Is FT3 useful for longevity?

Whether FT3 (free triiodothyronine) is useful for longevity lacks robust evidence. FT3 is the active thyroid hormone that regulates basal metabolic rate, mitochondrial function, and protein turnover. While maintaining euthyroid status is essential for health, there are no high-quality studies demonstrating that elevating FT3 above normal ranges extends lifespan. In fact, chronically high FT3 (e.g., in hyperthyroidism) is associated with increased mortality, atrial fibrillation, and muscle wasting. Low FT3 may indicate hypothyroidism or non-thyroidal illness, both linked to higher mortality. The evidence is limited to observational and mechanistic data; no RCTs have tested FT3 optimization for longevity. Thus, FT3 is not a standalone longevity biomarker but should be interpreted within the full thyroid axis (TSH, fT4). Supplementation without medical supervision is dangerous.