25hydroxyergocalciferol

25-hydroxyergocalciferol, also called 25-hydroxyergocalciferol or 25-OH-ergocalciferol, is the hepatic metabolite of ergocalciferol (vitamin D2). It is formed when ergocalciferol undergoes 25-hydroxylation in the liver by enzymes that metabolize vitamin D. This metabolite is a secosteroid and shares the core vitamin D structure with an additional hydroxyl group at the 25-position on the side chain.

In human and animal physiology, 25-hydroxyergocalciferol circulates in the bloodstream as a major storage and circulating form of vitamin D derived from ergocalciferol. It can be further hydroxylated in the kidney by 1α-hydroxylase to produce 1,25-dihydroxyergocalciferol, the active hormonally form. The same metabolic pathway exists for vitamin D3, yielding 25-hydroxycholecalciferol and then 1,25-dihydroxycholecalciferol.

Clinically, measurement of 25-hydroxyergocalciferol serves as a biomarker of vitamin D status in individuals who obtain vitamin D from ergocalciferol-containing sources. Some assays distinguish between the D2 and D3 forms (25-OH-D2 and 25-OH-D3) to assess contributions from ergocalciferol versus cholecalciferol, while others report total 25-hydroxy vitamin D. This metabolite is not itself an active drug, but its levels reflect vitamin D inputs and metabolism.

See also: Vitamin D, ergocalciferol, cholecalciferol, 25-hydroxyvitamin D, calcidiol, calcitriol.