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 (Ca2+) 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.