Vitamin D (25-OH) - Serum

A vitamin D test measures the level of vitamin D in your blood to make sure you have enough for your body to work well. Vitamin D is essential for healthy bones and teeth. It also helps keep your muscles, nerves, and immune system working normally.

Having low levels of vitamin D is a common problem that can lead to bone disorders and other medical problems. Vitamin D testing can let you know if you need to increase your vitamin D levels.

You get vitamin D in three ways:

• Your body makes vitamin D when your bare skin is exposed to sunlight.
• You get vitamin D from certain foods. Only a few foods, such as egg yolks and fatty fish, naturally contain vitamin D. That's why vitamin D is added to foods, including breakfast cereals, milk, and other dairy items.
• You can take vitamin D supplements.

Before your body can use vitamin D, your liver must change it into another form called 25 hydroxyvitamin D, or 25(OH)D. Most vitamin D blood tests measure the level of 25(OH)D in your blood.

Your kidneys use 25(OH)D to make "active vitamin D." Active vitamin D lets your body use calcium to build bone and helps other cells work properly. If you have kidney problems or abnormal calcium levels in your blood, your health care provider may order a test of active vitamin D. But this test is not generally used to check whether you have enough vitamin D.

Testing 25(OH)D is the most accurate way to measure how much vitamin D is in your blood.

Other names: 25-hydroxyvitamin D, 25(OH)D, cholecalciferol test, ergocalciferol test, calcidiol test, vitamin D2 test, vitamin D3 test.

Source: National Institutes of Health (NIH) U.S. Department of Health and Human Services.