An Uncommon Cause of Asymptomatic Primary Hyperparathyroidism

Asymptomatic hyperparathyroidism developed in a 65-yr-old man with a history of metastatic papillary thyroid carcinoma. Pertinent laboratory values were: calcium, 10.8 mg/dl (2.70 mmol/liter) [reference range, 8.9– 10.1 mg/dl (2.22–2.52 mmol/liter)]; intact PTH, 135.1 pg/ml (14.3 pmol/liter) [15–50 pg/ml (1.6–5.3 pmol/liter)]; and albumin, 4.5 g/dl (45 g/liter) [3.5–5.0 g/dl (35–50 g/liter)]. Phosphorus, creatinine, and 25-hydroxyvitamin D levels were within normal limits. Thyroid ultrasound for follow-up of his thyroid cancer revealed a 3.0 1.3 0.7-cm cystic mass in the left thyroid bed (Fig. 1A). Twenty-five milliliters of clear fluid were removed from the cyst, revealing an intact PTH of 821 pg/ml (87.1 pmol/liter). Serum calcium was 9.0 mg/dl (2.25 mmol/liter) at 6 months and 9.1 mg/dl (2.27 mmol/liter) at 24 months, with intact PTH 56.5 pg/ml (6 ng/liter) and reduction of the cyst to 0.7 0.6 0.7 cm (Fig. 1B). Parathyroid cysts are rare. An uncommon cause of primary hyperparathyroidism is a functional parathyroid cyst (1). Few parathyroid cysts (10–15%) are functional (presenting with hyperparathyroidism) (2). Functional cysts are treated with surgical excision (3, 4), although successful treatment with aspiration has been described in one case report (3). They mimic thyroid and thymic cysts, thus presenting a diagnostic challenge (4). The presence of elevated intact PTH in aspirated fluid is diagnostic. Parathyroid cysts have absent uptake on radioiodine thyroid scans and rarely are positive on sestamibi parathyroid scans (5). Ultrasound, computed tomography, and magnetic resonance imaging show a cystic cervical or mediastinal mass (5). Aspiration of a functioning parathyroid cyst can be both diagnostic and therapeutic. Acknowledgments