Vitamin D

What is being tested?
There are two forms of vitamin D that can be measured in the blood: 25-hydroxyvitamin D [25(OH)D] and 1,25-dihydroxyvitamin D [1,25(OH)2D]. 25-hydroxyvitamin D is the main circulating form of the hormone and the inactive precursor of the active form, 1,25-dihydroxyvitamin D. Because of its long half-life and high concentration, 25-hydroxyvitamin D is commonly measured to monitor a patient’s vitamin D levels.
Vitamin D is maintained in the body by two mechanisms: an endogenous one, in which the hormone is produced through exposure to the sun, and an exogenous one, through vitamin D in the diet. The chemical structure of these types of vitamin D differs, and they are called vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). D2 is found in some foods, in food supplements and in vitamin preparations. Vitamin D3, on the other hand, is produced by the body and can be found in some food supplements. Vitamins D2 and D3 have the same effect.
Many 25-hydroxyvitamin D tests do not distinguish between the D2 and D3 forms and measure the total hormone. Other methods do distinguish the two forms. The 25-hydroxyvitamin D concentration is used to estimate vitamin D levels.

Sources and forms of vitamin D

Vitamin D regulates the absorption of calcium, phosphorus and magnesium.

It is vital for bone growth and bone health; without this hormone, bones are fragile, misshapen and unable to repair themselves, a condition called rickets in children and osteomalacia in adults. Vitamin D is also involved in autoimmune mechanisms, metabolic functions and cancer prevention.

How is the sample collected?
The blood sample is obtained by drawing blood from a vein in the patient’s arm.

Is any preparation needed for the test?
No.

How is it used?
When is the test requested?
How is the test result interpreted?
Is there anything else important to know?

How is it used?
The test is used to find out whether bone fragility, bone deformities or abnormal calcium metabolism (abnormal calcium, phosphorus or PTH values) are due to a lack or excess of vitamin D.
Because vitamin D is fat-soluble and is absorbed as such from the intestine, measuring it is useful in conditions such as cystic fibrosis and Crohn’s disease and in patients who have had gastric surgery and cannot absorb enough of the hormone. The test is also used to monitor the effectiveness of treatment with vitamin D, calcium, phosphorus and/or magnesium.

When is the test requested?

25-hydroxyvitamin D
It is requested when calcium is low and the patient has symptoms of vitamin D deficiency, such as bone deformities in children and rickets, osteomalacia or bone fragility. Vitamin D deficiency is quite common; some studies have shown that about 50% of older women treated for osteoporosis are deficient in this hormone. The test is also requested before starting treatment for osteoporosis (which may also include vitamin D)

1,25-dihydroxyvitamin D
It is requested when calcium is high or the patient has a disease that can produce excess vitamin D, such as sarcoidosis or lymphoma. Rarely, it is requested when an abnormality of 1α-hydroxylase is suspected.

Vitamin D measurement is also used to diagnose or monitor disorders of the parathyroid glands, as PTH is essential for activating vitamin D. When vitamin D, calcium, phosphorus or magnesium are given, levels of this hormone are used to monitor treatment.

How is the test result interpreted?
There are different methods for measuring vitamin D, which makes it difficult to establish universal reference levels.

25-hydroxyvitamin D
Low blood levels indicate too little exposure to sunlight, too little dietary intake or intestinal malabsorption. Sometimes antiepileptic drugs such as phenytoin (Dilantin) can interfere with vitamin D production in the liver.

There is evidence that low vitamin D levels are linked to an increased risk of cancer, autoimmune diseases and heart disease.

High levels of 25-hydroxyvitamin D indicate an overdose of this hormone from supplements or the diet.

1,25-dihydroxyvitamin D
Low levels of this molecule are found in kidney disease.

High levels, on the other hand, are found in conditions such as lymphoma and sarcoidosis, or when there is overproduction of parathyroid hormone.

Is there anything else important to know?
High levels of vitamin D and calcium can lead to calcification and damage organs, especially the kidneys and blood vessels
If magnesium levels are low, calcium levels may fall and resist correction by vitamin D and PTH. In this case, calcium and magnesium supplements are needed to restore normal function.

Is it a good idea to eat milk and cereals with added vitamin D?
Can I get enough vitamin D from yoghurt and cheese?
Are there other uses for vitamin D?
Is vitamin D a necessary part of calcium supplements?

Is it a good idea to eat milk and cereals with added vitamin D?
Yes. The amount of vitamin D produced by the body is not enough, especially when there is little exposure to sunlight, and for many people dietary intake is not enough either. In the United States, vitamin D is commonly added to foods such as milk, cereals and fruit juices, which has drastically reduced the incidence of rickets.

Can I get enough vitamin D from yoghurt and cheese?
Although vitamin D is almost always added to milk, this is not the case for dairy products made from it. The recommended daily intake is 2000 IU of vitamin D.

Are there other uses for vitamin D?
Yes, in a cream for psoriasis.

Is vitamin D a necessary part of calcium supplements?
Because calcium absorption depends on vitamin D, some manufacturers also add the hormone to calcium supplements. In fact, if the body has enough vitamin D, this addition is not needed. However, the amount of vitamin D in these products is not enough to cause an excess of the hormone in the body.