The exact mechanism of action of vitamin D is unknown. It is assumed that since it is a steroid, it would act via a typical steroid receptor resulting in increased protein synthesis. It is known that vitamin D will increase the synthesis of calcium binding protein, presumably by this mechanism. However, vitamin D also increases calcium transport across cellular and mitochondrial membranes by a mechanism that is not dependent upon gene activation. Vitamin D will increase bone resorption, thus increasing the loss of bone calcium and phosphorus to the serum.
Vitamin D increases calcium and phosphorus reabsorption passively, by decreasing their secretion.
Vitamin D will increase the intestinal absorption of dietary calcium and phosphorus. The net effect of vitamin D is to increase serum levels of both calcium and phosphorus. Vitamin D may also be involved with PTH release, insulin secretion, cytokine production, and cell proliferation. In addition to the naturally occurring forms of vitamin D, some analogues are used. Calcifediol is used more than the endogenous form secalcifediol in clinical situations.