Vitamin D metabolism in hypophosphatemic rickets.

Vitamin D metabolites were measured in 21 patients with hypophosphatemic osteomalacia of juvenile onset. In eight patients who had not received any antirachitic treatment, serum 25-hydroxyvitamin D(25-OH-D) and 1,25-dihydroxyvitamin D (1,25-[OH]2D) values were normal, whereas serum 24,25-dihydroxyvitamin D (24,25-[OH]2D) values were lower than in normal subjects. In 13 patients who were receiving ergocalciferol and oral elemental phosphorus, serum 25-OH-D and 24,25-(OH)2D concentrations were elevated and serum 1,25-(OH)2D values were low. The findings in untreated patients supported the hypothesis that vitamin D metabolism is abnormal in hypophosphatemic rickets/osteomalacia. The reduction of serum 1,25-(OH)2D levels with ergocalciferol and phosphate therapy gives further support to a therapeutic role for 1,25-(OH)2D in this disorder.