What role does FT4 play in longevity?

FT4 (free thyroxine) is a thyroid hormone that regulates metabolism and influences aging processes. Epidemiological studies suggest that both low and high FT4 levels within the normal range may be associated with longevity, but the relationship is complex and not fully understood. Some research indicates that slightly lower FT4 levels correlate with reduced metabolic rate and slower aging, while others show increased mortality at extremes. However, these are population-level observations, not causal evidence. FT4 testing alone is insufficient for longevity assessment; it must be interpreted alongside TSH, T3, and clinical context. Consumer tests offering FT4 as a 'longevity marker' often oversimplify. The evidence is mixed and not strong enough for personalized recommendations. Safety note: Do not attempt to alter FT4 levels without medical supervision, as thyroid dysfunction can have serious health consequences.