1,25-Dihydroxycholecalciferol Deficiency: The Probable Cause of Hypocalcemia and Metabolic Bone Disease in Pseudohypoparathyroidism

Pseudohypoparathyroidism (PsH) is a genetic disease characterized by hypocalcemia, hyperphosphatemia, and metabolic unresponsiveness to parathyroid hormone (PTH). The administration of PTH elicits neither a significant rise in serum calcium (calcemic response) nor a decrease in the renal tubule reabsorption of phosphorus (phosphaturic response). The diminished phosphaturic response is due to an inability of PTH to generate cyclic AMP in renal tubule cells. We investigated the question of whether hypocalcemia and deficient calcemic response to PTH are due to a similar cyclic AMP defect in bone or to an acquired vitamin D deficiency. Four patients were studied. The active form of vitamin D (1,25-dihydroxycholecalciferol) was measured in 3 and was low. Treatment with vitamin D2 restored the serum calcium and the calcemic response to PTH to normal without changing the impaired renal response. Bone biopsy was performed in 2 patients and showed morphologic evidence of increased osteoclastic activity and osteomalacia. The data indicate that the hypocalcemia and bone disease in PsH are due to active vitamin D deficiency, possibly resulting from the genetic renal lesion.