[A case of functioning parathyroid cyst].

A case of functioning parathyroid cyst is reported. A 63-year-old woman consulted our hospital with the chief complaint of neck and joint pain. At that time, laboratory data showed a serum calcium level of 12.8 mg/dl and a phosphorus level of 2.2 mg/dl. Plasma levels of intact PTH were elevated to 278 pg/ml. Computerized tomography, ultrasonography and magnetic resonance imaging suggested parathyroid cyst on the left side of the thyroid gland. We performed left superior parathyroidectomy. The cyst measured 30 x 40 x 30 mm and was chocolate colored. The histopathological diagnosis was a functioning parathyroid cyst. Her postoperative course was uneventful and she was discharged on the 10th postoperative day without symptoms. To our knowledge, only 38 cases of functioning parathyroid cyst have been reported in the Japanese literature.