How are Vitamin D 25-OH and fatigue related?

The relationship between vitamin D (25-OH) and fatigue is not clearly established scientifically. Some observational studies suggest an association between low vitamin D levels and increased fatigue, especially in individuals with deficiency. However, robust randomized controlled trials demonstrating a causal effect are lacking. Fatigue is a multifactorial symptom that can be caused by sleep deprivation, stress, iron deficiency, thyroid dysfunction, or other underlying conditions. A vitamin D (25-OH) test may be reasonable if deficiency is suspected, but correcting vitamin D levels alone does not necessarily improve fatigue. The evidence level is moderate to unclear. For persistent fatigue, a comprehensive medical evaluation is recommended.