Evidence check: FT3 in the context of fatigue

FT3 (free triiodothyronine) is the active thyroid hormone and is often tested in the context of fatigue. Hypothyroidism can cause fatigue, with TSH and FT4 being the primary screening markers. FT3 is mainly useful when T3 thyrotoxicosis or conversion disorders are suspected. The evidence for isolated FT3 testing in non-specific fatigue is limited: in clinical practice, FT3 is usually ordered as a follow-up, not a first-line test. Direct-to-consumer tests frequently include FT3 in 'energy panels,' which can lead to misinterpretation due to reference range variability and individual fluctuations. A normal FT3 does not rule out treatable thyroid disease, and a mildly elevated FT3 without TSH abnormality is rarely clinically significant. Evidence is based on clinical guidelines (e.g., ATA) and observational studies, not on RCTs for this specific indication. Conclusion: FT3 can be useful in fatigue when thyroid dysfunction is suspected, but as a standalone screening marker it lacks strong evidence.