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Diagram showing the synthesis of 1,25-hydroxy-cholecalciferol (1,25-hydroxyvitamin D3) in humans. Cholecalciferol (vitamin D3) is converted into 25-hydroxycholecalciferol by 25-hydroxylase, and this 25-hydroxyvitamin D3 is then transformed into 1,25-hydroxycholecalciferol (1,25-hydroxyvitamin D3) by 1α-hydroxylase in the liver and kidney, respectively. Both reactions can nevertheless take place in the epidermis.
