Is FT3 useful for Langlebigkeit?

Based on the provided context, there is no direct evidence that FT3 (free triiodothyronine) is useful as a longevity biomarker. FT3 is a thyroid hormone reflecting metabolic activity. Some observational studies in elderly populations show an association between low FT3 and increased mortality, often in the setting of non-thyroidal illness (euthyroid sick syndrome). However, this does not establish FT3 as a causal or actionable marker for healthy aging. The evidence is primarily mechanistic and observational, not from interventional trials. Moreover, thyroid hormone supplementation without clinical indication carries risks (e.g., cardiac arrhythmias). Therefore, FT3 alone is not a validated longevity marker. A comprehensive thyroid panel (TSH, fT4, fT3) and clinical context are necessary. Given the lack of robust evidence in the provided knowledge, a conservative assessment is warranted.