What to watch with FT3 and longevity?

FT3 (free triiodothyronine) is the active thyroid hormone and a key metabolic regulator. Regarding longevity, the evidence is mixed: some observational studies suggest that higher FT3 levels within the normal range may be associated with increased mortality in older adults, possibly reflecting higher metabolic rate or subclinical hyperthyroidism. Conversely, low FT3 (euthyroid sick syndrome) is common in chronic illness and linked to worse outcomes. The evidence is observational and not causal. FT3 alone is insufficient to assess thyroid status – TSH, fT4, and antibodies are needed. MyBody-X includes FT3 in its thyroid panel, but interpretation requires clinical context. For longevity, stable, age-appropriate thyroid function likely matters more than chasing a specific FT3 level. Safety note: Do not self-medicate with thyroid hormones without medical supervision.