At a glance

Why get tested?

To check for vitamin D deficiency or to investigate a problem related to bone metabolism, mineral levels in the blood or parathyroid function.

When to get tested?

Your doctor may request a vitamin D 25 OH measurement as part of a general check-up because vitamin D deficiency appears to be very common in Australia. Vitamin D 25 OH measurement may also be requested if you have an abnormal calcium, phosphate, and/or parathyroid hormone level, as part of the investigation of some forms of bone disease or if you have kidney disease or a disease of the gastrointestinal tract that may result in malabsorption.

Sample required?

A blood sample drawn from a vein in your arm.

What is being tested?

Vitamin D is vital for the growth and health of bone; without it, bones will be soft, malformed, and unable to repair themselves normally, resulting in the disease called rickets in children and osteomalacia in children and adults. Vitamin D also helps to control the absorption of the minerals calciumphosphate and (to a lesser extent) magnesium from food as it passes through the intestine. Vitamin D may also be important in preventing many other diseases including disorders of the immune system, some forms of cancer and heart and blood vessel disease and it has been shown that the Vitamin D receptor (VDR) can affect a wide range of other gene expression, and may be involved in many cellular functions.

Vitamin D is a fat soluble vitamin found naturally in only a few foods, liver and fatty fish such as salmon, and so the major source of the vitamin is from sunlight skin exposure. A recommended time of sun exposure is difficult to say as it depends on a number of factors including skin type, latitude, time of year and time of day.

Vitamin D is now  also contained in fortified foods; it is added to milk, cereals and a variety of other foods, to ensure adequate intake by the general population.

The body is able to form vitamin D by exposure to sunlight. This is why vitamin D is sometimes described as the sunshine vitamin - it is formed from 7-dehydrocholesterol in the skin when the skin is exposed to light. Vitamin D can also be ingested - either in a few foods or in vitamin supplements. Vitamin D produced in the bodies of humans and other animals is slightly different to that produced in plants – the animal form is known as vitamin D3 or cholecalciferol and the plant form as vitamin D2 or ergocalciferol. All supplements sold in Australia are now vitamin D3, however both forms vitamin D2 and D3 are active in the human body.

Both vitamin D2 and vitamin D3 are converted in the liver into 25-hydroxy-vitamin D. This is the main active form of vitamin D stored in the body. When a doctor asks for a vitamin D 25 OH level to be tested on a person’s blood, this is the form the laboratory will measure. The test for 25-hydroxy-vitamin D is used to check that the body has an adequate supply of vitamin D.

Occasionally, the doctor will ask the laboratory to measure another active form of vitamin D known as 1,25 dihydroxy-vitamin D. 1,25 dihydroxy-vitamin D is a form of vitamin D produced in a number of tissues, mainly the kidney, but also skin, colon, pancreas, adrenal and brain, from 25-hydroxy-vitamin D, via an enzyme 1 alpha-hydroxylase also known as (CYP27B1). It is tested in special circumstances such as in people with kidney disease who may not be able to make enough 1,25-dihydroxy-vitamin D. The doctor may also ask the laboratory to measure 1,25-dihydroxy-vitamin D if he or she suspects you have a condition in which the body produces too much of this form of the vitamin such as in sarcoidosis or some lymphomas.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

The Test

How is it used?

Vitamin D 25 OH tests are often used as part of a general check-up for people who may be vitamin D deficient, such as frail elderly people who do not get outside in the sunshine very much. Vitamin D tests are also used to see if bone weakness, bone malformation, or abnormal metabolism of calcium (reflected by abnormal calcium, phosphate or PTH tests) is occurring as a result of a deficiency or excess of vitamin D.

Since vitamin D is a fat-soluble vitamin and is absorbed from the intestine like a fat, vitamin D 25 OH tests are sometimes used in people with diseases that interfere with fat absorption, such as cystic fibrosismalabsorption and Crohn's disease, to make sure they have an adequate level of vitamin D. 

