Radioreceptor Assay for 1α,25-Dihydroxyvitamin D 3

A competitive protein binding assay with a sensitivity of 80 picograms has been developed for 1α,25-dihydroxyvitamin D 3 , the hormonal form of vitamin D 3 . 1α,25-Dihydroxyvitamin D 3 displaced tritiated hormone from a cytosol-chromatin receptor preparation isolated from chick small intestine, providing a simple assay for the hormone. The concentration of 1α,25-dihydroxyvitamin D 3 in human plasma, as determined by this assay, is approximately 6 nanograms per 100 milliliters; in patients with renal disease the concentration of this kidney-produced hormone is significantly lower.