<prompt> Active form of Vitamin DCholecalciferolCalciferolCalcitriol25-hydroxy cholecalciferol  <response>: C Calcitriol Vitamin D Is Metabolized to the Active Metabolite, Calcitriol, in Liver Kidney. Cholecalciferol, either synthesized in the skin or from food, undergoes two hydroxylations to yield the active metabolite, 1,25-dihydroxyvitamin D or calcitriol. Ergocalciferol from fortified foods undergoes similar hydroxylation to yield calcitriol. In the liver, cholecalciferol is hydroxylated to form the 25-hydroxy-derivative, calcidiol. This is released into the circula-tion bound to a vitamin D binding globulin, which is the main storage form of the vitamin. In the kidney, calcidiol undergoes either 1 -hydroxylation to yield the active metabolite 1,25-dihydroxyvitamin D calcitriol, or 24-hydroxylation to yield a probably inactive metabolite, 24,25-dihydroxyvitamin D 24-hydroxycaicidiol. Some tissues, other than those that are involved in calcium homeostasis, take up calcidiol from the circulation and synthesize calcitriol that acts within the cell in which it was synthesized. 