What role does HbA1c play in fatigue?

HbA1c (glycated hemoglobin) reflects average blood glucose over the past 2–3 months. Elevated HbA1c indicates prediabetes or diabetes, both of which can cause fatigue through osmotic diuresis, impaired cellular energy metabolism, brain insulin resistance, and low-grade inflammation. Conversely, hypoglycemic episodes (common in treated diabetes) also trigger fatigue. However, fatigue is highly nonspecific; a normal HbA1c does not rule out other common causes like iron deficiency, thyroid disorders, sleep apnea, or depression. The evidence linking HbA1c directly to fatigue is moderate: strong in diabetic populations, but no validated cutoffs exist for fatigue in non-diabetic individuals. HbA1c testing is a reasonable component of a fatigue workup, but it should not be used in isolation. Caveat: home tests are screening-only; confirm with venous blood draw. Evidence grade: human-moderate.