PTH and Hypocalcemia: The activity of 1α-hydroxylase (CYP27B1) and/or 24-hydroxylase (CYP24) is increased by parathyroid hormone (PTH), PTH-related peptide (PTHrP), prostaglandins, hypocalcaemia and hypophosphataemia, and inhibited by fibroblast growth factor-23 (FGF23) and by 1,25(OH)2D alone.