risk stratification, and longitudinal management. The identification of germline pathogenic variants in genes such as MEN1, RET, CDKN1B, and MAX not only defines disease classification but also guides the entire diagnostic workflow. A key strength of genetic testing lies in its ability to anticipate disease before clinical manifestation. In MEN2, for example, genotype-driven risk stratification has enabled the implementation of prophylactic thyroidectomy at mutation-specific ages, dramatically improving outcomes in MTC. Similarly, in MEN1, early identification of mutation carriers allows structured biochemical and imaging surveillance long before tumour-related morbidity develops. This paradigm exemplifies precision medicine, where genetic information directly translates into tailored screening intervals, biomarker selection, and imaging strategies. Importantly, genetic testing also acts as a unifying framework for integrating biochemical and imaging data. Biomarkers such as calcitonin, metanephrines, or hormone profiles provide dynamic insights into tumour activity, but their interpretation is significantly strengthened when contextualized within a known genetic background. Likewise, imaging strategies are increasingly genotype-oriented: for instance, the selection of molecular imaging tracers in pheochromocytoma/paraganglioma is influenced by the underlying mutation cluster, reflecting tumour biology at the molecular level. This genotype–phenotype–imaging correlation represents a major advancement in integrated diagnostics, reducing diagnostic ambiguity and improving sensitivity. Despite these advances, several limitations remain. Much of the imaging evidence derives from small, single-centre, retrospective cohorts, and the data on MEN4 rests largely on isolated case reports, which constrain the strength of any quantitative conclusion. Reported sensitivities and detection rates vary widely due to differences in patient selection, calcitonin or PTH thresholds, imaging protocols, and reference standards. Positive studies are also more likely to be published than negative ones, and the literature may therefore overstate diagnostic performance. Incomplete assay standardization for several of the biomarkers discussed further limits cross-study comparison. These constraints reflect the rarity of MEN syndromes and underline the need for prospective, multicentre studies with harmonized methodology. Variants of uncertain significance continue to pose interpretative challenges, particularly in genes with incomplete penetrance or limited phenotype characterization, such as CDKN1B or MAX. Moreover, mosaicism and rare intragenic rearrangements may escape detection with standard sequencing approaches, necessitating complementary techniques. From a clinical standpoint, the variability in phenotypic expression—even among carriers of the same mutation—limits the predictive accuracy of genetic data alone and underscores the need for continuous integration with biochemical and imaging findings. The current state of the art, therefore, supports a fully integrated diagnostic model in which genetic testing provides the backbone, while biochemical and imaging modalities offer temporal and spatial resolution of disease. Multidisciplinary collaboration among endocrinologists, geneticists, laboratorians, and imaging specialists is essential for implementing this model effectively. Looking ahead, several perspectives are emerging. First, the expansion of next-generation sequencing panels and whole-exome/genome approaches will likely uncover additional susceptibility genes and refine genotype–phenotype correlations. Second, advances in liquid biopsy and circulating tumour DNA may complement germline testing by enabling real-time monitoring of tumour evolution. Third, artificial intelligence-driven integration of genetic, biochemical, and imaging datasets holds promise for improving predictive modelling and individualized risk assessment. Finally, the development of standardized algorithms for interpreting complex genetic findings, including VUS, will be crucial to translating molecular data into clinical decision-making. In conclusion, genetic testing has transitioned from a diagnostic adjunct to the central pillar of MEN syndrome management. Its integration with biochemical and imaging modalities defines the modern approach to these complex disorders, enabling earlier detection, personalized surveillance, and improved patient outcomes.

Author Contributions

Conceptualization, L.G. and P.P.O.; methodology, L.G., P.P.O., M.C., M.W.H., M.T., J.J.H. and E.L.; investigation, L.G., P.P.O., M.C., M.W.H., M.T., J.J.H. and E.L.; data curation, L.G., P.P.O., M.C., M.W.H., M.T., J.J.H. and E.L.; writing—L.G., P.P.O., M.C., M.W.H., M.T., J.J.H. and E.L.; writing—review and editing, L.G., P.P.O., M.C., M.W.H., M.T., J.J.H. and E.L.; visualization, L.G.; supervision, L.G. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created.

Conflicts of Interest

The authors declare no conflicts of interest.

Funding Statement

This research received no external funding.