What to watch with TSH and fatigue?

When TSH is outside the reference range (typically 0.4–4.0 mU/l) and fatigue is present, thyroid dysfunction should be evaluated. Elevated TSH suggests hypothyroidism, which can cause fatigue, lethargy, and cold intolerance. Low TSH points to hyperthyroidism, which may also present with exhaustion. Importantly, TSH alone is insufficient – free T3, free T4, and thyroid antibodies (TPO, Tg) should be measured to identify the underlying cause (e.g., Hashimoto's thyroiditis). Even subclinical changes (TSH 4–10 mU/l) can trigger symptoms in sensitive individuals. Other causes of fatigue (iron deficiency, vitamin D deficiency, sleep apnea, depression) should not be overlooked. A direct-to-consumer test can provide initial clues but does not replace a medical diagnosis. The evidence for TSH as a marker of fatigue is strong (human-strong). Caveat: reference ranges vary by lab and population; individual symptoms and follow-up are critical.