FGF23 and hypophosphatemia: Since FGF23 stimulates CYP24A1 and inhibits CYP27B1 (25-OH-D3 1α-hydroxylase) in kidney and extra-renal tissues, all FGF23-dependent OM, such as XLH, will have low calcitriol levels (despite hypophosphatemia), secondary hyperparathyroidism, and normal or low serum calcium.