Evidence check: FT4 in the context of Langlebigkeit

FT4 (free thyroxine) is a key thyroid marker regulating metabolism. In the context of longevity, some human observational studies suggest that low-normal FT4 levels are associated with extended lifespan, possibly due to reduced metabolic rate and oxidative stress. However, the evidence is moderate: most data come from cohort studies, not RCTs, and effects vary by age, sex, and genetic background (e.g., APOE genotype). An isolated FT4 value without TSH and fT3 is of limited use. Both hyper- and hypothyroidism clearly increase mortality risk. For longevity assessment, FT4 alone is insufficient; it must be interpreted within a full thyroid panel and other biomarkers. Evidence is rated as human-moderate.