Growth hormone increases serum 1,25-dihydroxyvitamin D levels and decreases 24,25-dihydroxyvitamin D levels in children with growth hormone deficiency.

The influence of growth hormone (GH) on calcium-phosphorus metabolism and modulation of vitamin D metabolism has been demonstrated, but the mechanism remains unclear. We investigated the effect of a 6-month course of GH therapy on vitamin D and mineral metabolism in twelve GH-deficient children. Before GH therapy, levels of vitamin D metabolites and other biochemistry data were within normal ranges. All patients responded to GH therapy with increased growth velocity. 1,25-Dihydroxyvitamin D levels increased after 1 month of treatment and remained at these higher levels, with a significant increase found at 3 months (P < 0.05), whereas 24,25-dihydroxyvitamin D levels were decreased at 1 and 3 months, the latter being a significant decrease (P < 0.05), and then returned to the baseline levels at 6 months. 25-Hydroxyvitamin D levels did not change significantly. A significant increase in serum insulin-like growth factor-I (IGF-I) levels occurred during the 6 months of treatment (1 month, P < 0.01; 3 and 6 months, P < 0.001). Serum parathyroid hormone (PTH) levels decreased significantly at 3 and 6 months (3 months, P < 0.01; 6 months, P < 0.05). Serum calcium and phosphorus levels did not change significantly. Significant increases were found in the urinary calcium/urinary creatinine ratio (3 and 6 months, P < 0.05) and the percent tubular reabsorption of phosphorus levels (1 and 3 months, P < 0.05). The results of this study confirmed the actions of GH on renal tubules with increases in calcium excretion and phosphorus reabsorption, and indicate that the action of GH on modulating vitamin D metabolism may be IGF-I mediated, not PTH mediated.