Calcitriol (INN), also called 1,25-dihydroxycholecalciferol or 1,25-dihydroxyvitamin D₃, is the hormonally active metabolite of vitamin D with three hydroxyl groups (abbreviated 1,25-(OH)₂D₃ or simply 1,25(OH)₂D), It was first identified by Michael F. Holick in work published in 1971. Calcitriol increases the level of calcium (Ca) in the blood by increasing the uptake of calcium from the gut into the blood, and possibly increasing the release of calcium into the blood from bone.