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Vitamin D25 is a shortened name for 25-hydroxy-vitamin D, one of two types of vitamin D that can be measured using a blood test. When a doctor orders a blood test, vitamin D25 will be analyzed, and sometimes the second form of vitamin D, called 1,25D-dyhydroxy-vitamin D (D1,25), will be assessed as well. Known as an inactive element in the body, vitamin D25 is sometimes called a pre-hormone because it converts into the hormone D1,25. This conversion process takes place in the kidneys, and the active form, D1,25, helps control the levels of calcium and phosphate in the body.
Two main components of vitamin D25 are D3, also called cholecarciferol, and D2, also called ergocalciferol. Human beings take in natural D3 when they eat animal-based foods, such as fatty fish and egg yolks, while synthetic D3 is often added to yogurt and milk as well as breads and cereals. D3 also can be obtained through exposure to sunlight. D2 comes from plant foods such as mushrooms.
Physicians order blood analysis of vitamin D when they suspect a patient has a deficiency of or a problem with the parathyroid glands, which play a role in converting vitamin D25 to vitamin D1,25. When levels of vitamin D25 are low, the patient might not be getting enough D3 and D2 from dietary sources and/or the sun. Low levels also can mean the patient has a problem absorbing vitamin D25 even if he or she is taking in adequate amounts. High levels of this type of vitamin D sometimes indicate a patient is ingesting too much of the vitamin in the form of supplements.
When a doctor orders a concurrent test of vitamin D1,25, and the result shows a low reading, the patient is assessed for kidney disease or parathyroid problems. When the D1,25 reading is very high, some diseases can be the cause, including lymphoma and an autoimmune condition called sarcoidosis. In cases where the vitamin D25 reading is very low and vitamin D1,25 is very high, doctors try to determine which disease process might be causing the results. Supplemental vitamin D should be avoided until the doctor understands the cause of very low vitamin D25 combined with a very high D1,25.
The relationship between the two forms of vitamin D is a complex one. Without both elements being tested, a diagnosis cannot be reached. In addition, when vitamin D malabsorption is suspected, some medications might be to blame, including the anticonvulsants phenytoin and phenobarbital and the antibiotic rifampicin.