25 hydroxy vitamin D is an inactive pre-hormone that is produced through the conversion of vitamin D in the liver. Vitamin D-25 is converted to 1,25 dihydroxy vitamin D, which helps the body absorb calcium and maintain the correct levels of phosphate. There are not many foods that naturally contain vitamin D, and a deficiency can be dangerous and cause health issues. A 25 hydroxy vitamin D blood test will determine if a person’s vitamin D levels are below the recommended amount.
There are two main types of vitamin D: D2 and D3. Vitamin D2 is found in fortified foods, some plants, and supplements, and D3 is found naturally in food sources such as tuna and salmon, and is produced through the exposure of the skin to sunlight or ultraviolet (UV) rays. D3, also known as calcidiol, is more easily converted to the active 25 hydroxy vitamin D and is more efficiently used for body processes.
Once vitamin D has been transformed into the active 1,25 dihydroxy vitamin D in the kidneys, the hormone helps to absorb calcium in the body and regulate levels of phosphate. It is involved in bone growth, mineralization and remodeling, nerve and muscle function, and the immune system, and it prevents hypocalcemic tetany. Rickets, a disease that causes bone deformity and stunted growth, and osteomalcia, a bone-thinning disease that causes muscle weakness, pain, and fractures, both are caused by a vitamin D deficiency.
A vitamin D deficiency can also signify that there are other diseases at work in the body. Cystic fibrosis and Crohn's disease may both interfere with the absorption of the vitamin. Nutritional deficiency and hyperparathyroidism after a gastric bypass surgery could signal that the body is not absorbing calcium and an increase in vitamin D is needed. The deficiency of vitamin D has been the subject of research that indicates that inadequate amounts of the vitamin could contribute to diseases such as multiple sclerosis, diabetes, and inflammatory bowel disease.
A 25 hydroxy vitamin D test only tests the levels of the inactive form of vitamin D. A blood test will be done to determine the levels of the vitamin in the body to determine if the person may have a deficiency. People more susceptible to a deficiency include those that are dark skinned, rarely exposed to sunlight, or are taking medications, such as phenytoin and phenobarbital, which may inhibit the body from absorbing vitamin D.