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Vitamin D Blood Test: When Testing Makes Sense

What a vitamin D blood test measures, who may benefit from one, how to prepare, and how clinicians read 25-hydroxy vitamin D results.

A vitamin D blood test measures how much vitamin D is in your blood. Most versions measure a form called 25-hydroxyvitamin D, or 25(OH)D, which MedlinePlus describes as the most accurate way to see whether you have enough vitamin D (MedlinePlus). Routine testing is not recommended for everyone, so the more useful question is often when testing makes sense for you.

This guide covers what the test measures, who a clinician may test, how to prepare, and how results are usually read. It is meant to help you follow the conversation with your clinician, not to replace it.

What vitamin D does in your body

Vitamin D helps your body absorb calcium to build healthy bones and teeth. MedlinePlus notes that it also helps keep your muscles, nerves, and immune system working normally (MedlinePlus). It also helps regulate calcium and phosphate levels in the body (MedlinePlus Medical Encyclopedia).

You can get vitamin D from three places:

Sunlight. Your body makes vitamin D when bare skin is exposed to sunlight outdoors.
Food. Only a few foods, such as egg yolks, liver, and fatty fish, naturally contain it. That is why it is added to many foods, including breakfast cereals and milk.
Supplements. Vitamin D supplements come as pills or liquid drops.

Cleveland Clinic adds that people with darker skin and older adults may not make enough vitamin D from sunlight, and that where you live can limit how much sun exposure you get (Cleveland Clinic).

What a vitamin D blood test measures

Before your body can use vitamin D, your liver changes it into 25-hydroxyvitamin D. Most vitamin D blood tests measure this form, because it gives the clearest picture of how much vitamin D your body has on hand (MedlinePlus). You may see the test called a 25-hydroxy vitamin D test, a 25(OH)D test, or a calcidiol test.

25-hydroxy vitamin D vs. "active" vitamin D

A different test measures what is often called active vitamin D, also known as calcitriol or 1,25-dihydroxyvitamin D. MedlinePlus explains that this test isn't usually used to check whether you have enough vitamin D. It may instead be used to monitor kidney problems or to help find the cause of abnormal calcium levels (MedlinePlus). If you are reading a lab report, check which of the two tests was run.

Vitamin D2, vitamin D3, and total vitamin D

Some reports list vitamin D2 and vitamin D3 separately. According to the MedlinePlus Medical Encyclopedia:

Vitamin D3 is the form your own body makes, or that you absorb from animal sources such as fatty fish or liver, or from a D3 supplement.
Vitamin D2 is the form you absorb from foods fortified with plant vitamin D, or from a D2 supplement.

The two work about the same in the body. The number that matters most is total vitamin D, which is D2 and D3 added together (MedlinePlus).

When vitamin D testing makes sense

MedlinePlus says routine vitamin D testing is not recommended for everyone, and that screening all adults for low vitamin D, including during pregnancy, is generally not recommended (MedlinePlus Medical Encyclopedia). Cleveland Clinic similarly notes that healthcare providers don't usually order routine checks. Instead, a clinician is more likely to order a test when there is a specific reason.

Conditions or symptoms that may point to low vitamin D

A clinician may order a vitamin D blood test if you have a condition that may be related to low vitamin D, such as low bone density, osteopenia, osteoporosis, or osteomalacia (soft bones, often with muscle weakness). Symptoms that may lead to a test include bone pain, muscle weakness or aches, and fractures (MedlinePlus).

In adults, low vitamin D isn't always obvious. Cleveland Clinic lists fatigue, bone pain, muscle weakness or cramps, and mood changes as possible signs, and notes that you may have no signs or symptoms at all (Cleveland Clinic). Those symptoms have many other possible causes, which is one reason a clinician looks at the whole picture.

Risk factors for vitamin D deficiency

Testing may also make sense for people at higher risk of vitamin D deficiency. The sources list several common risk factors:

Being over age 65, since skin production and gut absorption of vitamin D both drop with age
Darker skin, which has a harder time making vitamin D from sunlight
Rarely getting sun on your skin, for example staying indoors, covering up, or living somewhere with little sunlight
Obesity, or having had weight loss surgery
Conditions that make it hard to absorb nutrients, such as Crohn's disease, ulcerative colitis, or celiac disease
Kidney or liver disease, which can affect how your body converts vitamin D into a usable form
Taking certain medicines that affect vitamin D levels

On medicines, Cleveland Clinic names several that can lower vitamin D, including some steroids such as prednisone, certain cholesterol-lowering drugs, the seizure medicines phenobarbital and phenytoin, the tuberculosis drug rifampin, and the weight-loss drug orlistat (Cleveland Clinic).

