PTH and uremia: FGF-23 inhibits 1-α-hydroxylase in the kidney and stimulates 24-hydroxylase activity, thereby decreasing calcitriol synthesis and increasing calcitriol metabolism to the inactive 1,24,25(OH)3-vitamin D.105 In addition, FGF-23 inhibits the secretion of PTH before uremia is advanced, but this is a minor effect as the major regulator of PTH secretion is serum iCa.