Adult-Onset Vitamin D-Resistant Osteomalacia with the Unresponsiveness to Parathyroid Hormone

Serum immunoreactive parathyroid hormone (PTH) levels were increased in a 50-yr-old man with a unique variant of adult-onset vitamin D-resistant osteomalacia, presenting with high phosphate clearance, mild hypocalcemia, and blunted hypercalcemic and phosphaturic responses to exogenous parathyroid extract. After normalization of his serum calcium level with a large amount of 1 alpha-hydroxycholecalciferol, a synthetic vitamin D analog, serum PTH levels and phosphate clearance returned to normal, and hypercalcemia as well as phosphaturia appeared when parathyroid extract was administered. Further studies demonstrated that the renal response could be restored by the administration of calcium with the suppression of PTH. The evidence presented suggests that the resistance of PTH in this patient was due to secondary hyperparathyroidism rather than to a defect of PTH-sensitive receptors of the kidney and bone or to inadequacy of vitamin D per se.