Vitamin D is mainly synthesized in the skin from 7- dehydrocholesterol and is partially from dietary origin.
In the liver, Vitamin D is hydroxylated on carbon 25 to produce the obligatory intermediate 25OH Vitamin D. 25OH Vitamin D must be metabolized further before it can carry out the functions of Vitamin D on intestine, kidney and bone.
This subsequent reaction takes place exclusively in the kidney in the non-pregnant mammal.
Thus 25OH Vitamin D is further hydroxylated in the 1a-position to produce 1a,25 dihydroxyvitamin D (1,25(OH)2Vitamin D).