A study of Vitamin D Metabolism in Idiopathic Hypercalcemia of Infancy

Summary Serum vitamin D activity was determined in two cases of severe form of idiopathic hypercalcemia and the effect of a physiological dose of vitamin D3, was studied on this condition. The patients, 36 months of age, were uniovular female siblings, products of a triple normal pregnancy. Serum vitamin D activity was determined by a modification of the standard rachitic‐rat bioassay method, using radioactive calcium. The levels prior to the vitamin D test‐dose, in comparison with those found by the same method in 21 normal infants, was markedly elevated, about 4 times the mean normal value. A significant rise of the initial values was found 48 hours after the last test dose of vitamin D3. These findings suggest that the primary etiologic factor in idiopathic hypercalcemia, at least in some cases, is a genetically determined defect in the degradation mechanism of vitamin D.