Sunlight ( nm) stimulates skin cell to produce previtamin D3 which is then converted to vitamin D3. Over exposure to sunlight converts previtamin D3 to inactive products. Vitamin D has very little intrinsic biological activity and must undergo successive hydroxylations in order to act as a hormone. In liver, it is hydroxylated to 25-OH-D which is transported to kidney to form 1,25-(OH)2-D or 24,25-(OH)2-D. 1,25-(OH)2-D is the most potent vitamin D metabolite.