Hyperplastic thyroid nodule masquerading as parathyroid adenoma in a patient with tubercular lymphadenitis induced hypercalcaemia

Abstract

Serum intact parathyroid hormone (iPTH) levels are high or high normal in patients with parathyroid adenoma. Rarely these patients can have normal or low serum iPTH values. With sandwich immunometric assays, an exceptionally high serum iPTH level can lead to falsely low measurement due to the ‘hook effect’. Here, we describe the case of a 66-year-old female patient with PTH-independent hypercalcaemia which mimicked parathyroid adenoma. A multidisciplinary team approach helped in the diagnosis and management leading to complete recovery.

Background

Hypercalcaemia in clinical practice is largely due to primary hyperparathyroidism (PHPT) and malignancy, which together account for 90% of the cases. Investigation of hypercalcaemia begins with measurement of serum intact parathyroid hormone (iPTH) levels.¹ An inappropriately elevated serum iPTH suggests parathyroid-related causes of hypercalcaemia such as adenoma. In contrast, a suppressed serum iPTH indicates a PTH-independent aetiology such as malignancy (box 1).¹ In practice, this broad distinction does not always lead to correct diagnosis. In fact, patients with PHPT can have normal, low-normal and occasionally low serum iPTH.^2–9 Low or low-normal serum iPTH levels in patients with parathyroid adenoma have been described earlier due to multiple reasons.^7–11 The foremost is failure of maintenance of cold chain for the heat-labile PTH molecule. Coexistent sarcoidosis, magnesium deficiency and vitamin D toxicity can also lead to lower than the expected serum iPTH levels in patients with proven parathyroid adenoma.^{10 11} Other rare causes include parathyroid hormone-related protein (PTHrP) secreting parathyroid adenoma, intermittent release of PTH, PTH gene mutation and aberrant bioactive but immunologically inert PTH.^{3 6 7} Hook effect is another potential concern when serum iPTH levels are lower than the expected values in PHPT. Hook effect is characterised by false low serum levels of a hormone when its circulating levels are in fact very high. The effect is described with sandwich immunometric assays for several hormones including prolactin, beta human chorionic gonadotropin (β-hCG) and calcitonin.^12–14

Box 1. Causes of hypercalcaemia.

PTH-dependent hypercalcaemia (normal or high serum intact PTH level)

• Primary hyperparathyroidism (sporadic or familial; normal or elevated 24 hours urine calcium/UCCR)

• Tertiary hyperparathyroidism (usually associated with chronic renal dysfunction; normal or elevated 24 hours urine calcium/UCCR)

• Familial hypocalciuric hypercalcaemia (inherited defect in calcium sensing mechanism; low 24 hours urine calcium/UCCR)

• Lithium therapy (antagonism of calcium sensing receptor causing a shift in PTH inhibitory set point to a higher calcium concentration)

• Antagonistic autoantibodies to calcium sensing receptor

PTH-independent hypercalcaemia (suppressed serum intact PTH level)

• Malignancy (PTHrP production, local osteolysis)

• Multiple myeloma (local osteolysis)

• Granulomatous diseases, lymphoma (calcitriol mediated)

• Vitamin D toxicity (uncontrolled use of high doses of vitamin D or its metabolites)

• Adrenal insufficiency

• Pheochromocytoma

• Thyrotoxicosis

• Milk-alkali syndrome

• Immobilisation

• Vitamin A intoxication

• Thiazide diuretics

• Jansen’s disease (genetic disorder with constitutive activation of PTH/PTHrP receptor)

• Genetic defect in CYP24A1 gene encoding vitamin D 24-hydroxylase enzyme

Case presentation

A 66-year-old postmenopausal woman was admitted in the gastroenterology unit of our tertiary hospital with history of weight loss of 13 kg, dyspeptic symptoms and dry cough of 6 months duration. There was no history of haemoptysis, alteration in bowel habits, haematemesis, melena or haematochezia. She denied history of fever, night sweats or a recent contact with a person having tuberculosis. She was a known patient of hypertension and diabetes but well controlled on oral medications with daily dose of metoprolol 100 mg, amlodipine 10 mg and linagliptin 5 mg. On general physical examination, her vital parameters were normal and there was no evidence of pallor, icterus, cyanosis, clubbing or lymphadenopathy. On abdominal examination, hernial sites were free, and there was no organomegaly or palpable abdominal lump. Respiratory examination revealed bilateral equal vesicular breath sounds at all sites; there were no findings suggestive of pleural effusion, consolidation or cavitation.

