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Vitamin D is a family of compounds that is essential for the proper growth and formation of teeth and bones.


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


This test measures the level of 25-hydroxy vitamin D in the blood.

About

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 (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-dihydroxy vitamin 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 vitamin D that they need from food sources or supplements.

Preparation

No test preparation is needed.

Indications

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-hydroxy vitamin 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-hydroxy vitamin D is often requested before an individual begins drug therapy for osteoporosis.

Interpretation

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 a deficiency.
  • The Endocrine Society defines vitamin D deficiency as a 25-hydroxyvitamin D blood level below 20 ng/mL (50 nmol/litre) and vitamin D insufficiency as a level between 21–29 ng/mL (52.5–72.5 nmol/litre).
  • The U.S. Institute of Medicine has concluded that a 25-hydroxyvitamin D level of 20 ng/mL (50 nmol/litre) 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.
A low blood level of 25-hydroxy vitamin 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 can interfere with the production of 25-hydroxy vitamin 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.

Disclaimer

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario. Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider of the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

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