Vitamin D3: Products25-hydroxy-vitamin D3 (25(OH)D3) is synthesized in the liver and is the primary circulating form of vitamin D. Its blood concentration, which reflects 25(OH)D3 produced by exposure to ultraviolet B, as well as dietary and vitamin D supplementation, is felt to be the best indicator of vitamin D status. 25(OH)D3 is metabolized to 1-alpha,25(OH)2D3 in the proximal tubular cells of the kidney by the enzyme 1-alpha-hydroxylase. The vitamin D structure is similar to that of classic steroid hormones, such as estradiol, cortisol, and aldosterone in that they have the same root cyclopentanoperhydrophenanthrene ring structure. It has been shown that the active steroid hormone 1-alpha,25(OH)2D3 is essential for life in higher animals. Besides playing important roles in calcium homeostasis and bone mineral metabolism, it is now known to play a role in cellular differentiation, inhibition of cell growth, immune regulation and the prevention of neoplastic transformation. The active form of vitamin D3, 1-alpha,25(OH)2D3, acts both through its cellular receptor, the vitamin D receptor (VDR), and through other extrarenal targets in an autocrine and paracrine manner where 1-alpha-hydroxylase is present.