Whether it is made in the skin or ingested, vitamin D3 (cholecalciferol) is then hydroxylated in the liver to 25-hydroxycholecalciferol (25(OH)D3 or calcidiol) by the enzyme 25-hydroxylase produced by hepatocytes, and stored until it is needed. 25-hydroxycholecalciferol is further hydroxylated in the kidneys by the enzyme 1?-hydroxylase, into two dihydroxylated metabolites, the main biologically active hormone 1,25-dihydroxycholecalciferol (1,25(OH)2D3 or calcitriol) and 24R,25(OH)2D3. This conversion occurs in a tightly regulated fashion. Calcitriol is represented below right (hydroxylated Carbon 1 is on the lower ring at right, hydroxylated Carbon 25 is at the upper right end).

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