Is Vitamin D Test useful for fatigue?

Whether a vitamin D test is useful for fatigue depends on the context. Vitamin D deficiency is a known cause of fatigue and muscle weakness, supported by clinical evidence. However, fatigue is a highly nonspecific symptom with many potential causes (e.g., sleep deprivation, stress, iron deficiency, thyroid disorders, depression). Testing is only clinically justified if deficiency is likely due to risk factors such as limited sun exposure, dark skin, older age, or certain medical conditions. Routine screening for vitamin D in unselected individuals with fatigue is not supported by strong evidence. Furthermore, many direct-to-consumer (DTC) vitamin D tests, including those from DNA kits, lack standardization and clinical validation. The gold standard is a blood test for 25-hydroxyvitamin D ordered by a physician. Self-testing can lead to misinterpretation or unnecessary supplementation. In summary: a vitamin D test can be useful if deficiency is suspected based on risk factors; otherwise, it is not an evidence-based first step for fatigue.