Measurement of circulating 1,25-dihydroxyvitamin D in man. Changes in serum concentrations during treatment with 1 alpha-hydroxycholecalciferol.

A sensitive assay is described for measurement of circulating 1,25-dihydroxyvitamin D (1,25-(OH)2D) using rachitic chick intestinal cytosol binding protein. The metabolite was extracted from serum by diethyl ether and chromotographed on a Sephadex LH-20 column followed by high-pressure liquid chromatography. The concentration of 1,25-(OH)2D was then measured by a competitive protein-binding assay. The binding capacity of the protein was unchanged for 4 months. The inter- and intra-assay variations were both 11%. The standard curve was useful at serum concentrations ranging from 14 to 395 pmol/l (6 to 165 pg/ml). The normal mean concentration was 79.6 +/- 36.8 pmol/l (33.1 +/- 15.3 pg/ml). The serum 1,25-(OH)2D was studied in elderly subjects following administration of 1 alpha-hydroxycholecalciferol (1 alpha-OHD3) at different doses. Large variations occurred indicating individual rates of metabolism of 1,25-(OH)2D.