Mediastinal Ectopic Parathyroid Adenoma Mimicking Thyroid Follicular Adenoma

A 58-y-old woman presented with markedly elevated serum parathyroid hormone (777.7 pg/mL) and hypercalcemia. A 99mTc-sestamibi scan revealed focal radiotracer accumulation in the upper anterior mediastinum. The lesion was surgically removed. Initial histopathology suggested follicular thyroid adenoma. Because the laboratory findings strongly indicated hyperparathyroidism, further immunohistochemical staining for parathyroid hormone and GATA-binding protein 3 was performed, confirming mediastinal ectopic parathyroid adenoma. This case highlights the importance of correlating imaging, laboratory, and immunohistochemistry findings when interpreting mediastinal lesions with 99mTc-sestamibi uptake.