Primary hyperparathyroidism with low serum 1,25-dihydroxyvitamin D levels.

Primary hyperparathyroidism is usually associated with normal or elevated serum 1,25-dihydroxyvitamin D [1,25-(OH)2D] levels. We report a patient with extreme hypercalcemia (serum calcium, 19.4 mg/dl), primary hyperparathyroidism, and a very low plasma concentration of 1,25-(OH)2D. Surgical removal of a large parathyroid adenoma was associated with a decrease in the serum calcium and immuno- and bioactive PTH concentrations and normalization of the 1,25-(OH)2D level. The postoperative course was complicated by severe protracted hypocalcemia and cardiac arrest, requiring treatment with large doses of calcium iv. The low concentrations of 1,25-(OH)2D in this patient are an unusual manifestation of primary hyperparathyroidism, probably due to suppression of renal 1 alpha-hydroxylase activity by the severe hypercalcemia. We conclude that in severe hypercalcemia, a low serum 1,25-(OH)2D level does not exclude the diagnosis of primary hyperparathyroidism.