Hyperparathyroidism with a functioning parathyroid cyst.

A rare case of primary hyperparathyroidism with a functioning parathyroid cyst in whom Tc-99m MIBI scintigraphy failed to detect a parathyroid tumor is presented. A 62-year-old woman with primary hyperparathyroidism was referred for Tc-99m MIBI imaging to investigate a parathyroid adenoma. Plasma levels of intact parathyroid hormone were elevated to 2250 pg/mL. Neck ultrasonography revealed a cystic lesion measured 30 x 42 x 35 mm on the right inferior side of the thyroid gland. The cystic lesion was successfully removed at surgery. Pathologic diagnosis revealed a benign parathyroid cyst. The cyst contained clear fluid, and was lined by 1 layer of cuboidal epithelial cells. Her postoperative course was uneventful and plasma levels of intact parathyroid hormone normalized after operation.