Rare intrathyroid nodule-like lesions: Two case reports

Abstract

Thyroid nodules and nodule-like lesions, as well as other structural abnormalities of the thyroid gland, are often detected during ultrasonography. True thyroid nodules should be evaluated for sonographic features suggestive of malignancy and appropriately triaged for further diagnostic workup to reduce the risk of missed thyroid cancer. In addition to true thyroid nodules, other lesions may coexist with or mimic them. Among these are branchial cleft cysts and thyroglossal duct cysts, 2 common embryological remnants. Although these lesions are typically found in the lateral neck and midline, respectively, atypical presentations and aberrant anatomical locations may pose diagnostic challenges. Intrathyroidal occurrence of either entity is exceedingly rare. We report 2 cases of intrathyroidal congenital cystic lesions recognized during thyroid ultrasonography. In both patients, suspicious thyroid nodule-like lesions were identified accidentally. Subsequently ultrasound-guided fine-needle aspiration (FNA) was performed. Cytological examination suggested an intrathyroidal branchial cleft cyst in the first case, which was confirmed after surgical excision and histopathological examination, while in the second case suggested an intrathyroidal thyroglossal duct cyst, but that patient declined further diagnostic evaluation. These cases highlight the importance of considering cystic embryological remnants in the differential diagnosis of thyroid nodule-like abnormalities and underscore the value of ultrasound-guided FNA in establishing an accurate diagnosis.

Introduction

Thyroid nodules and nodule-like lesions are common incidental findings on thyroid ultrasonography [1]. Most true thyroid nodules are benign; however, approximately 10% (range, 4%-15%) finally proved to be malignant. This proportion has increased in recent years [[1], [2], [3]]. Imaging plays a pivotal role in the detection, characterization, and risk stratification of these lesions according to the Thyroid Imaging Reporting and Data System (TI-RADS) [4,5], which estimates the likelihood of malignancy based on sonographic features. When suspicious imaging characteristics are present, further diagnostic evaluation is needed. Ultrasound-guided fine-needle aspiration (US-FNA) is the recommended next step to determine the cytological nature of the lesion, with findings classified according to the Bethesda System for Reporting Thyroid Cytopathology [[6], [7], [8]]. Surgical excision and histopathological examination may be required to establish a definitive diagnosis.

In exceedingly rare cases, a lesion may appear as a thyroid nodule on imaging and exhibit suspicious sonographic features, while the underlying pathology is entirely different. Such lesions include branchial cleft cysts and thyroglossal duct cysts, which represent common embryologic remnants. Branchial cleft cysts are fluid-filled lesions resulting from incomplete involution of the branchial apparatus, while thyroglossal duct cysts arise from persistent remnants of the thyroglossal duct during embryogenesis. Typically, these lesions are located in the lateral neck and in the midline or paramedian neck, respectively. On ultrasonography, they usually demonstrate cystic characteristics and are most commonly diagnosed during childhood, although they may also present in adulthood. Intrathyroidal occurrence of these embryologic remnants is exceptionally rare and may pose a diagnostic challenge when they mimic suspicious thyroid nodules on ultrasonography.

Cases presentation

All ultrasound examinations were performed by the same radiologists to minimize interobserver variability. During ultrasound-guided fine-needle aspiration (FNA), at least 3 needle passes were obtained from different areas of the lesion to ensure representative sampling. Cytological specimens were reported according to the Bethesda System for Reporting Thyroid Cytopathology. Surgical specimens were processed using standard histopathological protocols.

Case A

A 46-year-old male with a known history of Hashimoto thyroiditis and otherwise unremarkable medical history underwent thyroid ultrasound as part of routine follow-up after a prolonged interval. Ultrasound demonstrated a slightly enlarged thyroid gland with diffusely heterogeneous echotexture, containing multiple hyperechoic and hypoechoic areas and small, well-circumscribed nodules. Despite these changes, vascularity was within normal limits. These findings were suggestive of chronic thyroiditis (Fig. 1). In addition, a 2.6-cm solid hypoechoic lesion with smooth margins and internal punctate echogenic foci was identified adjacent to the posterolateral aspect of the lower right thyroid lobe. The lesion was classified as ACR TI-RADS 5 (Fig. 2), and ultrasound-guided fine-needle aspiration was performed. Cytological examination showed no evidence of malignancy but revealed scattered crystals, abundant anucleate squamous cells, occasional mature squamous cells, and hyperkeratotic cells, findings most suggestive of a branchial cleft cyst (Fig. 3). The patient subsequently underwent total thyroidectomy. Intraoperatively, an ovoid cystic lesion with smooth margins was identified adjacent to the right thyroid lobe, deep to the thyroid capsule (Fig. 4). Histopathological examination demonstrated atrophic thyroid tissue and a cyst wall lined by squamous epithelium with adjacent lymphoid tissue (Fig. 5, Fig. 6), confirming the diagnosis of a branchial cleft cyst partially encased by a thin layer of thyroid parenchyma.

