FGF23 and secondary hyperparathyroidism: Because FGF23 stimulates CYP24A1 and inhibits CYP27B1 (25-hydroxyvitamin D3-1α-hydroxylase) expression in both renal and extrarenal tissues (40), FGF23-mediated forms of hypophosphatemia are associated with low 1,25(OH)2D levels (41), secondary hyperparathyroidism (42), and low-normal to slightly decreased calcemia.