25-Hydroxyvitamin D3 (25-OH vitamin D3) is measured worldwide to determine people’s vitamin D status by physicians. Its blood concentration is considered as the best indicator of vitamin D status in the body. Vitamin D is responsible for enhancing intestinal absorption of calcium. In humans, vitamin D3 and vitamin D2 are the most important compounds in the Vitamin D family. The deficiency of vitamin D can increase the risk of many diseases including autoimmune diseases, cancers and type II diabetes. 25-OH Vitamin D3 is formed in the liver by hydroxylation of vitamin D3 by the enzyme. It is converted in the kidneys into 1, 25-dihydroxyvitamin D3. 25-OH Vitamin D3 is also known as calcifediol, 25-Hydroxyvitamin D3, 25-Hydroxycholecalciferol.