Investigations

Her biochemical investigations at admission revealed serum calcium of 12.9 mg/dL (N: 8.5–10.5 mg/dL), serum inorganic phosphate of 3.7 mg/dL (N: 2.5–4.5 mg/dL) and serum alkaline phosphatase of 152 IU/L (N: 80–240 IU/L). Her blood urea and serum creatinine were elevated with values of 63 mg/dL (N: 10–40 mg/dL) and 3.0 mg/dL (N: 0.5–1.5 mg/dL), respectively. Blood sample for serum iPTH measurement was carefully sent to the laboratory under cold conditions. Serum iPTH value as assayed by electrochemiluminometric assay (Roche Diagnostics, Mannheim, Germany) was 9.1 ng/L (N: 15–65 ng/L). An endocrinology opinion was sought and a work-up for PTH-independent cause was advised. Vitamin D toxicity, thyrotoxicosis and multiple myeloma were excluded by relevant investigations (table 1). However, her serum 1,25-dihydroxyvitamin D (1,25(OH)2D3) value was higher than normal (235 pmol/L; normal 47.7–190.3 pmol/L).

Table 1.

Baseline laboratory investigations of the patient

Investigation	Result	Normal value
Serum urea/creatinine (mg/dL)	63/3.0	10–40/0.5–1.5
Serum total calcium (mg/dL)	12.9	8.5–10.5
Serum inorganic phosphate (mg/dL)	3.7	2.5–4.5
Serum sodium/potassium (mmol/L)	139/3.9	135–145/3.5–5.5
Serum albumin (gm/dL)	3.2	>3.5
SGOT/SGPT (IU/L)	14/7	10–30
Serum ALP (IU/L)	152	80–240
Fasting plasma glucose (mg/dL)	135	70–100
Glycated haemoglobin (%)	5.1	<6.5
Serum 25(OH)D3 (µg/L)	28	20–100
Serum Intact PTH (ng/L)	9.1	15–65
Serum free T4 (ng/dL)	1.15	0.9–1.8
Serum TSH (mIU/L)	3.59	0.27–4.2
Serum ACE (IU/L)	74	8–65
Serum 1,25(OH)2D3 (pmol/L)	235	47.7–190.3
Serum and urine electrophoresis	No M spike seen	Normal result
Serum free light chain assay kappa:lambda ratio	1.25	0.37–3.1
Bone marrow aspiration study	2%–3% plasma cells	<5%: normal

ALP, alkaline phosphatase; 25(OH)D3, 25-hydroxyvitamin D; 1,25(OH)2D3, 1,25-dihydroxyvitamin D; PTH, parathyroid hormone; SGOT, serum glutamic-oxaloacetic transaminase; SGPT, serum glutamic-pyruvic transaminase; T4, thyroxine; TSH, thyroid stimulating hormone.

She was advised 18F-labelled fluoro-2-deoxyglucose positron emission tomography (18F-FDG PET)/CT scan for significant weight loss (table 2), which revealed cervical and mesenteric adenopathy, omental thickening and a possible lesion in right inferior parathyroid gland location. A technetium-99m (Tc-99m) sestamibi parathyroid scan was carried to confirm presence of parathyroid adenoma. The scan revealed a significantly increased tracer uptake in the right inferior parathyroid location, indicative of a large parathyroid adenoma (figure 1). PTH gene sequencing was performed to exclude gene mutation as one of the possible reasons for low serum iPTH despite imaging suggestive of large parathyroid adenoma. The PTH gene sequence including its signal peptide region was found to be normal. A repeat serum iPTH measurement was also advised with serial dilution to consider hook effect-related low serum iPTH. At 1:64 serial serum dilutions, the calculated serum iPTH value was 1359 ng/L (table 3). A provisional diagnosis of right inferior parathyroid adenoma and associated granulomatous disorder was made. Ultrasound-guided fine needle aspiration cytology (FNAC) from the suspicious mass lesion was non-diagnostic. She underwent neck dissection and lesion in the right inferior parathyroid location was removed. A normal looking parathyroid gland located behind the lesion and an enlarged right supraclavicular lymph node was also resected. However, in postoperative period hypercalcaemia with low PTH persisted.

