Can FT3 help with fatigue?

FT3 (free triiodothyronine) is a thyroid hormone that regulates energy metabolism. Low FT3 can contribute to fatigue in diagnosed hypothyroidism, where thyroid hormone replacement is standard care. However, for general fatigue without confirmed thyroid dysfunction, taking FT3 is not evidence-based and may be harmful (e.g., cardiac arrhythmias, suppression of endogenous thyroid function). The evidence for FT3 as a fatigue treatment in euthyroid individuals is weak and mostly anecdotal. Fatigue has many causes—iron deficiency, sleep disorders, stress, infections—that should be investigated first. Consumer thyroid tests can measure FT3 but are not diagnostic; interpretation requires a physician. In summary: FT3 is a biomarker, not a supplement for fatigue. Do not self-medicate. Evidence: mixed/unclear.