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Vitamin D (25-OH)

Interpreting Results
Vitamin D (25-OH)

Overview of Vit D

What is Vit D?

Vitamin D helps our body absorb calcium and phosphorus, and maintain strong bones and teeth. Vitamin D (25-OH) test measures the level of Vitamin D (25-OH) in the blood which is a useful indicator of osteoporosis (bone weakness), rickets (bone malformation), and osteomalacia.

Why is Vit D done?

  • To determine vitamin D deficiency when calcium level is low 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).
  • Before starting therapy for osteoporosis. 
  • To monitor the adequacy of treatment in patients who are receiving Vitamin D supplements, calcium, phosphorus, and/or magnesium supplements.
  • To diagnose/monitor problems with the parathyroid gland functioning since the parathyroid hormone (PTH) is essential for vitamin D activation.
  • To monitor the health status of individuals with diseases that interfere with fat absorption, such as cystic fibrosis and Crohn’s disease and in patients who have undergone gastric bypass surgery.

What does Vit D Measure?

Vitamin D Test measures the levels 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.

The 25-hydroxyvitamin D test is done to determine the level of Vitamin D in your blood, whether it is low or higher than normal. Low levels can be seen if a person is not getting enough exposure to sunlight or enough dietary vitamin D to meet his or her body's demand or if there is a problem with its absorption from the intestines (cystic fibrosis, crohn’s disease, who have undergone gastric bypass surgery). Sometimes, medicines used to treat seizure (Phenytoin) can cause Vitamin D deficiency by interfering with transformation to 25-hydroxyvitamin D in the liver. Severe liver and kidney diseases can also cause vitamin D deficiency. High levels reflects excess supplementation of the vitamin.

Preparation for Vit D

  • No special preparation required

Sample Type for Vit D

The sample type collected for Vitamin D (25-OH) is: Blood

Interpreting Vit D results


Low vitamin D level than normal reference range indicates vitamin D deficiency which can be because of less sun exposure, dietary deficiency or due to decreased absorption from the intestine.

High vitamin D level than normal reference range indicates vitamin D intoxication and is usually due to excess supplementation of the vitamin.


The normal range of vitamin D is measured as nanograms per milliliter (ng/mL) or nmol/L and can vary from lab to lab.


Reference range (ng/mL)

Severe deficiency

<10 ng/mL

Mild to moderate deficiency




Potential intoxication



·      There can be seasonal variation in 25 (OH) vitamin D level, with values being 40-50% lower in winter than in summer. It is also influenced by sunlight, latitude, skin pigmentation, sunscreen use, and hepatic function.

·       25 (OH) vitamin D levels can vary with age

·       25 (OH) vitamin D level is increased during pregnancy.

Answers to Patient Concerns & Frequently Asked Questions (FAQs) about Vit D

Frequently Asked Questions about Vitamin D (25-OH)

Q. How is this test performed?
This test is performed on a blood sample. A syringe with a fine needle is used to withdraw blood sample from a blood vessel in your arm, generally from the inner side of the elbow area. The doctor, nurse or the phlebotomist will tie an elastic band around your arm which will help the blood vessels to swell with blood. This makes it easier to withdraw blood. You may be asked to tightly clench your fist. Once the veins are clearly visible, the area is cleaned with an antiseptic solution. The needle is then inserted into the blood vessel to collect the sample. You may feel a tiny pinprick during the procedure. Blood sample, once collected, is then sent to the laboratory.
Q. Is there any preparation required before the test?
Inform the doctor about the medications you may be taking. No other specific preparations are usually required before this test.
Q. What does the Vitamin D test result mean?
A below normal level can be due to a vitamin D deficiency, which can occur because of the following: · Lack of exposure to sunlight · Dietary deficiency of vitamin D · Liver and kidney diseases · Poor food absorption in cystic fibrosis and Crohn’s disease, and in patients who have undergone gastric bypass surgery · Use of certain medicines, including phenytoin, phenobarbital, and rifampin etc can cause vitamin D deficiency · In infants who are exclusively breastfed. Formula milk is fortified with vitamin D. A higher-than-normal level due to excess vitamin D is called hypervitaminosis D or vitamin D toxicity. This is most commonly caused by taking too much vitamin D can result high calcium level in the body (hypercalcemia) and can also cause kidney and blood vessel damage.
Q. Who should be tested for Vitamin D?
Vitamin D 25 (OH) should be monitored in following patients: · Individuals undergoing therapy to prevent or treat osteoporosis · Elderly people, especially those with minimal exposure to sunlight · Patients with signs and symptoms of low calcium level (hypocalcemia) or high calcium level (hypercalcemia) · Children and adults with suspected rickets and osteomalacia, respectively · Patients receiving vitamin D therapy who do not demonstrate clinical improvement · Infants who are exclusively breastfed. · Patients with cystic fibrosis, Crohn’s disease, and with gastric bypass surgery.
Q. What are the factors which can affect the 25-OH Vitamin D levels?
Following are the factors which can affect 25-OH Vitamin. · Seasonal variation: There can be seasonal variation in 25 (OH) vitamin D level, with values being 40-50% lower in winter than in summer. Ideally all samples should be collected in one season. · Age: Older adults have a reduced level of 7-dehydrocholesterol, so they cannot synthesize 25(OH) vitamin D and their kidneys are less able to produce the active hormone, 1,25 dihydro Vitamin D3. · Skin colour: Melanin in the darker skin reduces the ability to produce 25(OH)Vitamin D from sunlight exposure, because it absorbs the sunlight. · Body mass index/nutrition: Obese individuals (body mass index >30 kg/m2) are deficient in Vitamin D, as Vitamin D is fat-soluble, it is sequestered in the fat and not able to circulate. Obese patients cannot absorb Vitamin D as readily. · Certain medicines: phenytoin (seizure drug), phenobarbital, and rifampin, cholestyramine, orlistat, steroids, stimulant laxatives can reduce Vitamin D level. · It is also influenced by sunlight, latitude, sunscreen use, and hepatic function.
Q. What are additional tests needed to be performed in case of abnormal Vitamin D test results?
Additional tests which can help establish vitamin D deficiency are: 24-hour urine calcium Parathyroid hormone Total or bone alkaline phosphatase level Serum calcium and serum phosphorus level Radiological test
Q. What is osteoporosis?
Osteoporosis is a condition of fragile bones with increased susceptibility to fracture. Vitamin D and calcium deficiency are one of the leading cause of osteoporosis. · Other risk factors can be genetics, lack of exercise, personal history of fracture as an adult, cigarette smoking, excessive alcohol consumption, history of rheumatoid arthritis, low body weight, and family history of osteoporosis. · Usually, osteoporosis does not manifest itself until bone fractures occur. · X-rays and bone mineral density test can help diagnose osteoporosis.
Q. What is osteomalacia?
Osteomalacia is softening of bone due to demineralization (the loss of mineral) and most notably by the depletion of calcium from bone. · Osteomalacia is a characteristic feature of vitamin D deficiency in adults. Osteomalacia can also be caused by poor dietary intake or poor absorption of calcium and other minerals needed to harden bones.
Q. What are the sources of Vitamin D?
Vitamin D is derived from endogenous sources and exogenous sources. · Endogenous source: It is produced in our body on exposure of our skin to sunlight. · Exogenous source: Dietary source of the vitamin D includes fish, eggs, fortified dairy products and dietary supplement.
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