PTH and Hypocalcemia: The renal synthesis of 1,25-(OH)2D from its precursor, 25(OH)D is a rate-limiting step and is tightly regulated by serum1,25-(OH)2D, parathyroid hormone (PTH), FGF23, calcium, and phosphate, with renal 1α-hydroxylase being stimulated by PTH, hypophosphatemia, or hypocalcaemia, and inhibited by FGF23 [1, 14, 15].