Figure 3.

Vitamin D synthesis and effects. Most vitamin D is naturally synthesized in the skin from 7-dehydrocholesterol in response to ultraviolet radiation. 7-dehydrocholesterol is converted to Pre-Vitamin D3 that through the circulation reach the liver where it is metabolized to 25-hydroxyvitamin D (25(OH)D). This latter is then converted in the kidney by 1α-hydroxylase into 1,25-dihydroxyvitamin D3, the biologically active agonist for the Vitamin D receptor. The synthesis of 1,25-dihydroxyvitamin D3 is mainly regulated by PTH and serum calcium levels. 1,25-dihydroxyvitamin D3 has a several effects since it increases serum calcium levels by inducing calcium mobilization from bone, decreases renin-angiotensin-aldosterone system (RAAS) activity and inhibits PTH production.