What role does FT3 play in longevity?

FT3 (free triiodothyronine) is the active thyroid hormone that regulates metabolism, mitochondrial function, and cellular aging. In longevity research, low FT3 levels are observed in centenarians, suggesting a reduced metabolic rate may contribute to extended lifespan. However, the evidence is mixed: very low FT3 can also indicate illness and increased mortality (non-thyroidal illness syndrome). Most studies are observational, and causality is unclear. MyBody-X does not test FT3; it is a blood biomarker. A single FT3 value has limited predictive power for lifespan and must be interpreted alongside TSH, fT4, and clinical context. Evidence: human-moderate.