Vitamin D tests
are used to see if bone weakness, bone malformation, or abnormal
metabolism of calcium (reflected by abnormal calcium, phosphate, or PTH
tests) is occurring as a result of a deficiency or excess of vitamin D.
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 are used to determine the
effectiveness of treatment when vitamin D, calcium, phosphate, and/or
magnesium supplementation is prescribed
Either 25 hydroxy-vitamin
D or 1,25 dihydroxy-vitamin D tests may be requested when a patient has
an abnormal blood calcium, phosphate and/or magnesium level or evidence
of bone disorders.
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 calcitriol 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 is being tested?
These tests measure the concentrations of various forms of vitamin D in
your blood. The term vitamin D refers to a number of different but
related chemical compounds (termed sterols), some of which are inactive
and some of which are active forms. The two most common vitamin D tests
measure 25-hydroxy-vitamin D (calcidiol, an inactive form) and 1,25
dihydroxy-vitamin D (calcitriol, the active form). The test for
25-hydroxy-vitamin is used to check that the body has an adequate supply
of vitamin D. The test for 1,25 dihydroxy-vitamin D is used to test that
the kidney is converting an appropriate amount of 25-hydroxy-vitamin D
to the active 1,25 dihydroxy-vitamin D.
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 are 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.
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