Figure 3.

Metabolism and mechanism of action of vitamin D₃. Skin-synthesized or dietary vitamin D₃ is converted via a two-step hydroxylation process into the active hormonal form 1,25-(OH)₂D₃. The hormone binds to the vitamin D receptor (VDR) and regulates serum calcium (sCa²⁺) and serum phosphate (sPO₄) levels ensuring sufficient minerals for normal cellular activity around the body including bone. Insufficient vitamin D results in insufficient 1,25-(OH)₂D₃ and vitamin deficiency rickets. Circled in red are the proteins in the vitamin D-specific machinery that when mutated also result in some type of rickets. Circled in blue is the enzyme CYP24A1 that when mutated results in elevated 1,25-(OH)₂D₃ and hypercalcemia and/or kidney stones.