The metabolic transformation of vitamin D3. The parent vitamin D3 can be photochemically generated from 7-DHC (mammals, unlikely in fish) or absorbed from the diet (both in mammals and fish). In the liver vitamin D3 is hydroxylated to 25(OH)D3. In fish, further hydroxylation to form the most active metabolites 1,25(OH)2D3 and 24,25(OH)2D3 occurs in liver, kidney and several other tissues.