Table 2.

Baseline radiological investigations of the patient

Investigation	Result
Chest radiograph	Normal study
Ultrasound of kidney, ureters and bladder	Bilateral small sized kidneys with raised cortical echogenicity
Ultrasound of neck	Extrathyroidal lesion with increased vascularity in right inferior parathyroid location
18F-FDG PET/CT	FDG avid soft tissue mass of 2.1×3.5 cm seen posterior to right lobe of thyroid gland. FDG avid cervical and mesenteric lymph nodes with mesenteric streaking and omental thickening also seen
Tc-99m sestamibi scintigraphy	Tracer avid lesion of size 2.2×3.4 cm noted in region of right inferior parathyroid gland suggestive of parathyroid adenoma

18F-FDG PET, 18F-labelled fluoro-2-deoxyglucose positron emission tomography; Tc-99m, technetium 99 m.

Figure 1.

Technetium 99 m sestamibi scan showing focal accumulation of tracer in the location of right inferior parathyroid gland (arrow).

Table 3.

Post dilution, calculated serum intact parathyroid hormone (PTH) levels for the patient and a normal subject*

Dilution factor	Patient	Normal subject
Undiluted (ng/L)	9.1	42
1:16 dilution (ng/L)	264.9	309
1:32 dilution (ng/L)	598.7	721
1:64 dilution (ng/L)	1359.3	1712

*Normal serum intact PTH levels: 15–65 ng/L. Post dilution, the calculated serum intact PTH levels were found to be high in both the index patient and a normal subject.

Histopathological examination of the resected specimens revealed a hyperplastic thyroid nodule, normal parathyroid gland and tubercular necrotising granulomatous lymphadenitis. The latter showed epithelioid cell granulomas with Langhans giant cells and central necrosis. Ziehl-Neelsen staining revealed multiple acid-fast bacilli (figure 2). Considering her constitutional symptoms, elevated serum 1,25(OH)2D3 levels and histological features, the diagnosis was revised to extrapulmonary tuberculosis, that is, granulomatous lymphadenitis causing PTH-independent hypercalcaemia with a follicular thyroid nodule leading to positive Tc-99m sestamibi scan.

Figure 2.

Cytological and histopathological features of the right neck swelling. (A) Haemodiluted aspirate from the suspicious lesion showing occasional cell cluster arranged as follicle (inset). (B) Microscopic examination of the resected lesion showing features of hyperplastic thyroid nodule. (C, D) Microscopic examination of the right supraclavicular lymph node specimen showing central necrosis (arrow, (C)), surrounded by epithelioid histiocytes (C) and occasional giant cell (D). Ziehl-Neelsen staining revealed multiple acid-fast bacilli (inset, (D)).

Treatment

The patient was initiated on antitubercular therapy (ATT) in modified renal doses (rifampicin 450 mg once a day, isoniazid 300 mg once a day, pyrazinamide 1500 mg three times a week and ethambutol 800 mg three times a week).

Outcome and follow-up

Her serum calcium normalised to 9.7 mg/dL at 2 months after the initiation of ATT. However, serum iPTH remained suppressed to 7.1 ng/L After 6 months, patient continued to have normal calcium, but with normal serum iPTH value of 25.0 ng/L (table 4).

Table 4.

Serial biochemical investigations of the patient

Investigation	Preoperative	Postoperative	Two months post ATT	Six months post ATT
Serum total calcium (mg/dL)	12.9	11.7	9.7	9.2
Serum inorganic phosphate (mg/dL)	3.7	3.3	–	–
Serum iPTH (ng/L)	9.1	8.3	7.1	25.0

ATT, antitubercular therapy; iPTH, intact parathyroid hormone.

Discussion

The present case is a rare instance of hypercalcaemia due to tubercular necrotising granulomatous lymphadenitis with concomitant follicular thyroid nodule masquerading as parathyroid adenoma. The high serum iPTH calculated values obtained following unnecessary serial dilutions with normal saline further added to the lack of clarity in the diagnosis in this case.

