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.
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.
Your doctor will tell you not to eat anything for four to eight hours before the test.
The 25-hydroxy vitamin D test requires a common blood test. Your healthcare provider will draw blood from a vein in your arm using a needle. A quick finger prick will more than likely provide enough for a blood sample in children and infants.
Results will depend on your age, sex, and the testing methods used. Results can also slightly vary from lab to lab.
According to the Office of Dietary Supplements (ODS), levels of vitamin D are measured by the 25-hydroxy level in nanomoles/liter (nmol/L) or nanograms/milliliter (ng/mL). The results can indicate the following:
- deficiency: less than 30 nmol/L (12 ng/mL)
- potential deficiency: between 30 nmol/L (12 ng/mL) and 50 nmol/L (20 ng/mL)
- normal levels: between 50 nmol/L (20 ng/mL) and 125 nmol/L (50 ng/mL)
- high levels: higher than 125 nmol/L (50 ng/mL)
If your vitamin D levels are low and you’re having symptoms of bone pain, a doctor may recommend a special scan to check bone density. Doctors use this painless scan to evaluate a person’s bone health.
Low blood levels of 25-hydroxy vitamin D usually mean one (or more) of the following:
- you aren’t eating a balanced, complete diet
- your intestines aren’t absorbing the vitamin properly
- you’re not spending enough time outside to absorb adequate vitamin D levels through sun exposure
Some evidence links vitamin D deficiency to a higher risk of certain cancers, immune diseases, and cardiovascular disease.
High vitamin D blood levels generally result from taking too many vitamin pills and other nutritional supplements. High doses of vitamin D can result in a condition called hypervitaminosis D. Hypervitaminosis is a rare but serious condition that could put you at risk for liver or kidney problems.
High levels are rarely due to consuming too much of the vitamin through foods or sun exposure.
Your doctor will help explain the results of your test and determine if you have a vitamin D deficiency.
As with any routine blood test, risks of the 25-hydroxy vitamin test are minimal and include:
- excessive bleeding
- a slight chance of infection where the needle pierces your skin
Vitamin D is vital to the body. Deficiencies at any age can cause problems. Your doctor may recommend supplements or other treatment options if you’re very deficient. Eating foods that contain vitamin D in addition to adding supplements to your regimen can help keep your vitamin D levels stable.