Vitamin D is essential for many bodily processes, particularly the development of healthy bones. A 25-hydroxy vitamin D test can reveal whether the vitamin D levels in a person’s blood are within a normal range.

In this article, we will discuss what 25-hydroxy vitamin D tests are for, what to expect, what the results mean, and how to increase vitamin D levels.

A bandage is placed on a patient's arm after a vitamin D 25 hydroxy test.Share on Pinterest
By taking a blood sample for a 25-hydroxy vitamin D test, doctors can identify whether a person is vitamin D deficient.

Vitamin D is a nutrient that the body makes on exposure to sunlight. It is also present in some foods, such as oily fish and egg yolk.

Vitamin D performs several important roles throughout the body. For example, it helps the body absorb calcium to maintain bone health. It also supports the function of muscles and nerves.

A 25-hydroxy vitamin D test measures vitamin D levels in the body. Doctors use blood samples to carry out the test.

The 25-hydroxy vitamin D test involves providing a blood sample. To take a blood sample, a healthcare provider will first clean the skin on the person’s arm. Then they will draw blood using a needle.

When they have collected enough blood, the healthcare provider will remove the needle and apply pressure to the wound. They will then dress it with a Band-Aid or bandage.

A 25-hydroxy vitamin D test is safe and usually does not cause side effects. However, some people may feel faint or lightheaded after the test. Others may notice bruising or pain around the needle site, which may last several days.

Most people in the United States have adequate vitamin D. However, from 2001–2006, about one-quarter of adults in the U.S. were at risk of vitamin D deficiency.

Low vitamin D can cause a range of problems. In children, a vitamin D deficiency can cause rickets, which occurs when the bones become soft and bend. In adults, low vitamin D can lead to osteomalacia, which causes bone weakness and pain.

Certain groups are more at risk for vitamin D deficiency than others, so people belonging to them may wish to check their vitamin D levels with a test. These individuals include:

  • older adults and people with darker skin, as their skin is less able to synthesize vitamin D from sunlight
  • people with obesity, as fat cells can bind to vitamin D and prevent it from entering the bloodstream
  • people with osteoporosis, celiac disease, and other conditions that interfere with vitamin D absorption
  • people who have had gastric bypass surgery

A doctor may also recommend this test for someone with abnormal levels of calcium and phosphorous in their blood.

Some researchers argue that screening at-risk groups for vitamin D deficiency would be a cost effective way of preventing health conditions. However, the Preventive Services Task Force published a statement in 2014 saying that there is not enough evidence to justify screening.

A doctor will also recommend vitamin D testing if someone has the symptoms of a deficiency. These include:

The cost of a vitamin D test will vary depending on the healthcare provider. However, the average price in the U.S. is about $50.

The results of the 25-hydroxy vitamin D test are usually in nanograms per deciliter (ng/dl) of blood.

Currently, there is no consensus on a specific threshold for vitamin D deficiency. Generally, though, public health organizations agree that most people require at least 20 ng/dl of vitamin D in their blood to stay healthy and that lower results are a sign of a deficiency.

Some studies suggest that 25 ng/dl is ideal. However, there is little evidence that more than 30 ng/dl of vitamin D offers additional health benefits.

Exposing the skin to sunlight is a convenient way to get more vitamin D. In a study from Switzerland, researchers found that just 10–15 minutes of sunlight exposure per day during spring and summer gave participants adequate vitamin D levels.

To do this safely, it is important to apply sunscreen with a high SPF to the skin to protect it from sunburn and UV damage. A person cannot make vitamin D from sunlight that has filtered through glass, so sitting in sunlight indoors will not contribute to vitamin D levels.

During less sunny seasons, people may find it more difficult to get adequate vitamin D from sunlight alone. In these cases, it may help to eat more foods that contain vitamin D. People with darker skin and those who cover their skin for religious or cultural reasons may also benefit from doing this.

Few foods contain naturally occurring vitamin D. However, some manufacturers add vitamin D to foods, such as some cereals, milk, and orange juice. A person also can get vitamin D from:

  • oily fish, such as tuna or mackerel
  • beef liver
  • egg yolks
  • mushrooms

If a person still has low vitamin D levels after trying to increase them through food and sunlight, they may need to take vitamin D supplements. It is best to discuss any new supplements with a doctor before taking them.

Vitamin D performs a range of important functions in the body. Low vitamin D levels can cause health problems, such as rickets and osteomalacia. A 25-hydroxy vitamin D test can help someone determine whether they have a deficiency or are at risk of developing one.

It is possible to get enough vitamin D by going outside in the sunlight each day. Foods rich in vitamin D can also help.

People who struggle to get enough vitamin D or have underlying conditions that prevent them from absorbing vitamin D can speak to a doctor about taking a supplement.