Vitamin D (25-hydroxyvitamin D)
Also called 25(OH)D, 25-hydroxyvitamin D, calcidiol
Vitamin D is a nutrient the body needs to absorb calcium for bones and teeth, and to keep muscles, nerves and the immune system working normally. The skin makes it in sunlight, and it also comes from a few foods, fortified products and supplements. Before the body can use it, the liver converts it to 25-hydroxyvitamin D, written 25(OH)D, and that is what a standard vitamin D blood test measures, because it is the most accurate way to see whether you have enough.
A different test, for the active form called 1,25-dihydroxyvitamin D or calcitriol, is not used to check vitamin D status; it is used mainly to monitor kidney problems or investigate abnormal calcium levels. What a 25(OH)D result means is covered in Vitamin D: What the Number Means After 45.
Why it matters after 45
A common reason for a vitamin D test is bone health, and low bone density, osteopenia and osteoporosis come into focus in midlife. Several medicines common at this age can affect vitamin D levels, including certain statins, steroids and weight-loss medicines, as can weight-loss surgery and conditions that limit absorption. Older adults may also need more vitamin D than younger ones. Even so, routine testing is not recommended for everyone, and the decision rests on risk.
Where it comes up
Related terms
References
- MedlinePlus. Vitamin D Test. U.S. National Library of Medicine. https://medlineplus.gov/lab-tests/vitamin-d-test/
- MedlinePlus. Vitamin D. U.S. National Library of Medicine. https://medlineplus.gov/vitamind.html
Definitions here are educational and are not medical advice. They explain what a term means; the pages linked above cover what it means for you, and a clinician reviews your own results. Editorial & medical review policy · All glossary terms