To investigate a problem related to bone metabolism or parathyroid function, possible vitamin D deficiency, malabsorption and to monitor some patients taking vitamin D.
Your doctor may request a vitamin D measurement in the following situations:
- If you are found to have an abnormal calcium, phosphate, and/or parathyroid hormone (PTH) concentration in the blood.
- As part of the investigation of some forms of bone disease or muscle weakness/pain.
- If you have disease of the gastrointestinal tract that could result in malabsorption.
- If you are receiving certain anticonvulsant drugs.
A blood sample taken from a vein in your arm
The main role of vitamin D is to help regulate the absorption of calcium, phosphate, and (to a lesser extent) magnesium from the gut. 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 adults. Vitamin D also plays an important role in musculoskeletal health.
There are two forms of vitamin D that can be measured in the blood - 25 hydroxyvitamin D and 1,25 dihydroxyvitamin D. 25 hydroxyvitamin D is the major form of vitamin D and is the relatively inactive component from which the active hormone, 1,25 dihydroxyvitamin D is made. 25 hydroxyvitamin D is the most useful indicator of vitamin D status in individuals, and is the form most commonly measured. This is because 25 hydroxyvitamin D remains in the blood longer and is present at much higher concentrations than 1,25 dihydroxyvitamin D. Occasionally it may be necessary to measure 1,25 dihydroxyvitamin D to find out whether the kidney is converting an appropriate amount of inactive 25-hydroxyvitamin D to the active 1,25 dihydroxyvitamin D.
Vitamin D comes from 2 sources: It is 1) produced in the skin following exposure to sunlight (endogenous source) and 2) ingested from foods and supplements (exogenous source). The chemical structures of these types of vitamin D are slightly different. Vitamin D2 (ergocalciferol) is the form found in foods of vegetable origin and vitamin D3 (cholecalciferol) is the form produced in the skin and is also found in foods of animal origin. Both forms of vitamin D may be present in vitamin preparations and supplements. For most people, the majority (up to 90%) of vitamin D is formed following the action of sunlight upon the skin. Both vitamin D2 and vitamin D3 are converted to 25-hydroxyvitamin D and then to 1,25 dihydroxyvitamin D.
Some vitamin D blood tests do not distinguish D2 and D3 forms of the vitamin and the result therefore gives an indication of the total amount of vitamin D present within the bloodstream.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
25-hydroxy vitamin D may be requested as part of the investigation of abnormal bone metabolism (reflected by abnormal calcium, phosphate or PTH blood test results). Recent guidelines from the National Osteoporosis Society (2013) recommend that 25-hydroxy vitamin D should also be measured in:
- Patients with bone diseases that may be improved by giving vitamin D supplementation.
- Patients with bone diseases that may need supplementation of vitamin D prior to starting a specific form of treatment.
- Patients with musculoskeletal symptoms that may be due to vitamin D deficiency (e.g. long term wide spread pain).
Routine monitoring of vitamin D is usually unnecessary, but may be required for patients that continue to have symptoms of vitamin D deficiency despite supplementation. Since vitamin D is a fat-soluble vitamin and is absorbed from the intestine like a fat, it may also need to be monitored in individuals with diseases that interfere with fat absorption, such as cystic fibrosis and Crohn’s disease, It is also used to monitor people who have had gastric bypass surgery and may not be able to absorb enough vitamin D.
Screening for vitamin D deficiency is not recommended in asymptomatic individuals.
When is it requested?
25-hydroxyvitamin D may be requested:
- If there is evidence of a defect in bone metabolism reflected by an abnormal calcium, phosphate or PTH concentration 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 may need supplementation of vitamin D prior to starting a specific form of treatment for bone disease.
- If a patient suffers from malabsorption. Malabsorption, causing vitamin D deficiency may occur in individuals following a gastric bypass or in patients with fat malabsorption such as in cystic fibrosis or Crohn’s disease.
- If a patient that is supplemented with vitamin D continues to experience symptoms of vitamin D deficiency.
1,25 dihydroxyvitamin D may be requested if the calcium concentration is high or the patient has a disease that might produce excess amounts of 1,25 dihydroxyvitamin D, such as sarcoidosis or some forms of lymphoma.
What does the test result mean?
The National Osteoporosis society (NOS) guidelines (UK, 2013) and the Institute of Medicine (US) classify vitamin D results as follows:
- 25-hydroxyvitamin D of less than 30 nmol/L is deficient
- 25-hydroxyvitamin D of 30-50 nmol/L may be inadequate in some people
- 25-hydroxyvitamin D of greater than 50 nmol/L is sufficient for almost the whole population.
Low blood levels of 25-hydroxyvitamin D may mean that you are not getting enough exposure to sunlight or enough vitamin D in your food to meet your body's demand or that there is a problem with its absorption from the intestines. Occasionally, drugs used to treat seizures, particularly phenytoin (epanutin), can interfere with the liver's production of 25-hydroxyvitamin D.
High levels of 25- hydroxyvitamin D usually reflect excess supplementation from vitamin pills or other nutritional supplements.
Low levels of 1,25-dihydroxyvitamin D can be seen in patients with kidney disease.
High levels of 1,25-dihydroxyvitamin D can be seen in patients with sarcoidosis or some lymphomas, as a result of 1,25 dihydroxyvitamin D being made outside of the kidneys.
Is there anything else I should know?
High levels of vitamin D and calcium can lead to the calcification and damage to organs particularly the kidneys and blood vessels.
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.
How much vitamin D should I be getting each day?
It depends on your age and gender. The European Union has set a Recommended Daily Allowance (RDA) of 5 micrograms for food labelling purposes. However the UK Department of Health recommend higher Vitamin D intake in the following groups who are at risk of vitamin D deficiency:
- Infants and children from 6 months – 5 years: 7 micrograms/day
- Pregnant and breastfeeding women, people who are not exposed to sunlight, and the elderly (over 65): 10 micrograms/day
The Department of Health recommends that these groups of people take food supplements to ensure sufficient vitamin D.
Is fortifying milk and cereals 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. Since dietary vitamin D is found naturally only in a few foods, such as cod liver oil, dietary intake may not be sufficient for most people. In the UK vitamin D is added to many fortified cereals and infant preparations and, by law, to margarine. Vitamin D was originally added to margarine to reduce rickets and osteomalacia in some parts of the UK (as butter is a source of these nutrients and margarine is often used instead of butter). Most vitamin D (up to 90%) is obtained from the action of sunlight on the skin, but some groups of people have less exposure to sunlight, e.g. some children and older people, particularly if they live in the northern part of the UK, or amongst people who habitually cover most of their skin surfaces for cultural or religious reasons.
Can I get my vitamin D from yogurt and cheese?
Unlike some countries, milk is not routinely fortified with vitamin D in the UK but in general, dairy products do contain some vitamin D.
Are there other uses for vitamin D?
Yes, there is a topical form of vitamin D cream that is used to treat psoriasis.
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.
On This Site
Tests: Calcium; Phosphate; Parathyroid Hormone (PTH); Magnesium; Bone Markers; Trace Elements
Conditions: Kidney disease, lymphoma, autoimmune conditions, cystic fibrosis, coeliac disease