Functioning parathyroid cyst extending from neck to anterior mediastinum. Diagnosis by sonography and computed tomography.

A parathyroid cyst was evaluated by sonography and computed tomography (CT) in a patient with hyperparathyroidism. Sonography demonstrated a cystic lesion near the left lower pole of the thyroid, extending into the anterior mediastinum. On CT, the parathyroid cyst had a slightly high attenuation (30 H), with a smooth wall demonstrated after intravenous administration of contrast material. Fine-needle aspiration biopsy of the cyst showed elevated parathyroid hormone (PTH) level. The lesion probably represents cystic degeneration of an adenoma.