Fig. 1.

Ultrasound images of the thyroid isthmus (A) and left lobe (B1, transverse; B2, longitudinal) demonstrating diffuse heterogeneous echotexture and mild enlargement.

Fig. 2.

Ultrasound images of the right thyroid lobe (A, transverse; B, longitudinal; C, longitudinal with microvascular mode) demonstrating a solid hypoechoic lesion (calipers) with smooth margins and internal punctate echogenic foci, located adjacent to the posterior aspect of the lower third of the lobe (ACR TI-RADS 5).

Fig. 3.

Cytological findings: Giemsa stain (A and B) and Papanicolaou stain (C) demonstrating abundant anucleate squamous cells with occasional mature squamous cells and focal crystalline material. No cytological features of malignancy are present.

Fig. 4.

Surgically excised specimen showing an ovoid lesion with smooth margins (yellow arrow) protruding just beneath the thyroid capsule in the right lobe (yellow dotted line).

Fig. 5.

Histological examination (H&E stain, ×25 magnification) showing atrophic thyroid tissue on the right and the cyst wall on the left (asterisk), partially surrounded by a thin layer of atrophic thyroid tissue.

Fig. 6.

Histological examination (H&E stain, ×200 magnification, right panel) showing the cyst wall lined by squamous epithelium (blue arrow) and adjacent lymphoid tissue.

Case B

A 63-year-old male underwent thyroid ultrasound as part of a routine check-up. No previous thyroid imaging was available, and the patient reported no relevant medical history. Ultrasound demonstrated a slightly enlarged thyroid gland with multiple scattered spongiform nodules of varying size (Fig. 7). In addition, a 1.0-cm, markedly hypoechoic (almost anechoic) nodule with smooth margins was identified in the inner aspect of the right lobe, near the border to the isthmus (Fig. 8). The lesion showed subtle posterior acoustic enhancement; however, it did not fulfill the criteria for a simple cyst, thus it was classified as ACR TI-RADS 4, and further evaluation was recommended. Ultrasound-guided fine-needle aspiration was performed. Cytological analysis revealed no evidence of malignancy and demonstrated mature and anucleate squamous cells, occasional mast cells, and small cuboidal cells, raising a differential diagnosis of either branchial cleft cyst or thyroglossal duct cyst. Given the paramedian location, a thyroglossal duct cyst was considered more likely (Fig. 9). Surgical excision was recommended; however, the patient declined further intervention, and no additional information is available.

Fig. 7.

Ultrasound images of the thyroid gland: right lobe (A and B; longitudinal and transverse), isthmus (C; transverse), and left lobe (D-F; longitudinal and transverse) demonstrating mild thyroid enlargement and multiple spongiform nodules of varying size.

Fig. 8.

Ultrasound images of the right thyroid lobe (A, transverse; B, longitudinal with color Doppler) showing a very hypoechoic lesion with smooth margins and non–purely cystic features, located in the lower pole near the isthmus and measuring approximately 1.0 cm (ACR TI-RADS 4).

Fig. 9.

Cytological findings: Giemsa stain (A) and Papanicolaou stain (B) showing abundant anucleate squamous cells with occasional mature squamous cells and focal crystalline material. No cytological features of malignancy are present.

Discussion

Thyroid nodules are common incidental findings during ultrasound examinations [1]. Although most thyroid nodules are benign, a significant proportion are malignant, and the incidence of thyroid cancer has increased in recent decades [2,3]. Ultrasound evaluation using the Thyroid Imaging Reporting and Data System (TI-RADS) enables risk stratification based on specific imaging characteristics associated with malignancy [4,5]. Accurate characterization is essential for guiding clinical management and determining the need for further diagnostic procedures, including fine-needle aspiration (FNA).

However, not all lesions detected within or adjacent to the thyroid gland represent true thyroid nodules. Rare embryological remnants, such as branchial cleft cysts and thyroglossal duct cysts, should be considered in the differential diagnosis of thyroid and perithyroidal lesions, particularly when imaging findings are atypical.

Branchial cleft cysts (BCCs) result from incomplete involution of the branchial apparatus during embryonic development [9,10]. Histologically, they are lined by stratified squamous or pseudostratified columnar epithelium and characteristically contain abundant subepithelial lymphoid tissue with prominent lymphoid follicles.

On imaging, BCCs typically present as well-circumscribed cystic lesions in the lateral neck, with thin, smooth walls and homogeneous fluid content. However, their imaging appearance is not pathognomonic and may vary depending on internal content, particularly in the presence of infection, hemorrhage, or proteinaceous debris. In addition, they may occur in atypical locations and, rarely, may be partially surrounded by thyroid parenchyma [10], findings that can pose a diagnostic challenge. One proposed mechanism for this rare intrathyroidal location is the incomplete involution of the third or fourth branchial pouches during fetal development. Consequently, residual embryological structures may persist closely proximal, or rarely within, the thyroid gland [10,11]. When atypical imaging features are present, the differential diagnosis should include thyroid cysts and nodules, branchial cleft cysts, and parathyroid lesions. Notably, when such lesions are identified adjacent to the thyroid in patients with chronic thyroiditis, particularly Hashimoto’s thyroiditis, this finding is clinically significant and branchial cleft cysts should be included in the differential diagnosis, as their recognition may influence surgical decision-making and long-term follow-up.

