Radioreceptor assay for 1 alpha,25-dihydroxyvitamin D3.

A competitive protein binding assay with a sensitivity of 80 picograms has been developed for 1alpha,25-dihydroxyvitamin D(3), the hormonal form of vitamin D(3). lalpha,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 lalpha, 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.