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 calcium, phosphate 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 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 fibrosis, malabsorption 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 may be requested:

• If a patient is at high risk of vitamin D deficiency such as people who have limited sun exposure for medical, occupational, cultural or residential reasons. Also included in this group are people with darker skin and exclusively breastfed infants.
• If there is evidence of a defect in bone metabolism (e.g. osteoporosis) reflected by abnormal levels of calcium, phosphate, and/or magnesium in the blood.
• If a patient has symptoms of vitamin D deficiency, such as bone malformation in children (rickets) and bone weakness, softness, or fracture in adults (osteomalacia).
• If a patient suffers from malabsorption. Malabsorption causing vitamin D deficiency may occur in individuals following bariatric surgery or in patients with fat malabsorption such as in cystic fibrosis or Crohn's disease.
• In patients with kidney disease or who have received a kidney transplant
• To help diagnose or monitor problems with parathyroid gland functioning since parathyroid hormone (PTH) is essential for vitamin D activation and there is an inverse relationship between vitamin D 25OH and PTH levels.
• If a patient that is supplemented with vitamin D continues to experience symptoms of vitamin D deficiency.

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.

What does the result mean?

The Australian and New Zealand Bone and Mineral Society (ANZBMS) and Osteoporosis Australia (OA) guidelines (2012) classify vitamin D results as follows:

• 25-hydroxy-vitamin D of less than 30 nmol/L is deficient
• 25-hydroxy-vitamin D of 30-50 nmol/L is mildly deficient
• 25-hydroxy-vitamin D of greater than 50 nmol/L is sufficient for the health of bones

Vitamin D levels should be higher at the end of summer to allow for a decrease in levels over winter. Your doctor may recommend a vitamin D supplement if your vitamin D level is below this target level.

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.

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.

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

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.

• How much vitamin D should I be getting each day?

This depends on your age and sex. The Australian and New Zealand Bone and Mineral Society (ANZBMS) and Osteoporosis Australia (OA) guidelines (2012) recommend that when there is minimal sun exposure vitamin D intake from dietary and supplementation sources for adults should be at least 600 IU (15ug) per day for people aged equal to or less 70 years and 800 IU (20ug) per day for those aged above 70 years.

The amount of daily sun exposure required to aid in maintaining adequate vitamin D levels depends on a number of factors including skin type, latitude, time of year and time of day. The following table gives recommendations of daily sun exposure to around 15 per cent of the body surface (e.g. hands, arms and face) for moderately fair-skinned people. People with dark skin are likely to need 3-6 times more sun exposure.

_{Source: Nowson et al, Australian guideline MJA 2012;196: 1-7}

• 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.

• Is fortifying foods with vitamin D a good practice?

Yes. The amount of vitamin D produced by the body may be insufficient, especially when there is limited exposure to sunlight and routine use of sunscreens. Since dietary vitamin D is found naturally only in a few foods dietary intake would not be sufficient for most people. In Australia it is mandatory that edible oil spreads (e.g. margarine) are fortified with vitamin D. Voluntary fortification of vitamin D is allowed for some products such as skim milks, powdered milk, yoghurts, dairy desserts, butter, breads, cereals and cheese.

• Is vitamin D a necessary component of calcium supplements?

Since absorption of calcium is dependent on vitamin D, many manufacturers of calcium supplements add vitamin D to assure optimal calcium uptake. If you have adequate amounts of vitamin D from other sources, the additional vitamin D is not necessary. The amount of vitamin D in these tablets is not likely to lead to excess vitamin D or be harmful either.

• Who are the adult groups at increased risk of vitamin deficiency?

• People who have low or no exposure to the sun, for example those who:
  - are housebound or who are confined indoors for long periods (e.g. the elderly, hospitalised patients)
  - wear concealed clothing (e.g. for cultural reasons)
  - work in enclosed environments (e.g. factory and office workers) or night-shift workers
• People with naturally dark skin; the melanin in dark skin doesn't absorb as much UV radiation and their bodies are not able to make as much vitamin D
• People with fair skin and those who are at risk of skin cancer who avoid sun exposure
• People with a disability or chronic disease that affects vitamin D metabolism
• People who are obese

More Information

RCPA Manual: Vitamin D

RCPA Manual: Vitamin D 1,25-dihydroxy