VDR and Hypocalcemia: Vitamin D3 is first metabolized to 25-hydroxyvitamin D3 (25(OH)D3) in the liver and then to 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3) in the kidney.1, 2 1α,25(OH)2D3 is now recognized as a steroid hormone that has a role in maintaining calcium homeostasis through the vitamin D receptor (VDR).3 VDR knockout (VDR−/−) mice grew normally before weaning.4 However, they developed severe hypocalcemia after weaning.