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.
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 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 , 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.