Vitamin D is metabolized by the liver to 25 (OH) D (calcifediol, calcidiol, 25-hydroxycholecalciferol, or 25-hydroxyvitamin D), which is then converted by the kidneys to 1, 25-dihydroxyvitamin D (1, 25-dihydroxycholecalciferol, calcitriol, or active vitamin D hormone).