(εR,1R,3aS,4E,7aR)-Octahydro-4-[(2Z)-2-[(5R)-5-hydroxy-2-methylenecyclohexylidene]ethylidene]-α,α,ε,7a-tetramethyl-H-indene-1-pentanol-d6; (3α,5Z,7E)-9,10-Seco-|cholesta-5,7,10(19)-triene-3,25-diol-d6; 3-epi-25-Hydroxyvitamin D3-d6; |

The labeled epimer metabolite of Calcifediol or 25-hydroxyvitamin D3. Calcifediol (INN), also known as calcidiol, 25-hydroxycholecalciferol, or 25-hydroxyvitamin D (abbreviated 25(OH)D),[1] is a prehormone that is produced in the liver by hydroxylation of vitamin D3(cholecalciferol) by the enzyme cholecalciferol 25-hydroxylase which was isolated by Michael F. Holick. Physicians worldwide measure this metabolite to determine a patient's vitamin D status.[2] At a typical daily intake of vitamin D3, its full conversion to calcifediol takes approximately 7 days.[3] Calcifediol is then converted in the kidneys (by the enzyme 25(OH)D-1α-hydroxylase) into calcitriol (1,25-(OH)2D3), a secosteroid hormone that is the active form of vitamin D. It can also be converted into 24-hydroxycalcidiol in the kidneys via 24-hydroxylation.[4][5]