In this case, though the initial evaluation suggested PTH-independent hypercalcaemia, the 18F-FDG PET/CT and Tc-99m sestamibi scan suggested the possibility of a large parathyroid adenoma. Though Tc-99m sestamibi scan can show false positive uptake in a variety of lesions other than parathyroid, such as reactive or metastatic lymph nodes,^{15 16} benign or malignant thyroid nodules,¹⁷ cervical paraganglioma,¹⁸ remnant thymic tissue,¹⁹ brown tumour of hyperparathyroidism²⁰ and brown fat,²¹ these possibilities got ignored in the present case in the view of high calculated serum iPTH levels obtained on serial dilution. Further, the presence of normal serum inorganic phosphate levels in the face of renal insufficiency also suggested PTH-mediated phosphaturic effect.

The persistence of hypercalcaemia with low PTH levels following surgery and histopathology suggesting the presence of a hyperplastic nodule of thyroid led us to re-review the sequence of events leading to misdiagnosis before operation. The clinching point of high calculated iPTH levels consequent to hook effect was reviewed in hyperparathyroidism.

Hook effect due to high concentration of hormone antigen can lead to unusually low serum iPTH in a patient with proven parathyroid adenoma. Such low serum iPTH values have been described with immunometric assays. Lafferty et al have reported a case of parathyroid adenoma with low serum iPTH levels, where dilution of the preoperative serum produced progressive rise in calculated serum iPTH levels. The dilution effect was not seen in the postoperative sample, thus suggesting hook effect as the cause for false low PTH result preoperatively.⁵ In immunometric assay, a sandwich is formed by solid-phase antibody, antigen and the excess labelled antibody. In assays with simultaneous addition of capture antibody, antigen and a second labelled detecting antibody, falsely low values can be observed in cases with extremely high antigen concentrations. Thus, in cases with very high hormone level, the excess of hormone antigen in solution would bind with the labelled antibody and make it unavailable to form a sandwich with the antigen bound to the capture antibody. Following washing, this would lead to reduction in detection signal, and therefore be falsely interpreted as low serum hormone concentration.^{22 23} In the present case, the reason for false positive hook effect leading to increase in calculated serum iPTH values in serially diluted sample was not clear. The same dilution, when applied to another individual with normal serum calcium and PTH levels also showed a similar increase of calculated serum iPTH values (table 3). A review of assay manual revealed that the electrochemiluminescence assay used in this case for PTH measurement does not show hook effect even when the serum value is as high as 17 000 ng/L. Therefore, serial dilution is not recommended unlike other hormones like prolactin, calcitonin and β-hCG. Thus, it seems that the rising serum values obtained with progressive dilution were reflective of a matrix effect due to the use of normal saline as a diluent in this study. This hypothesis was based on the fact that for samples which do not exhibit dilutional linearity, that is, sample concentration deviates beyond the accepted range of 80%–120% of the expected after dilution, a matrix interference should be suspected.²⁴

The surgical exploration proved to be decisive in this case as it provided a histopathological diagnosis of tubercular lymphadenitis which was the cause for PTH-independent hypercalcaemia. Both serum calcium and serum iPTH normalised following the initiation of ATT. In retrospect, the normal serum inorganic phosphate observed in the present case which also contributed to the initial diagnosis of parathyroid adenoma could be a manifestation of phosphaturic effect of her high serum 1,25-hydroxyvitamin D level through fibroblast growth factor 23 (FGF23).

Patient’s perspective.

The multidisciplinary team approach has helped in the diagnosis of my clinical problem leading to complete recovery.

Learning points.

• Hypercalcaemia due to tubercular lymphadenitis recovered completely following antitubercular therapy with rise of parathyroid hormone (PTH) levels at 6 months.

• Thyroid hyperplastic nodule caused a strong tracer uptake of 18F-FDG PET/CT and technetium-99m sestamibi in the right inferior parathyroid region leading to a false diagnosis of parathyroid adenoma.

• A systematic approach should be followed for patients with hypercalcaemia while using serum PTH levels to subtype them into two broad categories.

• With the currently used electrochemiluminescence assay, false positive hook effect can occur due to matrix effect following the use of normal saline as diluent. When suspecting a hook effect, only a standard/recommended diluent which does not interfere with the matrix of clinical sample should be used.