The present method for the measurement of 24,25-dihydroxyvi- tamin D (24,25-(OH)2D) in serum associates the highly sensitive competitive protein binding assay of Preece et al and a doublestep chromatography of the serum lipid extract (Sephadex LH 20 column and high pressure liquid chromatograph). With this assay, values observed in 16 normal children, 10 adolescents and 5 neonates, were not significantly different from those found in 14 normal adults (25-(OH)D: 20 ± 7.4 ng/ml, 24,25-(OH)2D: 1.4 ± 0.8 ng/ml). No correlation was found between 25-(OH)D and (OH)2D serum concentrations. Measurements of 25-(OH)D and 24,25-(OH)2D serum concentrations in some pathologic states demonstrated the existence of a 25-(OH)D-24-hydroxylase in children with three types of vitamin D resistant rickets (hereditary hypophosphatemia, pseudodeficiency rickets, and vitamin D resistance associated with Recklinghausen’s neurofibromatosis). Finally, results observed during vitamin D administration suggest a regulation of the 24,25-(OH)2D concentration in human serum: 1) (OH)2D serum concentration increased after vitamin D administration, yet it was not found higher in a 3-wk-old child with vitamin D2 intoxication (25-(OH)D: 900 ng/ml, 24,25-(OH)2D: 6 ng/ml) than in three children with vitamin D deficiency rickets in the 2nd wk after administration of vitamin D2 and calcium (25- (OH)D: 25.5 ± 9.2 ng/ml, 24,25-(OH)2D: 13.4 ± 3.7 ng/ml; 2) In a child with pseudodeficiency rickets, 24,25-(OH)2D concentrations were found elevated before treatment. They decreased to normal values during treatment with 1α-(OH) cholecalciferol, whereas 25-(OH)D concentrations were not significantly different before and after treatment. Speculation Development of methods for the measurement of 24,25–(OH)2D concentration in serum should provide valuable informations for a better understanding of vitamin D metabolism in physiologic and pathologic states in children. © 1979 International Pediatric Research Foundation, Inc.
