1,25-Dihydroxycholecalciferol in human serum and its relationship with other metabolites of vitamin D-3.

A competitive protein binding assay for 1,25-dihydroxycholecalciferol has been developed using the hormone's nuclear receptor protein from chick intestinal mucosa. This nuclear receptor protein can be stored at -70 degrees C for several months and bound and free hormone can be separated easily with dextran coated charcoal. Results obtained using this assay agree well with those reported by other groups of workers. Serum levels of other vitamin D-3 metabolites, namely 25-hydroxycholecalciferol and 24,25-dihydroxycholecalciferol have also been measured and are shown in relation to 1,25-(OH)2D3 levels.