How are FT4 and Langlebigkeit related?

The provided context does not contain any information about the relationship between FT4 (free thyroxine) and longevity. Generally, thyroid hormones influence metabolism, and both hyper- and hypothyroidism are linked to increased mortality. An optimal FT4 level may support longevity, but evidence is mixed and largely observational. Without specific sources from the given context, the evidence grade is unclear. Consumer tests for such associations have limited validity; clinical interpretation is required.