Low vitamin D is not rare. Cleveland Clinic states that approximately 35% of adults in the United States have vitamin D deficiency (Cleveland Clinic).

Follow-up testing

If you are already taking vitamin D for a low level, your clinician may repeat the test to see whether your level is changing. A test may also be ordered if someone taking supplements has symptoms that could point to too much vitamin D (MedlinePlus).

How to prepare for a vitamin D blood test

Preparation is usually simple:

Fasting is usually not needed. MedlinePlus says you usually won't need to fast, though this depends on the lab and the testing method. If your blood is also being used for other tests, such as a fasting insulin test, follow the instructions for those tests.
List your medicines and supplements. Tell your clinician about all the medicines, vitamins, and supplements you take, because they may affect your results. Don't stop taking any medicine unless your clinician tells you to.

The test itself is a standard blood draw from a vein in your arm and usually takes less than five minutes. There is very little risk. You may feel a brief sting and have slight soreness or a small bruise afterward (MedlinePlus).

Vitamin D blood test results: what they mean

Your report will show the reference range the lab used, and ranges can vary slightly from lab to lab. Your clinician reads your result alongside your health history, symptoms, and other labs.

Units: ng/mL and nmol/L

Vitamin D results are reported in one of two units: nanograms per milliliter (ng/mL) or nanomoles per liter (nmol/L). The MedlinePlus Medical Encyclopedia says many experts recommend a level from 30 to 80 ng/mL, which is the same as 75 to 200 nmol/L (MedlinePlus Medical Encyclopedia). Because the two units give very different numbers, check which one your report uses before comparing results.

Result categories

MedlinePlus explains that results are usually described in one of four ways:

Deficient: very low levels that are likely to affect your bones and general health
Insufficient: low levels that may weaken your bones and affect your health, even without symptoms
Sufficient or optimal: levels that are enough for good bone and general health for most people
Toxic or possible toxicity: very high levels that might cause health problems

If your level is low

A low result may mean you aren't getting enough vitamin D from food or sunlight. It can also mean your body has trouble absorbing vitamin D, which may be a sign of a malabsorption disorder, or trouble converting it into a usable form, which may be a sign of kidney or liver disease. Some medicines can also lower levels (MedlinePlus).

If your level is low, your clinician decides what, if anything, to do next. MedlinePlus notes that a provider may recommend supplements or more foods rich in vitamin D, and that this is usually a safer approach than getting more sun, which can raise the risk of skin cancer. Your clinician determines whether you need a supplement and how much.

If your level is high

A high level is most often caused by taking too much vitamin D from supplements. It can lead to too much calcium in the blood, called hypercalcemia (MedlinePlus Medical Encyclopedia). Cleveland Clinic says vitamin D toxicity is rare, that you can't get too much vitamin D from the sun, and that symptoms can include nausea, increased thirst and urination, poor appetite, constipation, weakness, and confusion. Cleveland Clinic also advises against taking higher-than-recommended amounts without first talking with your healthcare provider (Cleveland Clinic).

How vitamin D fits with your other labs

Vitamin D works closely with calcium, so a clinician may read your result alongside calcium and other related labs. If you are reviewing other lab work, our guides to the hormone panel test and the thyroid blood test results explain how those numbers are read in context as well.

When to talk to a clinician

Consider asking a licensed clinician about a vitamin D blood test if:

You have low bone density, osteoporosis, or a history of fractures
You have ongoing bone pain, muscle weakness, or muscle aches
You have a risk factor such as limited sun exposure, a digestive condition that affects absorption, kidney or liver disease, or past weight loss surgery
You take a medicine that may affect vitamin D levels
You take vitamin D supplements and want to know whether your level is in a healthy range

Some symptoms should not wait for a routine lab visit. Confusion, trouble with balance or coordination, or unclear speech need prompt medical attention. Call 911 or seek urgent care if symptoms are severe or come on suddenly.

If you are exploring telehealth, Age Edit coordinates access to lab testing through independent licensed providers in our partner network, available in all 50 states. The treating clinician reviews your history, decides which tests make sense for you, explains the results, and makes all clinical decisions, including whether any supplement is appropriate. You can start at /get-started.

Sources

This article is for general education and is not medical advice. A licensed clinician in your state decides whether any test or treatment is right for you.

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