Summary

• Vitamin D is vital to maintaining healthy bones, and plays an important role in muscle movement, nerve function and the immune system.
• Your supplies of vitamin D come from exposing your skin to sunlight, and from your diet.
• Before your body can use vitamin D, your liver must change it into another form called 25 hydroxyvitamin D, or 25(OH)D. 
• Most vitamin D blood tests measure the level of 25(OH)D in your blood because this is the most accurate way to see if you have enough vitamin D.

What is vitamin D?

Vitamin D helps your body absorb calcium and phosphate from your diet. You need this to build healthy bones and teeth. Without it, your bones would be soft, malformed, and unable to repair themselves normally. Vitamin D has been found to be important in keeping your immune system, heart, nervous system and muscles healthy.

There are two forms of vitamin D. These are vitamin D2 and vitamin D3. 

• Vitamin D² comes mainly from foods like fortified breakfast cereals and plants such as mushrooms grown in UV light.
• Vitamin D³, which accounts for 90 per cent of your vitamin D supplies, is mostly made by your body when skin is exposed to sunlight. It is also found in some foods, including fatty fish, fish liver oils, egg yolks and fortified dairy products.

When you get vitamin D through sunlight exposure or from food and supplements, it is in an inactive form. In order for your body to be able to use it, this inactive vitamin D is first converted in your liver to another form called 25-hydroxyvitamin D or 25(OH)D.

Next, when your body needs to use calcium, your kidneys convert 25(OH)D into another form of vitamin D called calcitriol or 1,25 dihydroxy-vitamin D (1,25(OH)D). As well as the kidneys, some other tissues like the skin, prostate and immune cells also produce small amounts of 1,25(OH)D, which are used locally.

When your doctor asks for a vitamin D level to be measured in your blood sample, it is 25(OH)D – the form made in your liver – that the laboratory measures. This form is measured because it is a good indicator of overall vitamin D levels in your body.

Occasionally, the type of vitamin D made in the kidneys, calcitriol or 1,25(OH)D, is measured. This happens in special circumstances such as if you have kidney disease because you may not be able to make enough calcitriol to maintain healthy bones. Calcitriol is also tested if you have a condition in which the body produces too much, such as in sarcoidosis or some lymphomas (a type of cancer).

Why get tested?

The test is used to check for vitamin D deficiency:

• As part of a general health check, especially if you have limited sun exposure.
• If you have symptoms of a bone disease such as osteoporosis and/or blood tests show abnormal levels of calcium, phosphate and/or magnesium.
• When there are signs of bone malformation in children (rickets) and bone weakness, softness or fracture in adults.
• If you have a malabsorption disorder. This can occur after bariatric surgery or in conditions such as cystic fibrosis or Crohn's disease that prevents fat being absorbed from the intestine.
• If you have kidney disease. 
• If your parathyroid gland is not working properly. Parathyroid hormone (PTH) is important for vitamin D activation.
• To monitor your vitamin D levels if you are taking supplements.

Certain people are at greater risk of vitamin D deficiency:

• People who have low or no exposure to the sun through being housebound or confined indoors for long periods, such as the elderly and hospitalised patients, wearing head to toe clothing, such as for cultural reasons, working in enclosed environments, for example factory and office workers, or night-shift workers.
• People with naturally dark skin. This is because the melanin in dark skin doesn't absorb as much UV radiation as those with fair skins and their bodies are not able to make as much vitamin D.
• Those with fair skin who avoid sun exposure because they are at risk of skin cancer.
• Those with a disability or chronic disease that affects vitamin D metabolism.
• People who are obese.

Having the test

Sample

Blood.

Any preparation?

None.

Your results

Reading your test report

Your results will be presented along with those of your other tests on the same form.  You will see separate columns or lines for each of these tests.

Vitamin D levels are normally higher at the end of summer and become lower over winter due to lack of sunlight exposure. Your doctor may recommend a vitamin D supplement if your vitamin D level is below the target level.

Low vitamin D levels may mean:

• you are not getting enough exposure to sunlight or enough dietary vitamin D to meet your body's requirements,
• there is a problem with its absorption from the intestines, or 
• not enough is being converted to 25-hydroxy-vitamin D in the liver, which means that it is not making it into the bloodstream.

High vitamin D levels are usually caused by taking too many vitamin pills or other nutritional supplements and can lead to calcification and damage of organs, such as the kidneys. As the body tries to lower blood calcium levels calcium phosphate compounds can be deposited into organs.
 

Reference intervals

Your results will be compared to reference intervals (sometimes called a normal range). 

• Reference intervals are the range of results expected in healthy people.
• When compared against them your results may be flagged high or low if they sit outside this range.
• Many reference intervals vary between labs so only those that are standardised or harmonised across most laboratories are given on this website.

If your results are flagged as high or low this does not necessarily mean that anything is wrong. It depends on your personal situation. Your results need to be interpreted by your doctor.

Any more to know?

Tell your doctor about medicines, vitamins or supplements you are taking, because they may affect your test results.

Occasionally, drugs used to treat seizures, particularly phenytoin (Dilantin), can interfere with the liver's production of 25-hydroxy-vitamin D. 

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help. 

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely. 

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record.

You and your healthcare provider can now access your results whenever and wherever needed. Get further trustworthy health information and advice from healthdirect.