A vitamin D test measures the levels of vitamin D in your blood. Vitamin D helps your body absorb calcium to build healthy bones and teeth. It also helps keep your muscles, nerves, and immune system working normally. Having vitamin D deficiency (very low levels of vitamin D) can lead to bone disorders and other medical conditions.

A vitamin D test is used to check the levels of vitamin D in your blood. The test is usually done if your health care provider thinks that a bone or other health condition you have could be caused by very low vitamin D levels.

Source: MedlinePlus, U.S. National Library of Medicine