FIGURE 7-2 Photosynthesis of vitamin D3 and the metabolism of vitamin D3 to 25(OH)D3 and 1,25 (OH)2D3. Once formed, 1,25(OH)2D3 carries out the biologic functions of vitamin D3 on the intestine and bone. Parathyroid hormone (PTH) promotes the synthesis of 1,25(OH)2D3, which, in turn, stimulates intestinal calcium transport and bone calcium mobilization, and regulates the synthesis of PTH by negative feedback. Reproduced with permission, Holick (1996).



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