The Vitamin D Receptor (VDR) is a nuclear hormone receptor that is activated by the active form of Vitamin D, calcitrol. VDR is expressed by most cells in every organ of the body. More than 2,000 genes are regulated by VDR activation.

The primary role of Vitamin D is to regulate blood levels of calcium and phosphorus by promoting absorption in the intestines and reabsorption in kidneys. Calcium and phosphorus levels are important for bone mineralization and growth as well as for the prevention of hypocalcemic tetany. Vitamin D is also an important immune regulator. It promotes phagocytosis, anti-tumor activity, and immunomodulary functions that play a role in autoimmune disease.