Vitamin D metabolism in chronic childhood hypoparathyroidism: evidence for a direct regulatory effect of calcium.

We noted the presence of elevated levels of circulating 1,25-dihydroxyvitamin D (83 pg/ml (200 pmol/L), with low total serum calcium concentration (6.5 mg/dl (1.88 mmol/L), in an untreated adolescent boy with hypoparathyroidism. Furthermore, an inverse relationship between total serum calcium and circulating 1,25-dihydroxyvitamin D levels was evident during treatment with 1,25-dihydroxyvitamin D3. We examined this relationship with a 33-hour intravenous infusion of calcium gluconate in the absence of exogenous 1,25-dihydroxyvitamin D3 therapy. The infusion was accompanied by a gradual increase of both total serum calcium and blood ionized calcium concentrations from hypocalcemic to normocalcemic ranges, and produced a 50% reduction in circulating 1,25-dihydroxyvitamin D values, with minimal changes in circulating phosphorus, magnesium, and 25-hydroxyvitamin D values. These results suggest that calcium-dependent, parathyroid hormone-independent regulation of 1,25-dihydroxyvitamin D production may exist in human beings.