What role does TSH play in fatigue?

TSH (thyroid-stimulating hormone) is secreted by the pituitary gland and regulates thyroid hormone production. Elevated TSH indicates hypothyroidism (underactive thyroid), which classically causes fatigue, lethargy, and cold intolerance. Low TSH suggests hyperthyroidism (overactive thyroid), which can also cause fatigue, often accompanied by sleep disturbances and restlessness. The evidence for this link is strong (human-strong), based on decades of clinical practice and large epidemiological studies. However, fatigue is a non-specific symptom; many other conditions (iron deficiency, sleep apnea, depression, chronic inflammation) can mimic thyroid-related fatigue. A single TSH value is insufficient for diagnosis – free T3, free T4, and antibodies are needed. Subclinical thyroid dysfunction (borderline TSH) is controversial regarding treatment for fatigue. Consumer DNA tests analyzing TSH-related genes (e.g., PDE8B, TSHR) have limited predictive value and cannot replace blood testing. For persistent fatigue, medical evaluation with TSH measurement is reasonable, but interpretation must consider the full clinical picture.