Vitamin D, 25-hydroxy, serum or plasma (25[OH]D)
SST or green
The vitamin D system functions to maintain serum calcium levels. Vitamin D is a fat-soluble steroid hormone. Two molecular forms exist: D3 (cholecalciferol), synthesized in the epidermis, and D2 (ergocalciferol), derived from plant sources. To become active, both need to be further metabolized. Two sequential hydroxylations occur: in the liver to 25(OH) D and then, in the kidney, to 1,25[OH]2D.
Besides consequences for bone health, vitamin D deficiency reportedly is associated with a number of conditions such as cardiovascular disease, autoimmunity and cancer; however, evidence-based cause-and-effect relationships have not been established.
Increased in: Heavy milk drinkers (up to 64 ng/mL), vitamin D intoxication, sun exposure.
Decreased in: Dietary deficiency, malabsorption, rickets, osteomalacia, biliary and portal cirrhosis, nephrotic syndrome, renal failure, inadequate sun exposure, advanced age (> 70), primary hyperparathyroidism. Drugs: phenytoin, phenobarbital.
Serum or plasma total 25(OH)D is an integrated marker of vitamin D status, incorporating endogenous synthesis from solar exposure, dietary intake, fortified products and/or supplements.
There is no universal or strong evidence-based consensus on the appropriate level of 25(OH)D. However, according to a 2011 US Institute of Medicine Report, a 25(OH)D level of 20–30 ng/mL is all that is needed for bone and general health, and nearly everyone (97.5%) in the general population is in that range. A 25(OH)D level above 30 ng/mL has not been consistently associated with increased health benefits, and, in fact, risks have been identified for outcomes at levels above 50 ng/mL.
Routine screening for vitamin D deficiency is not necessary. Patients with the following conditions should be considered for testing: osteoporosis, osteomalacia, malabsorption, liver disease, pancreatic insufficiency, chronic kidney disease, COPD, bariatric surgery, cancer, bedridden or home-bound, obesity, taking anticonvulsants or long-term glucocorticoids, atraumatic fractures, elderly (> 70 years old), and chronic inflammatory conditions.
Vitamin D toxicity can occur after taking excessive doses of vitamin D, a condition that is manifested by hypercalcemia, hyperphosphatemia, soft tissue calcification, and renal failure.
Bikle DD. Vitamin D metabolism, mechanism of action, and clinical applications. Chem Biol 2014;21:319. [PMID: 24529992]
LeBlanc E et al. Screening for vitamin D deficiency: systematic review for the U.S. Preventive Services Task Force Recommendation [Internet]. Rockville (MD): Agency for Healthcare Research and Quality (US); 2014 Nov. [PMID: 25521000]
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