Evidence check: Vitamin D Test in the context of fatigue

The evidence for using a Vitamin D test in the context of fatigue is moderate at best. While severe deficiency can cause fatigue, most individuals with fatigue have normal levels. Supplementation only benefits those with confirmed deficiency, and even then the effect on fatigue is modest. Population-wide testing is not recommended; targeted testing for at-risk groups (e.g., limited sun exposure, darker skin, elderly) is more justified. Marketing often overstates the link, and many fatigue causes remain undiagnosed. The test itself is reliable, but its clinical utility for fatigue is limited. Evidence: human-moderate. Caveat: Do not rely solely on vitamin D testing; consider broader workup.