What Is the Vitamin D 25-Hydroxy Test?

man studying vitamin bottle
Expert Fact Checked

Vitamin D 25 hydroxy is actually the name of a blood test used to determine how much vitamin D is in the body. Your doctor may order this test for a number of reasons such as determine if you're deficient in vitamin D or at risk for complications due to a deficiency. The test is conducted on blood samples taken in your doctor's office or at a laboratory, and it's thought by most medical practitioners to be a fairly reliable way of measuring the amount of vitamin D in the body.

About Vitamin D 25 Hydroxy

Vitamin D 25 hydroxy circulates in the blood. Once it reaches the kidneys, they transform it into an active form of the vitamin that your body uses for various functions.

How the Test is Performed

If your doctor orders this test, you'll have blood drawn from a vein on your arm or on your hand. Your doctor, a specialist at his office, or a phlebotomist at a laboratory will clean the area with antiseptic, then use a sterile needle attached to a vial to draw the blood. They send the blood sample to a laboratory for analysis. You should fast according to your doctor's directions prior to having this test performed; the usual time to refrain from eating and drinking is four hours before the test.

Normal Readings

A normal reading on the test is considered any number within the range of 30 to 74 ng/mL (nanograms per milliliter.) Anything below this number means you are deficient in vitamin D. Anything above the number means you have too much vitamin D.

Why the Test Is Used

Quest Diagnostics, a major laboratory in the United States, cites the reasons for this test as:

  • Determining if the patient has a vitamin deficiency
  • As a diagnostic tool for various musculo-skeletal disorders
  • For homebound or elderly patients, who may not be getting adequate sunlight to make their own vitamin D.

Another reason is that low levels of vitamin D are suspected as one of the many possible factors implicated in autoimmune diseases such as multiple sclerosis and type I (juvenile) diabetes. With multiple sclerosis, there's an increase in the frequency of the diseases the higher the latitude, leading to the speculation that the decreased amount of sunlight available in northern latitudes may negatively affect vitamin D levels, which in turn act with other factors to set the stage for the disease.

How Low Levels Are Treated

If the laboratory test results indicate that you have too little vitamin D, your doctor may recommend that you take a vitamin D supplement, either a tablet or a liquid supplement. Judicious sun exposure may also be recommended unless you have a history of skin cancer or other problems that may limit the amount of sun exposure you receive. Your doctor may also recommend nutritional counseling, since many people with low vitamin levels could benefits from improving their diets.

If your doctor recommends a vitamin D 25 hydroxy test, don't panic. It doesn't mean that you're sick. The cost of the test varies considerably, and insurance may in some cases pick up the entire amount or offset it somewhat. It's worth having done, especially if it detects low levels of vitamin D in the early stages. With proper nutrition, you can easily get those levels back up and ward off potentially dangerous deficiency diseases and conditions that may lead to other diseases.

Was this page useful?
Related & Popular
What Is the Vitamin D 25-Hydroxy Test?