Hyperparathyroidism due to Eutopic PTH Secretion from an Ectopic Intrathymic Parathyroid Cyst

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Parathyroid cysts rarely cause hyperparathyroidism. In addition, they rarely occur in the thymus. We report a 56-year-old woman with hypertension on telmisartan and hydrochlorothiazide, who developed symptomatic hypercalcemia for a month. Initial serum calcium was 15.6 mg/dL, together with inappropriately elevated intact PTH at 437 pg/mL. Coincidentally, an anterior mediastinal mass on chest imaging was found. Biopsy of the mass revealed parathyroid tissue. She was treated sequentially with saline rehydration and frusemide calciuresis, intravenous pamidronate and calcitonin, which lowered her calcium to 11.1 mg/dL. Normocalcemia was finally achieved following surgical extirpation of the mass. Histology confirmed an intrathymic parathyroid cyst. Definitive treatment by resection of the mass is potentially curative.