Vitamin D Blood Level Test

Bone, Muscle, & Joint
Digestive Health
Contributed byMaulik P. Purohit MD MPHMay 07, 2018

What are the other Names for this Test? (Equivalent Terms)

  • 1,25 Dihydroxy-Vitamin D (Calcitriol) Test
  • 25-Hydroxycholecalciferol Test
  • Vitamin D(Cholecalciferol) Test

What is Vitamin D Blood Level Test? (Background Information)

  • Vitamin D is important in the formation of bones and teeth, since it helps in the absorption of calcium, phosphorus, and magnesium (to a lesser degree)
  • A lack of vitamin D make bones brittle, malformed, and they tend to lose their capacity for self-repair; leading to tooth decay and/or diseases, such as rickets (in children) and osteomalacia (in adults)
  • Excess vitamin D in the body, could also lead to hormonal imbalances, abnormal calcium metabolism, etc. This condition is called hypervitaminosis of vitamin D
  • Research has also indicated that vitamin D helps in body metabolic functions, fights cardiovascular diseases, helps in suppression of certain types of cancer cells, and in regulating auto-immunity
  • The blood contains two types of vitamin D:
    • 1,25-dihydroxyvitamin D [1,25(OH)(2)D]
    • 25-hydroxyvitamin D [25(OH)D]
  • 25-hydroxyvitamin D changes in the kidney to the active form of 1,25-dihydroxyvitamin D that is responsible for controlling calcium and phosphate levels in the body. Normally 25-hydroxyvitamin D is measured, by virtue of its higher concentration and longer half-life in blood, in order to indicate the quantity of vitamin D in the body

Vitamin D source: The body receives vitamin D through the food (or supplements) one takes, which is termed as exogenous. It is also produced on the skin when exposed to sunlight, called endogenous process.

Since, this leads to two types of vitamin D with differing chemical structures, they are named vitamin D2 and vitamin D3. Both vitamin forms have equal significance, though vitamin D3 is formed in the skin and vitamin D2 is found in foods and vitamin preparations.

  • Vitamin D Blood Level Testing: Test reports normally indicate only total 25-hydroxyvitamin D quantity, which basically is a measure of both vitamin D2 and vitamin D3. Sometimes, individual amounts of both are reported that add up together to indicate the total vitamin D status

What are the Clinical Indications for performing the Vitamin D Blood Level Test?

Individuals in the following scenario may be prescribed vitamin D testing and/or those requiring calcium, phosphorous, magnesium, or vitamin D supplementation. Those with:

  • Low levels of calcium or showing symptoms of vitamin D deficiency, such as bone weakness or malformation. In this scenario blood level of 25-hydroxyvitamin D is tested
  • In cases of abnormally high levels of calcium and/or phosphorus,blood levels of  1,25-dihydroxyvitamin D is tested
  • The high levels may be due to individuals having diseases, such as sarcoidosis, or certain types of lymphoma that cause to produce, excess amounts of calcium. Rarely, testing is also performed, where 1-alphahydroxylase enzyme abnormalities are suspected
  • Parathyroid gland problems: Parathyroid hormone is required for vitamin D activation and parathyroid hormone (called parathormone) helps in determining the amount of vitamin D in blood
  • Weakness in the bones, or bone-disease causing frequent signs and symptoms of calcium deficiency, such as frequent fractures
  • Before starting on medication for osteoporosis, testing for 25-hydroxyvitamin D is performed to establish baseline levels. Studies have indicated that almost 50% of those treated for osteoporosis, especially the elderly and women, are vitamin D deficient
  • Women at high-risk are tested:The National Osteoporosis Foundation, includes women with dark skin, older women, women with malabsorption and institutionalized/homebound women in the high-risk group
  • Testing is performed in individuals with diseases, such as cystic fibrosis, Crohn’s disease, and patients who undergo surgery for gastric bypass. These conditions interfere with, or reduce, fat absorption in the body. Since,vitamin D is fat-soluble and absorbed in the intestine, measuring its concentration helps monitor such conditions

How is the Specimen Collected for Vitamin D Blood Level Test?

Sample required: Blood

Process: Insertion of needle into a vein (arm).

Preparation required: None

What is the Significance of the Vitamin D Blood Level Test Result?

There is a general disagreement, all over the world, on what constitute a lower limit of normal vitamin D range, to definitively diagnose a deficiency of vitamin D. However, as explained previously, vitamin D status is usually indicated by the total 25-hydroxyvitamin D (D& D3) content in blood.

Low level of 25-hydroxyvitamin D is seen in:

  • Exposure to sunlight or dietary intake of vitamin D,is insufficient
  • Difficulty in fat absorption from the intestine
  • Use of seizure drugs (phenytoin – ‘Dilantin’) is interfering with liver production of 25-hydroxyvitamin D

Low level of 1,25-hydroxyvitamin D is seen in:

  • Kidney-related medical problems, kidney failure etc.

High level of 25-hydroxyvitamin D is seen in:

  • Supplementation from vitamin D preparations/drugs, is in surplus

 High level of 1,25dihydroxy-vitamin D is seen in:

  • Parathyroid hormone (PTH) produced is in excess
  • Vitamin D is being prepared outside of kidneys, due to conditions like sarcoidosis, or tumors like lymphoma

Too much of vitamin D and calcium could damage the kidney, blood vessels, or other organs, due to the process of calcification.

Too less of magnesium may cause reduced calcium levels in the body, leading to problems in parathyroid hormone regulation.

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.

Additional and Relevant Useful Information:

  • Including cereals, milk, and fruit juices, fortified with vitamin D in one’s regular diet is highly advised, as this would ensure adequate levels of body vitamin D. Normally milk products, like cheese and yoghurt, are not fortified with this vitamin
  • Vitamin D is naturally found only in a few foods, like cod liver oil, and fishes like salmon, mackerel, tuna, etc.
  • Lack of exposure to sunlight in regions, where weather conditions do not permit prolonged periods of sunshine, indicates that skin production of vitamin D is hampered or may be inadequate
  • Vitamin D is usually added to calcium supplements, in order to ensure proper absorption of calcium. But, even if the body receives the vitamin from other sources, an additional supplementation from these drugs, may not cause high or dangerous levels of body build-up of vitamin D

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, 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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Maulik P. Purohit MD MPH

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