Is FT4 useful for longevity?

FT4 (free thyroxine) is a key biomarker of thyroid function. Optimal thyroid activity is linked to better metabolic regulation, cardiovascular health, and potentially longer lifespan. However, direct evidence for FT4 as a longevity marker is weak. Studies show conflicting results: both low and high FT4 levels may be associated with increased mortality, especially in older adults. Most data come from observational studies, not randomized controlled trials. Reference ranges also vary by lab and population. A single FT4 value without context (TSH, T3, symptoms) is insufficient for longevity claims. For individual assessment, thyroid values should always be interpreted by a physician. In summary, FT4 is useful for evaluating thyroid function but not as a standalone longevity test. The evidence is mechanistic and observational, not causal.