24,25-Dihydroxycholecalciferol, also known as 24,25-dihydroxyvitamin D<sub>3</sub>, refers to a mixture of epimers hydroxycalcidiol and hydroxycalcidiol (abbreviated as and ) closely related to 1,25-dihydroxyvitamin D<sub>3</sub>, the active form of vitamin D<sub>3</sub>. Like vitamin D<sub>3</sub> itself and calcifediol (25-hydroxyvitamin D<sub>3</sub>), it is inactive as a hormone both in vitro and in vivo. It was first identified in 1972 in the laboratory of Hector DeLuca and Michael F. Holick.
24(R),25-(OH)<sub>2</sub>D<sub>3</sub> is formed from 25-hydroxyvitamin D<sub>3</sub> by the action of CYP24A1 (25-hydroxyvitamin D3-24-hydroxylase). CYP24A1 appears to be "a multicatalytic enzyme catalyzing most, if not all, of the reactions in the C-24/C-23 pathway of 25-OH-D<sub>3</sub> metabolism." 24(S),25-(OH)<sub>2</sub>D<sub>3</sub> is derived from reduction of 25-OH-24-oxo-D3, a metabolite of 24(R),25-(OH)<sub>2</sub>D<sub>3</sub>.
The proportion of serum 24,25-dihydroxyvitamin D<sub>3</sub> that is the 24(R) isomer was found to be 96.8% in rats under standard conditions, decreasing to 89.5% several days after injection with a large dose of vitamin D<sub>3</sub>.
It has been proposed that 24,25-dihydroxyvitamin D<sub>3</sub> is a metabolite of 25-hydroxyvitamin D<sub>3</sub> which is destined for excretion.
It is not known whether the compound might also have some physiologically significant activity. Some evidence of a possible receptor has been obtained.