A vitamin D blood test is a type of test that measures the levels of vitamin D in the blood. Vitamin D is a fat-soluble vitamin that is essential for many important functions in the body, such as maintaining strong bones, regulating the immune system, and controlling the levels of calcium and phosphorus in the blood. This test is commonly ordered when a person has symptoms that may be caused by a deficiency of vitamin D, such as weak bones, muscle weakness, and fatigue. It can also be used to monitor the effectiveness of treatment for these conditions. Vitamin D deficiency is common, especially in older adults, people who have limited exposure to sunlight, and those with dark skin.
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