Vitamin D
At a Glance
Why Get Tested?
When to Get Tested?
Sample Required?
The Test Sample
What is being tested?
The main role of the active hormone is to help regulate the absorption of calcium, phosphate, 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 the disease called rickets in children and osteomalacia in adults.
Vitamin D comes from two sources. The body is able to form vitamin D by exposure to sunlight. This is why vitamin D is sometimes described as the sunshine vitamin - it is formed from 7-dehydrocholesterol in the skin when the skin is exposed to light. Vitamin D also can be ingested - either in foods or in vitamin supplements. The different compounds of vitamin D are distinguished by the use of subscript numbers. Vitamin D2 comes from diet and vitamin preparations. Vitamin D3 is produced in the body. Vitamins D2 and D3 have slightly different chemical structures, but both lead to production of the active hormone 1,25 dihydroxy-vitamin D.
Vitamin D3 as formed in the skin and vitamin D2 supplied in the diet are inactive forms that are converted by the liver into yet another inactive form called 25-hydroxy-vitamin D - this is the main form of vitamin D stored in the body and measured in the circulation. 25-hydroxy-vitamin D is converted to the active form 1,25 dihydroxy-vitamin D by the kidney. About 99.9% of vitamin D in the blood is 25-hydroxy-vitamin D and 0.1% is 1,25 dihydroxy-vitamin D.
How is the sample collected for testing?
The Test
How is it used?
Since vitamin D is a fat-soluble vitamin and is absorbed from the intestine like a fat, vitamin D tests are sometimes used to monitor and assure individuals, with diseases that interfere with fat absorption, such as cystic fibrosis and Crohn's disease, that they have adequate amounts of vitamin D. Vitamin D tests are also used to determine the effectiveness of treatment when vitamin D, calcium, phosphate, and/or magnesium supplementation is prescribed.
When is it requested?
If calcium concentration is low or the patient has symptoms of vitamin D deficiency, such as bone malformation in children (rickets) and bone weakness, softness, or fracture in adults (osteomalacia), the 25 hydroxy-vitamin D test usually is requested to identify a possible deficiency in vitamin D.
If calcium concentration is high or the patient has a disease that might produce excess amounts of 1,25 dihydroxy-vitamin D, such as sarcoidosis or some forms of lymphoma, the 1,25 dihydroxy-vitamin D test may also be requested.
Vitamin D tests also may be used to help diagnose or monitor problems with parathyroid gland functioning since parathyroid hormone is essential for vitamin D activation. 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?
High levels of 25 hydroxy-vitamin D usually reflect excess supplementation from vitamin pills or other nutritional supplements.
Low levels of 1,25 dihydroxy-vitamin D are often seen in kidney disease and are one of the earliest changes to occur in persons with early kidney failure.
High levels of 1,25 dihydroxy-vitamin 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 dihydroxy-vitamin D outside of the kidneys.Is there anything else I should know?
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.
Common Questions
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