How are FT3 and fatigue related?

The relationship between FT3 (free triiodothyronine) and fatigue is well established in physiology: FT3 is the active thyroid hormone that regulates basal metabolic rate and cellular energy production. Low FT3 levels—as seen in hypothyroidism or non-thyroidal illness syndrome—can cause significant fatigue, lethargy, and reduced exercise tolerance. However, fatigue is a non-specific symptom; stress, poor sleep, nutrient deficiencies (e.g., iron, vitamin B12, vitamin D), and chronic inflammation are also common contributors. The provided context does not contain specific data on FT3 and fatigue. Therefore, this answer relies on general medical knowledge, not on the referenced sources. Testing FT3 alone is insufficient for fatigue workup; a comprehensive thyroid panel (TSH, fT4, fT3, antibodies) plus lifestyle and biomarker assessment is recommended. Caveat: Without clinical context, a single FT3 value can be misleading.