Ectopic Mediastinal Thyroid-Simulating Cancer in a Post - Thyroidectomy Patient

INTRODUCTION

Ectopic thyroid tissue (ETT) is defined as the presence of thyroid tissue outside its normal pretracheal location and is typically the result of aberrant embryologic migration of the thyroid gland. While the majority of ETT cases are located in the lingual or sublingual regions, ectopic tissue within the mediastinum is exceptionally rare, accounting for <1% of all cases of ETT.^[1,2] The posterior mediastinum is an especially uncommon site for ectopic thyroid, and lesions arising in this location may pose diagnostic challenges due to their atypical radiological features and proximity to vital mediastinal structures.^[3,4]

Mediastinal thyroid masses can be classified into two types: substernal extension of cervical thyroid tissue, and truly ETT with no connection to the cervical gland.^[5,6] While most substernal goiters are found in the anterior mediastinum, posterior mediastinal goiters – particularly those arising from ectopic tissue – are exceedingly rare and often misdiagnosed as lymphomas, neurogenic tumors, or metastatic lesions.^[4,7] Clinical manifestations are frequently due to mass effect, such as dyspnea, dysphagia, or superior vena cava syndrome, depending on the size and exact location of the mass.^[6,8] Notably, a history of prior thyroid surgery does not exclude the presence of residual cervical thyroid tissue, particularly after subtotal procedures.

Diagnostic workup typically includes cross-sectional imaging (computed tomography [CT] or magnetic resonance imaging) and thyroid function tests. In some cases, functional imaging with iodine-123 or technetium-99 m may support the diagnosis by demonstrating uptake in the mediastinal mass, although this is not always reliable, especially in nonfunctioning tissue.^[1,9] When malignancy cannot be excluded, or when compressive symptoms are present, surgical excision is indicated. The surgical approach – cervical, sternotomy, or thoracotomy – depends on the location and extent of the mass.^[5,10]

Herein, we present a rare case of a 77-year-old woman with a remote history of thyroidectomy that developed progressive dyspnea and dysphagia. Imaging revealed a hypervascular posterior mediastinal mass with elevated thyroglobulin levels, raising suspicion for thyroid carcinoma. However, histopathological examination confirmed a benign multinodular goiter arising from ETT. This case underscores the importance of considering ETT in the differential diagnosis of posterior mediastinal masses, particularly in postthyroidectomy patients.

CASE REPORT

A 77-year-old woman presented with a several-month history of progressive dyspnea and a recent onset of dysphagia for 2 weeks. She denied weight loss, chest pain, voice changes, or recent infections. Her medical history was notable for two thyroid surgeries approximately 20 years earlier for benign goiter, although no operative or pathology reports were available. Although the patient reported two previous thyroid operations for benign goiter, both thyroid lobes were still identifiable on ultrasound. This pattern is most consistent with prior subtotal/incomplete resections rather than total thyroidectomy.

On examination, the patient was hemodynamically stable. There was no cervical mass or lymphadenopathy. Pulmonary examination revealed decreased breath sounds over the right lower chest.

Initial laboratory tests revealed normal hematologic and renal function (white blood cell: 6200/μL, hemoglobin: 16.2 g/dL, platelets: 194,000/μL, creatinine: 0.7 mg/dL). Thyroid function tests showed a thyroid-stimulating hormone level of 0.56 μIU/mL (normal range [NR]: 0.27–4.2), free T4: 1.01 ng/dL, and free T3: 2.36 pg/mL. The patient reported irregular use of levothyroxine, typically taking a single 100 μg tablet approximately once every 10 days. She was not on a regular replacement regimen. Her euthyroid biochemical profile was therefore most likely maintained by residual functioning thyroid tissue rather than consistent exogenous hormone therapy. Ultrasound showed the right lobe measuring 29 mm × 15 mm × 30 mm and the left lobe 14 mm × 8 mm × 9 mm, with a small lesion (20 mm × 21 mm) in the postero-inferior aspect of the right lobe, likely a postsurgical change. These findings support a history of incomplete/subtotal thyroid resection, explaining the persistence of cervical thyroid tissue. The lesion with surrounding edema and inflammation was noted in the posteroinferior aspect of the right lobe, suggestive of postsurgical or abscess changes. A single round lymph node measuring 5 mm × 5 mm was seen in the right supraclavicular area.