Vitamin D 25 OH tests may also be used in people on vitamin D supplements, to make sure they have an adequate level of vitamin D.

When is it requested?

Vitamin D 25 OH tests are often used as part of a general check-up, especially in people at high risk of vitamin D deficiency such as frail elderly people. They are also particularly used to monitor people with kidney disease and some gastrointestinal disorders. Vitamin D 25 OH tests may also be done in people who are taking vitamin D supplements.

Vitamin D 25 OH tests may also be done when a person has an abnormal level of the minerals calcium, phosphate, and/or magnesium in the blood. They are also done in people with bone disorders.

If the calcium level in the blood is low or the patient has symptoms of vitamin D deficiency, such as bone malformation in children (rickets) and bone weakness, softness, or fracture in adults (osteomalacia), the vitamin D 25 OH test is usually requested to identify a possible deficiency in vitamin D.

If the calcium level in the blood is high or the patient has a disease that might produce excess amounts of 1,25 dihydroxy-vitamin D, such as sarcoidosis or some forms of lymphoma, the 1,25 dihydroxy-vitamin D test may also be requested.

Vitamin D 25 OH tests may also be used to help diagnose or monitor problems with parathyroid gland functioning since parathyroid hormone (PTH) is essential for vitamin D activation.

There is an inverse relationship between Vitamin D (25,hydroxyl-vitamin D) and parathyroid hormone (PTH) and this is sometimes useful to detect a Vitamin D deficiency.

What does the test result mean?

Low blood levels of 25-hydroxy-vitamin D may mean that you are not getting enough exposure to sunlight or enough dietary vitamin D to meet your body's demand; that there is a problem with its absorption from the intestines; or that not enough is being converted to 25-hydroxy-vitamin D in the liver (which means that it is not making it into the bloodstream). Occasionally, drugs used to treat seizures, particularly phenytoin (Dilantin), can interfere with the liver's production of 25-hydroxy-vitamin D. Australian experts believe that a vitamin D level of at least 50 nmol/L is needed for the health of the bones. The level should be higher at the end of summer because vitamin D levels fall over the winter. Your doctor may recommend a vitamin D supplement if your vitamin D level is below this level.

Low levels of 1,25-dihydroxy-vitamin D are often seen in kidney disease and are one of the earliest changes to occur in persons with kidney failure.

High levels of vitamin D and calcium can lead to calcification and damage of organs, such as the kidneys, as the body tries to lower blood calcium levels by depositing calcium phosphate compounds into the organs.

High levels of 25-hydroxy-vitamin D usually reflect excess supplementation from vitamin pills or other nutritional supplements.

High levels of 1,25-dihydroxy-vitamin D may occur when there is excess parathryoid hormone or when there are diseases, such as sarcoidosis or some lymphomas, in which the body makes too much 1,25-dihydroxy-vitamin D.

About Reference Intervals

Is there anything else I should know?

Vitamin D may also be important in preventing many diseases including disorders of the immune system, some forms of cancer and heart and blood vessel disease. It is not clear yet what level of vitamin D is needed to prevent such diseases, but it may be higher than the level needed for bone health and normal mineral metabolism.

If magnesium levels are low, they can cause a low calcium level that is resistant to vitamin D and parathyroid hormone regulation. It may be necessary to supplement both magnesium and calcium to regain normal function.

Common Questions

Are there other uses for vitamin D?

Yes, there is a topical form of vitamin D cream that is used to treat psoriasis.

If I am vitamin D deficient, do I need to take a supplement containing calcium as well as vitamin D?

Many supplements contain both vitamin D and calcium, because both are needed for bone health. However, for most people with vitamin D deficiency, it is not necessary to take a supplement that also contains calcium. It is possible that such combined supplements are bad for the heart and blood vessels and it is probably better to ensure your diet contains enough calcium. However, there are some people for whom combined calcium and vitamin D supplements may be of overall benefit, such as frail elderly people in nursing homes in whom it is very important to prevent bone fractures.

Last Review Date: February 11, 2020