PTH and Hypocalcemia: The first hydroxylation occurs in the liver, converting cholecalciferol to 25hydroxyvitamin D [25(OH)D], and the second one takes place in the kidneys, where 1α-hydroxylase converts 25(OH)D to 1,25(OH)2D (or calcitriol), the active form of vitamin D. Calcitriol production is strictly controlled; it is stimulated by PTH and hypocalcemia and inhibited by fibroblast growth factor-23 (FGF-23) and hyperphosphatemia (11,18–20).