25(OH)D, are measured in serum to assess a person's vitamin D status. Calcifediol is further hydroxylated by the kidneys and certain immune cells to form calcitriol (1,25-dihydroxycholecalciferol; 1,25(OH)₂D), the biologically active form of vitamin D. Calcitriol attaches to vitamin D receptors, which are nuclear receptors found in various tissues throughout the body.

The discovery of the vitamin in 1922