Primary Hyperparathyroidism Secondary to a Sestamibi-Negative Cystic Parathyroid Adenoma

The most common cause of primary hyperparathyroidism (PHPT) is a solid parathyroid adenoma. Less than 2% of cases of PHPT are caused by cystic parathyroid adenomas formed from degeneration of pre-existing solid parathyroid adenomas. Cystic parathyroid adenomas are non-functional in over 90% of cases. In this case we describe management of a 56-year-old man with acute-onset polydipsia, polyuria, xerostomia, nausea, and constipation. Serum chemistry upon admission revealed hypercalcemia, hyperparathyroidism, and reduced serum phosphorus. Neck sonogram revealed a predominantly anechoic lesion later confirmed by pathology to be a cystic parathyroid adenoma in the right thyroid lobe. Sestamibi scan did not show uptake in parathyroid gland, and parathyroid hormone (PTH) was elevated in fine-needle aspiration sample. Otolaryngology removed the cystic lesion via surgical excision, which led to normalization of PTH level. This case demonstrates the importance of evaluation of cystic components for PTH levels and if confirmed should be treated as a parathyroid adenoma.