Vitamin D Test: what are common causes?

A vitamin D test measures 25-hydroxyvitamin D (25(OH)D) in blood, the main storage form. Common causes of low levels include: inadequate sun exposure (especially in northern latitudes or with darker skin), low dietary intake (fatty fish, eggs, fortified foods), malabsorption (e.g., celiac disease, Crohn's), kidney or liver disease impairing conversion, obesity (vitamin D sequestered in fat tissue), and certain medications (antiepileptics, glucocorticoids). The test itself is a blood test, but immunoassays can be inaccurate. Cutoffs: deficiency <20 ng/mL, insufficiency 20–30 ng/mL, sufficiency ≥30 ng/mL – though thresholds are debated. A single low result should be evaluated by a physician before supplementing. Evidence for these causes is strong (human-observational), but test accuracy varies.