Ectopic Thyroid Tissue Presenting as an Anterior Mediastinal Mass in an Elderly Male

A rare developmental defect called ectopic thyroid tissue (ETT) is caused by abnormal embryonic migration. Less than 1% of ectopic thyroid instances are mediastinal ETT, which might radiologically resemble malignant neoplasms. We report a 72-year-old male with hepatocellular carcinoma who was incidentally found to have a large anterior mediastinal mass (8.4 cm × 7.5 cm × 8.2 cm) on staging computed tomography (CT). CT-guided biopsy revealed benign colloid-filled thyroid follicles, and immunohistochemistry confirmed ETT with positive thyroid transcription factor-1 (TTF-1), thyroglobulin, and PAX8 staining. Despite having a normally positioned cervical thyroid gland with normal function, the patient had mediastinal ectopic thyroid. Given his asymptomatic status and high operative risk, conservative management was adopted, and the patient remained clinically stable with no progression of the mediastinal lesion during 6 months of follow-up. This case highlights the importance of considering ETT in the differential diagnosis of mediastinal masses to avoid misdiagnosis and unnecessary intervention.