Figure 3.

Relationship of vitamin D, PTH, FGF-23 and phosphate handling-vitamin D3 is produced in skin from a UVB-mediated conversion of 7-dehydrocholesterol. In liver, vitamin D is converted to 25(OH) vitD3, which is released in blood. In the kidney, 25(OH) vitD3 is converted to 1-dihydroxyvitamin D [1,25(OH) 2vitD]. Two hormones regulate this step: PTH and FGF23, a hormone that is synthesized by osteocytes and osteoblasts. In kidney, PTH stimulates whereas FGF23 represses 25(OH) D-1α-hydroxylation. In kidney, FGF23 increases expression of 24-hydroxylase an enzyme that inactivates calcitriol