What role does FT3 play in fatigue?

FT3 (free triiodothyronine) is the biologically active thyroid hormone and a key regulator of energy metabolism. Low FT3 levels—as seen in overt or subclinical hypothyroidism—are strongly associated with fatigue, lethargy, and reduced exercise tolerance. However, FT3 alone is not diagnostic; the entire hypothalamic-pituitary-thyroid axis (TSH, FT4, FT3) must be evaluated. Additionally, non-thyroidal illness (e.g., chronic inflammation, caloric restriction) can lower FT3 without true hypothyroidism. The evidence linking FT3 to fatigue is human-strong, supported by decades of clinical and epidemiological research. Caveat: Direct-to-consumer DNA tests cannot measure FT3; it requires a blood test. MyBody-X does not offer thyroid hormone analysis. If you experience persistent fatigue, consult a physician for proper thyroid function testing. Do not self-diagnose based on isolated FT3 values.