Vitamin D helps your body absorb calcium and maintain strong bones throughout your entire life. Your body produces vitamin D when the sun’s UV rays contact your skin. Other good sources of the vitamin include fish, eggs, and fortified dairy products. It’s also available as a dietary supplement.

Vitamin D must go through several processes in your body before your body can use it. The first transformation occurs in the liver. Here, your body converts vitamin D to a chemical known as 25-hydroxyvitamin D, also called calcidiol.

The 25-hydroxy vitamin D test is the best way to monitor vitamin D levels. The amount of 25-hydroxyvitamin D in your blood is a good indication of how much vitamin D your body has. The test can determine if your vitamin D levels are too high or too low.

The test is also known as the 25-OH vitamin D test and the calcidiol 25-hydroxycholecalcifoerol test. It can be an important indicator of osteoporosis (bone weakness) and rickets (bone malformation).

Your doctor may request a 25-hydroxy vitamin D test for several different reasons. It can help them figure out whether too much or too little vitamin D is causing bone weakness or other abnormalities. It can also monitor people who are at risk for having a vitamin D deficiency.

Those who are at high risk of having low levels of vitamin D include:

• people who don’t get much exposure to the sun
• older adults
• people with obesity
• babies who are breastfed only (formula is usually fortified with vitamin D)
• people who have had gastric bypass surgery
• people who have a disease that affects the intestines and makes it difficult for the body to absorb nutrients, such as Crohn’s disease

Your doctor may also want you to do a 25-hydroxy vitamin D test if they’ve already diagnosed you with a vitamin D deficiency and want to see if treatment is working.