Cystic parathyroid adenoma within a multinodular goiter: a rare cause of primary hyperparathyroidism.

Parathyroid cysts are uncommon lesions that do not have a specific sonographic appearance. We report a rare case of a functionally active cystic parathyroid adenoma as a cause of primary hyperparathyroidism and discuss the difficulties associated with the diagnosis. In this case, the association with a multinodular goiter made preoperative differentiation from thyroid cysts difficult. Demonstration of a high intact parathyroid hormone level in the aspirated cyst fluid was critical for the correct diagnosis.