What to watch with Vitamin D 25-OH and fatigue?

Vitamin D 25-OH is the primary biomarker for vitamin D status. Low levels are commonly linked to fatigue, but the evidence is mixed: some studies show improvement in fatigue after supplementation in deficient individuals, while others find no effect in those with sufficient levels. Fatigue is multifactorial – iron deficiency, vitamin B12 deficiency, thyroid dysfunction, poor sleep, and stress are often more relevant. Home tests for 25-OH can provide a screening indication but are not diagnostic. Caution is needed with high-dose self-supplementation, as toxicity can cause hypercalcemia. Interactions with other fat-soluble vitamins or medications (e.g., thiazide diuretics) require medical supervision. Evidence: mixed – the association is observed, but causal effects are not universal.