Demonstration of circulating 1,24,25-trihydroxyvitamin D3 in man by radioimmunoassay.

1,24,25-trihydroxyvitamin D3 has been detected in human serum using a sensitive radioimmunoassay. Tritiated 1,24,25-trihydroxyvitamin D3 was synthesized biologically and used as tracer to monitor the recovery of endogenous metabolite during isolation from serum. Circulating 1,24,25(OH)3D3 in normal subjects ranged from 9.3 to 18.5 pmol/l but was not detectable (less than 2.3 pmol/l) in serum from nephrectomized subjects. The trihydroxymetabolite was elevated in three out of four vitamin D deficient subjects who were being treated with vitamin D3.