Mediastinal thyroglossal duct cyst after thyroidectomy mimicking recurrent thyroid disease: a case report.

Thyroglossal duct cysts (TGDCs) are common congenital midline neck lesions, but mediastinal extension in adults is exceptional and may mimic recurrent thyroid disease after thyroidectomy. We report a 55-year-old woman with dysphagia and exertional dyspnoea one year after near-total thyroidectomy for benign multinodular hyperplasia. Computed tomography demonstrated a 76-mm heterogeneous cervico-mediastinal mass extending to the aortic arch with tracheal displacement. Scintigraphy showed no uptake, and fine-needle aspiration revealed squamous epithelial elements with atypia, creating concern for malignancy. The lesion was removed en bloc through the previous cervical incision with thoracic surgical assistance and recurrent laryngeal nerve monitoring. Histology confirmed a benign TGDC with squamous/columnar lining and ectopic thyroid follicles. In post-thyroidectomy patients, a cold mediastinal cystic mass with squamous cytology should prompt consideration of TGDC to guide safe surgery.