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

Ectopic thyroid tissue (ETT) is a rare embryological anomaly, with mediastinal involvement comprising <1% of all cases. Posterior mediastinal ETT is particularly uncommon and may mimic other mediastinal pathologies, including malignancies, leading to diagnostic dilemmas. We report the case of a 77-year-old woman with a history of two prior thyroid operations approximately 20 years earlier for benign goiter. Ultrasound at presentation demonstrated both thyroid lobes, consistent with prior incomplete resections rather than total thyroidectomy. She presented with progressive dyspnea and dysphagia. Imaging revealed a large, hypervascular posterior mediastinal mass displacing adjacent structures. Laboratory tests showed a markedly elevated thyroglobulin level (>300,000 ng/mL) with normal thyroid function and negative anti-Tg antibodies. Whole body scintigraphy with iodine-131 demonstrated a large area of functional thyroid tissue in the upper thoracic, beyond the neck. The mass was surgically excised via right thoracotomy. Histopathological examination confirmed a benign multinodular colloid goiter arising from ETT. This case emphasizes the importance of including ectopic thyroid in the differential diagnosis of posterior mediastinal masses, especially in patients with prior thyroid surgery. Multimodal evaluation and surgical resection are essential for definitive diagnosis and symptom resolution.