Calcitriol ($1,25\text{-dihydroxycholecalciferol}$ or $1,25(\text{OH})_2\text{D}_3$) is the biologically active hormonal form of vitamin D. It is produced by two-step hydroxylation: first in the liver (25-hydroxylation → calcidiol) and then in the kidney (1α-hydroxylation → calcitriol). Calcitriol acts on intestine, bone, and kidney to regulate calcium and phosphate homeostasis. Calciferol is the general term for vitamin D₂/D₃; retinol is vitamin A.