Chest CT with contrast showed a large, hypervascular mediastinal mass in the superior and posterior right mediastinum, measuring approximately 105 mm × 67 mm × 84 mm, with scattered calcifications and areas of necrosis. The mass displaced the esophagus laterally and compressed the trachea anteriorly. Major mediastinal vessels were preserved. The findings raised the differential diagnosis of ectopic thyroid malignancy, Castleman disease, or hypervascular lymphadenopathy [Fig 1]. Furthermore, serum thyroglobulin was markedly elevated (>300,000 ng/mL; NR: 1.6–59.9), raising a strong suspicion for functioning thyroid tissue or thyroid carcinoma. Anti-thyroglobulin antibody was within normal limits (3.35 IU/mL; NR <4.11). Serum calcitonin level was 0.62 pg/mL (NR <7.4), excluding medullary thyroid carcinoma. A thyroid scintigraphy with technetium-99 m pertechnetate displayed a large mediastinal focus of activity. The uptake appeared attenuated because of the low administered dose and chest wall attenuation [Fig 2].

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Fig 1:

Contrast-enhanced chest computed tomography. Axial and coronal images show a well-circumscribed, hypervascular mass in the right posterior mediastinum (~105 mm × 67 mm × 84 mm) with coarse calcifications and focal necrotic/cystic areas. The lesion displaces the esophagus laterally and indents the posterior tracheal wall; major mediastinal vessels are preserved (a) Mediastinal view, (b) Lung view

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Fig 2:

(a) Whole body iodine 131 scintigraphy reveals considerable amounts of iodine avid tissue, (b) The Technetium 99 m displayes a large area of activity which is expected to be subtle due to low dosage and attenuation with chest wall

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The patient underwent surgical resection through right thoracotomy. Intraoperatively, the mass was encapsulated and well-demarcated, located deep in the posterior mediastinum, without adherence to surrounding structures. No continuity was noted with cervical thyroid tissue.

Gross pathology revealed a 12 cm × 7 cm capsulated mass with a heterogeneous cut surface containing colloid-rich areas and hemorrhagic foci. Microscopic examination showed thyroid follicles of varying size separated by fibrous bands, with areas of cystic degeneration and hemosiderin-laden macrophages. Scattered lymphoid cells were noted. The diagnosis was consistent with a multinodular goiter arising from ETT.

Postoperative recovery was uneventful. The patient’s dyspnea and dysphagia resolved, and at 3-month follow-up, she remained asymptomatic with no evidence of recurrence on follow-up imaging.

Imaging timeline. Contrast-enhanced chest CT was performed first and was followed 4 days later by whole-body scintigraphy using iodine-131. In addition, a separate Tc-99 m pertechnetate scan was obtained to localize functioning tissue further. We acknowledge that iodinated contrast may attenuate or transiently reduce radioiodine uptake; however, the 4-day interval likely limited this effect. The mediastinal focus demonstrated activity on both functional studies but appeared relatively subtle on planar imaging due to chest-wall attenuation and the large soft-tissue mass. Single-photon emission computed tomography (SPECT) or SPECT-CT would have provided better localization and sensitivity; these modalities were not available in our center for this case, which we now state as a limitation.

DISCUSSION

ETT is a rare congenital anomaly, resulting from aberrant embryologic migration of the thyroid anlage. The most common location for ETT is lingual (accounting for up to 90% of cases), while intrathoracic and mediastinal locations comprise <1% of all ectopic thyroid presentations.^[1,2] Within the mediastinum, most thyroid tissue extends from the cervical gland (secondary intrathoracic goiter), but truly ETT with no cervical continuity, as seen in this case, is exceedingly rare.^[3,4]

Posterior mediastinal ETT is especially uncommon, with only a few dozen cases reported to date.^[5,6] The clinical presentation is typically related to compressive symptoms, including dyspnea, dysphagia, hoarseness, or superior vena cava syndrome, depending on the size and location of the mass.^[7] In this case, the patient’s progressive dyspnea and recent onset dysphagia were in line with mass effect caused by the large hypervascular lesion noted on imaging.

