Effect of vitamin D2 on the assay of 24,25-dihydroxyvitamin D3 in man

Serum levels of the dihydroxylated metabolite of vitamin D3, 24,25-dihydroxycholecalciferol (24, 25(OH)2D3) were measured by competitive protein-binding assay in 2 anephric patients while undergoing therapy with 25-hydroxycholecalciferol (25(OH)D3) and, at a later date, vitamin D2, ergocalciferol. Whereas there was no increase in serum levels of 24, 25 (OH)2D3 when the patients were treated with 25 (OH)D3, a significant increase was apparently seen when vitamin D2 was given. These results suggest that a metabolite of vitamin D2, non-renal in origin, may be interfering in the assay for 24, 25 (OH)2D3. The different levels of serum 24, 25 (OH)2D3 in anephric man found by various groups of workers are explained by these results.