A Patient with Pseudohypoparathyroidism with Increased Serum Calcium and lα,25-Dihydroxyvitamin D after Exogenous Parathyroid Hormone Administration*

The cause for the normal calcemic response to exogenous parathyroid hormone (PTH) was explored in a case of pseudohypoparathyroidism. The diagnosis of pseudohypoparathyroidism in this 29-yr-old woman was well established, since she had hypocalcemia (6.6 mg/dl), hyperphosphatemia (5.0 mg/ dl), high serum immunoreactive PTH, and negligible urinary cAMP response to exogenous PTH. Treatment with Amphojel slightly increased serum calcium (to 7.4 mg/dl) and lowered serum phosphorus (to 4.1 mg/dl) without altering the serum concentration of 1 alpha, 25-dihydroxyvitamin D [1,25(OH)2D]. After im administration of parathyroid extract (400 U/day for 4 days), serum calcium increased to 9.2 mg/dl, commensurate with significant elevations in serum 1,25(OH)2D (from 1.2 to 2.6 ng/dl) and fractional (intestinal) calcium absorption (from 0.22 to 0.43). The results suggest that the synthesis of 1,25(OH)2D upon PTH challenge was relatively intact and may have accounted for the normal calcemic response.