In case A, the coexistence of Hashimoto’s thyroiditis, a predominantly solid sonographic appearance, and a partially intrathyroidal location—an extremely rare presentation for a BCC—initially raised suspicion for a malignant thyroid nodule. Definitive diagnosis was confirmed by histopathological examination.

Thyroglossal duct cysts (TGDCs) are the most common congenital malformation of the head and neck and are typically diagnosed in childhood. Despite their relative frequency among congenital neck lesions, they affect approximately 7% of the general population [12,13]. Histologically, they are characterized by an epithelial lining composed of squamous or pseudostratified ciliated columnar epithelium and may contain ectopic thyroid tissue within their walls [13]. Importantly, although TGDCs are benign, less than 1% may harbor carcinoma, most commonly arising from thyroid tissue remnants within the cyst [12].

TGDCs arise from aberrations in embryological development, specifically incomplete regression of the thyroglossal duct, a transient epithelial tract that guides the descent of the thyroid gland from the foramen cecum at the base of the tongue to its final pretracheal position. Under normal circumstances, the thyroglossal duct involutes completely by the eighth to tenth week of gestation. However, persistence of ductal remnants may occur due to incomplete regression or residual epithelial tissue along its course. These remnants may subsequently become cystic, particularly following inflammation or infection [12,13]. Rarely, infection can alter their typical cystic appearance, leading to more complex imaging features.

The typical location of TGDCs is the midline of the neck, most commonly in a suprahyoid position, whereas infrahyoid lesions may present in a paramedian location. Intrathyroidal TGDCs are exceptionally rare, with only a few cases reported in the literature [12,13].

In Case B, the lesion was well-circumscribed but markedly hypoechoic and lacked clear posterior acoustic enhancement. Accordingly, it was classified as ACR TI-RADS 4. Cytological analysis excluded malignancy but introduced diagnostic uncertainty, suggesting either a branchial cleft cyst or a thyroglossal duct cyst. Given the paramedian location, an infrahyoid TGDC was considered more likely. The absence of distinctly cystic features was hypothesized to result from increased cyst viscosity associated with the patient’s age or from prior episodes of inflammation. Surgical excision was recommended for definitive diagnosis and management; however, the patient declined further intervention, and this decision was respected.

Although branchial cleft cysts and thyroglossal duct cysts typically occur in the lateral neck and the anterior midline or paramedian neck, respectively, they may rarely present as intrathyroidal lesions, creating a significant diagnostic challenge. Furthermore, while both entities are characteristically cystic, secondary infection, hemorrhage, or inflammatory changes may alter their imaging appearance and lead to features that mimic suspicious thyroid nodules. Although anatomical location remains the most useful clue for differentiating these lesions, branchial cleft cysts are generally more prone to infection than thyroglossal duct cysts.

Accurate diagnosis ultimately relies on correlation of imaging findings with cytological and histopathological examination. Awareness of these rare embryological remnants is important when evaluating atypical thyroid or perithyroidal lesions, as their inclusion in the differential diagnosis may help avoid misinterpretation and unnecessary invasive procedures or surgical intervention.

Conclusion

Accurate evaluation of intrathyroidal masses requires awareness of both true thyroid nodules and their malignant potential, as well as other entities such as embryological remnants that may mimic them on ultrasound. Recognition of distinguishing imaging features is essential for correct diagnosis. However, when imaging findings are inconclusive, ultrasound-guided fine-needle aspiration (FNA) and cytological analysis are still crucial for guiding appropriate management.

Ethics approval

The patients consented to undergo all processes described. No personal data or any information that could reveal patients’ identity were used. No animals were involved in this study.

Author contributions

All authors contributed to the study conception and design. Angelopoulou V.A., Papandreou E., Medinas P., and Triantafyllou E. performed the ultrasound examinations, while Angelopoulou V.A. and Triantafyllou E. also performed the ultrasound-guided fine-needle aspiration (FNA) procedures. Vidali M. and Berdelis V. performed the surgical excisions. Angelopoulou V.A. performed data collection and material preparation. Angelopoulou V.A. wrote the first draft of the manuscript. All authors critically revised the manuscript, commented on previous versions, and approved the final version. All authors agree to be accountable for all aspects of the work.

Patient consent

Written informed consent for publication of this case report and associated images was obtained from the patient. A copy of the written consent is available for review by the Editor-in-Chief of Radiology Case Reports upon request.