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  Back Blood-tests Explained


Vitamin D


At a Glance

Why Get Tested?

To determine if you have a vitamin D deficiency; if you are receiving vitamin D supplementation, to determine if it is adequate

When to Get Tested?

When you have an abnormal calcium, phosphorus, and/or parathyroid hormone level; when you have evidence of bone disease or bone weakness; when you are at high risk of deficiency or a healthcare practitioner suspects that you might have a vitamin D deficiency; prior to starting drug treatment for osteoporosis; periodically to monitor treatment of vitamin D deficiency

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

Vitamin D is a family of compounds that is essential for the proper growth and formation of teeth and bones. This test measures the level of vitamin D in the blood.

Two forms of vitamin D can be measured in the blood, 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D. The 25-hydroxyvitamin D is the major form found in the blood and is the relatively inactive precursor to the active hormone, 1,25-dihydroxyvitamin D. Because of its long half-life and higher concentration, 25-hydroxyvitamin D is commonly measured to assess and monitor vitamin D status in individuals.

Vitamin D comes from two sources: endogenous, which is produced in the skin on exposure to sunlight, and exogenous, which is ingested in foods and supplements. The chemical structures of the types of vitamin D are slightly different, and they are named vitamin D2 (ergocalciferol, which comes from plants) and vitamin D3 (cholecalciferol, which comes from animals). The D2 form is found in fortified foods and in most vitamin preparations and supplements. Vitamin D3 is the form produced in the body and is also used in some supplements. Vitamin D2 and D3 are equally effective when they are converted by the liver and the kidney into the active form, 1,25-dihydroxyvitamin D.

Some tests do not distinguish D2 and D3 forms of the vitamin and report only the total result. Newer methods, however, may report levels of both D2 and D3 and then add them together for a total level.

The main role of vitamin D is to help regulate blood levels of calcium, phosphorus, and (to a lesser extent) magnesium. 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 diseases called rickets in children and osteomalacia in adults. Vitamin D has also been shown to influence the growth and differentiation of many other tissues and to help regulate the immune system. These other functions have implicated vitamin D in other disorders, such as autoimmunity and cancer.

Based on data from the National Health and Nutrition Examination Survey, the Centers for Disease Control and Prevention (CDC) reports that two-thirds of U.S. population has sufficient vitamin D, while roughly one-quarter are at risk of inadequate vitamin D and 8% are at risk of deficiency, as defined by the Dietary Reference Intake (DRI) set by the Institute of Medicine.

People at higher risk of deficiency include the elderly or obese people, people who don't get enough sun exposure, people with darker skin, and people who take certain medications for long periods of time. Adequate sun exposure is typically estimated to be two periods per week of 5-20 minutes. People who do not have adequate sun exposure may obtain the vitamin D that they need from food sources or supplements.

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.

The Test

How is it used?

A vitamin D test is used to:

  • Determine if bone weakness, bone malformation, or abnormal metabolism of calcium (reflected by abnormal calcium, phosphorus, PTH) is occurring as a result of a deficiency or excess of vitamin D
  • Help diagnose or monitor problems with parathyroid gland functioning since PTH is essential for vitamin D activation
  • Screen people who are at high risk of deficiency, as recommended by the National Osteoporosis Foundation, the Institute of Medicine, and the Endocrine Society
  • Help monitor the health status of individuals with diseases that interfere with fat absorption, such as cystic fibrosis and Crohn disease, since vitamin D is a fat-soluble vitamin and is absorbed from the intestine like a fat
  • Monitor people who have had gastric bypass surgery and may not be able to absorb enough vitamin D
  • Help determine the effectiveness of treatment when vitamin D, calcium, phosphorus, and/or magnesium supplementation is prescribed

When is it ordered?

25-hydroxyvitamin D
When calcium is low and/or a person has symptoms of vitamin D deficiency, such as bone malformation in children (rickets) and bone weakness, softness, or fracture in adults (osteomalacia), 25-hydroxyvitamin D usually is ordered to identify a possible deficiency in vitamin D.

The test may be requested when an individual is known to be at risk of vitamin D deficiency. Older adults, people who are institutionalized or homebound and/or have limited sun exposure, those who are obese, who have undergone gastric bypass surgery, and/or who have fat malabsorption are at an increased risk of a vitamin D deficiency. Also included in this group are people with darker skin and breastfed infants.

25-hydroxyvitamin D is often requested before an individual begins drug therapy for osteoporosis.

1,25-dihydroxyvitamin D
This testing may be ordered when kidney disease or abnormalities of the enzyme that converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D is suspected. Rarely, this test may be done when calcium is high or a person has a disease that might produce excess amounts of vitamin D, such as sarcoidosis or some forms of lymphoma (because immune cells may make 1,25-dihydroxyvitamin D).

When vitamin D, calcium, phosphorus, or magnesium supplementation is necessary, vitamin D levels are sometimes measured to monitor treatment effectiveness.

What does the test result mean?

Although there are differences among vitamin D methods, most laboratories utilize similar reference intervals. Because toxicity is rare, the focus has been on the lower limit and what cut-off for total 25-hydroxyvitamin D (D2 + D3) indicates deficiency.

  • The Endocrine Society defines vitamin D deficiency as a 25-hydroxyvitamin D blood level below 20 ng/mL (50 nmol/liter) and vitamin D insufficiency as a level between 21–29 ng/mL (52.5–72.5 nmol/liter).
  • The U.S. Institute of Medicine has concluded that a 25-hydroxyvitamin D level of 20 ng/mL (50 nmol/liter) or above is adequate for bone health, and that levels greater than 30 ng/mL do not offer significantly better effects. However, it is fair to say that the level of 25-hydroxyvitamin D associated with protection from the many other disorders recently associated with a lack of vitamin D is controversial.

25-hydroxyvitamin D
A low blood level of 25-hydroxyvitamin D may mean that a person is not getting enough exposure to sunlight or enough dietary vitamin D to meet his or her body's demand or that there is a problem with its absorption from the intestines. Occasionally, drugs used to treat seizures, particularly phenytoin (Dilantin), can interfere with the production of 25-hydroxyvitamin D in the liver.

There is some evidence that vitamin D deficiency may increase the risk of some cancers, immune diseases, and cardiovascular disease.

A high level of 25-hydroxyvitamin D usually reflects excess supplementation from vitamin pills or other nutritional supplements.

1,25-dihydroxyvitamin D
A low level of 1,25-dihydroxyvitamin D can be seen in kidney disease and is one of the earliest changes to occur in persons with early kidney failure.

A high level of 1,25-dihydroxyvitamin D may occur when there is excess parathryoid hormone or when there are diseases, such as sarcoidosis or some lymphomas, that can make 1,25-dihydroxyvitamin D 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.



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