Calcitriol is synthesized from 7-dehydrocholesterol in humans via a non-enzymatic photochemical reaction with 290-310 nm UV light in the skin. Hydroxylation of the resulting cholecalciferol in the liver produces 25-hydroxy Vitamin D3, the principal circulating form of Vitamin D. A second, tightly regulated hydroxylation in the kidney produces calcitriol. Plasma calcitriol levels range from 10-70 pg/ml and are influenced by numerous dietary and hormonal factors. The main physiologic effects of calcitriol are to increase the absorption of calcium at the level of the intestinal epithelium, and to increase the mineralization of bone via the direct stimulation of osteoblasts.