Imaging plays a pivotal role in the evaluation of mediastinal masses. CT scans commonly reveal well-circumscribed, heterogeneous masses with calcifications or cystic changes, while MRI may help in assessing the relationship with adjacent structures.^[8,9] However, distinguishing ETT from other mediastinal masses – such as lymphoma, thymic tumors, neurogenic tumors, or Castleman disease – based solely on imaging can be difficult.^[10] Radionuclide imaging modalities, such as iodine-123 or technetium-99 m pertechnetate scintigraphy, may support the diagnosis by demonstrating functioning thyroid tissue. However, uptake may be absent in nonfunctioning or degenerated nodules. In our patient, Tc-99 m scintigraphy demonstrated a large area of activity in the posterior mediastinum, though the uptake appeared relatively subtle due to the low administered dose and chest wall attenuation.

A markedly elevated serum thyroglobulin level (as in this case, >300,000 ng/mL) in the absence of circulating anti-Tg antibodies and a normal cervical thyroid gland may strongly suggest ectopic or metastatic thyroid tissue.^[5,11] We hypothesize that the extreme Tg elevation reflects the large volume of functioning ectopic multinodular tissue with rich vascularity and active colloid production. Similar, albeit rare, extreme Tg levels have been anecdotally reported in massive benign multinodular goiter. Nevertheless, the magnitude of thyroglobulin elevation in our patient (>300,000 ng/mL) is remarkable and far exceeds typical levels even in widely metastatic differentiated thyroid carcinoma, where concentrations usually remain below 100,000 ng/mL. Several mechanisms may account for this extreme value: (1) the very large mass of functioning follicular tissue with abundant colloid production and vascular permeability, allowing massive Tg release into circulation; (2) possible disruption of follicular architecture within hyperplastic nodules, enhancing Tg leakage; and (3) assay-related factors, although anti-Tg antibodies were negative, reducing the likelihood of interference. These mechanisms together could explain the exceptional Tg elevation in this benign ectopic goiter. This finding highlights that markedly elevated Tg alone does not necessarily imply malignancy and must be interpreted in the proper clinical and radiologic context.

Furthermore, a normal calcitonin level can help exclude medullary thyroid carcinoma. However, due to the overlap in imaging features and laboratory findings between benign ectopic thyroid and thyroid carcinoma, definitive diagnosis requires histopathologic confirmation.^[6,12]

Depending on the extent and activity of functioning thyroid tissue, serum thyroglobulin levels can reach several hundred or even thousands of ng/mL. In our patient, Tg exceeded 300,000 ng/mL, consistent with a very large hyperfunctioning ectopic thyroid mass. Although a re-evaluation of the excised specimen was requested, it must be acknowledged that complete exclusion of malignancy in such a large mass would require extensive sampling with multiple sections. Nevertheless, both the initial and second pathology reviews confirmed a multinodular goiter arising from ETT, with no evidence of local or distant metastasis.

Surgical resection remains the cornerstone of management in symptomatic patients or when malignancy cannot be ruled out. Depending on the size and location, a transcervical approach may suffice for anterior mediastinal goiters, but posterior mediastinal lesions often require thoracotomy or video-assisted thoracic surgery.^[5,10,13] In this patient, a right thoracotomy enabled complete removal of the lesion without complication.

Histologic analysis of the excised mass confirmed a multinodular colloid goiter arising from ETT. The nodular architecture, cystic degeneration, hemosiderin-laden macrophages, and fibrous bands between follicles were consistent with benign hyperplasia. ^[6] No features of malignancy were observed.

This case highlights several key learning points: First, that posterior mediastinal ETT can present decades after prior thyroid surgery, even without cervical recurrence; second, that serum Tg may be extraordinarily elevated in benign lesions; and finally, that a definitive diagnosis often requires a combination of imaging, functional assessment, and histopathology. Clinicians should include ETT in the differential diagnosis of any mediastinal mass, particularly in patients with a history of thyroid disease or surgery.

CONCLUSION

This case illustrates a rare presentation of ETT as a posterior mediastinal mass in a patient with a remote history of thyroidectomy. Despite radiologic and laboratory findings suggestive of malignancy – including a large hypervascular lesion and markedly elevated serum thyroglobulin – the final diagnosis was a benign multinodular goiter arising from ETT. This highlights the importance of considering ectopic thyroid in the differential diagnosis of mediastinal masses, particularly in postthyroidectomy patients. Comprehensive evaluation using imaging, biochemical markers, and histopathological confirmation is essential to avoid misdiagnosis and ensure appropriate management.