What to watch with Vitamin D Test and fatigue?

A vitamin D test measures 25-hydroxyvitamin D in blood. Low levels can contribute to fatigue, but fatigue is multifactorial (iron deficiency, thyroid, sleep, stress). Pay attention to the lab's reference range – many use 30 ng/ml as cutoff, but optimal levels may be 40–60 ng/ml. Genetic variants like rs2228570 (VDR) may affect vitamin D metabolism, but evidence for clinical utility is weak (GWAS, incomplete). A single low vitamin D level rarely explains chronic fatigue; comprehensive workup is needed. Beware of tests that only measure IgG or SNP profiles without blood levels – they are not validated. Supplementation should be guided by a physician to avoid toxicity (hypercalcemia). Evidence: human-moderate for vitamin D deficiency and fatigue, but mixed for genetic testing.