Discrepancy between serum concentrations of 1,25-dihydroxyvitamin D metabolites measured by radio-immunoassay and thymus radioreceptor assay during vitamin D2 treatment.

A radio-immunoassay (RIA) for determination of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) and 1,25-dihydroxyergocalciferol (1,25(OH)2D2) in serum was compared with a competitive protein binding assay using calf thymus receptor to clarify the comparability of measurements in subjects treated with either vitamin D2 or vitamin D3 (4000 IU per day for 8 weeks). Before and during treatment with vitamin D3 the two assay techniques were concordant, but during vitamin D2 treatment the serum concentrations of 1,25(OH)2D2 and 1,25(OH)2D3 were higher when measured by RIA. The study demonstrates that during vitamin D2 treatment unknown compounds, recognized only by the antiserum, co-elute with 1,25(OH)2D2 and 1,25(OH)2D3 in high pressure liquid chromatography. It can be concluded that the radio-immunoassay used here cannot replace the calf thymus receptor assay.