25 Hydroxyvitamin D is the main form of vitamin D in the human body. Vitamin D is a steroid derivative and is a fat-soluble vitamin. Vitamin D is primarily synthesized by human skin after UV exposure, with a small percentage coming from food or supplements. Vitamin D not only affects the metabolism of calcium and phosphorus, but also has a wide range of physiological functions. It is necessary for the maintenance of human health, cell growth and development, and is closely associated with a wide range of diseases. Vitamin D in the body is converted to 25-hydroxyvitamin D through hydroxylation in the liver. 25-hydroxyvitamin D test is the basis for early diagnosis of rickets. There is evidence that vitamin D deficiency increases the risk of certain cancers, immune disorders, and cardiovascular disease, and if 25 hydroxyvitamin D is insufficient, supplementation is needed as soon as possible to reduce the risk of disease.