A vitamin D test is a blood test that is done to determine the level of vitamin D in the body. There are two different types of vitamin D tests. The most common is called 25-hydroxy-vitamin D and is performed when a vitamin D deficiency is suspected, for example because of bone weakness, bone disease, or abnormal metabolism of calcium. The second type of vitamin D test is called 1,25-dihydroxy-vitamin D and is usually done to test for elevated vitamin D levels that can be caused either by excess vitamin D intake or by disease. There is no clear consensus on what constitutes a normal vitamin D level or a vitamin D deficiency, so it is very important to consult a doctor about the results of a vitamin D test.
Vitamin D regulates the body's absorption of calcium, making it vital for healthy bones, and affects the immune system and blood vessels. There are several kinds of vitamin D, but the ones that are important for human health are vitamin D2 and vitamin D3. Vitamin D3 is produced by the body when it is exposed to sunlight, while vitamin D2 is available in foods like salmon, mackerel, sardines, eggs, and fish-liver oil. Both types of vitamin D are also available in supplements. The most accurate kind of vitamin D test is called "total 25-hydroxyvitamin D" and measures the levels of both vitamin D2 and D3.
Vitamin D deficiency can be caused by not getting enough sunlight or by a lack of vitamin D in the diet. It can also be caused by certain liver and kidney problems or by conditions like Crohn's disease and cystic fibrosis that interfere with vitamin D absorption. In children, vitamin D deficiency can lead to rickets, a condition characterized by malformed bones. Low levels in adults can lead to osteomalacia, a softening of the bones, and increase the risk of osteoporosis, a bone disease that increases the risk of fractures. Some groups, including women over 50 and women who are housebound, have a higher risk of vitamin D deficiency and can benefit from having a vitamin D test.
Excessive vitamin D levels can lead to vitamin D toxicity and can damage the heart, kidneys, and blood vessels. This condition is usually caused by an overdose of vitamin D supplements or by diseases such as sarcoidosis and lymphoma that make the body produce too much vitamin D. Sun exposure or eating foods rich in vitamin D are unlikely to cause excessive vitamin D levels.