Myths vs facts: FT4 and longevity

The idea that a specific FT4 (free thyroxine) level directly determines longevity is a myth. Research shows a U-shaped relationship: both very low and very high FT4 are linked to increased mortality. Some studies in centenarians suggest slightly lower thyroid activity, but evidence is observational and not causal. FT4 is influenced by iodine, medications, genetics, and illness. Attempting to manipulate FT4 through supplements or hormones without medical supervision is dangerous and unsupported by evidence. The provided context lacks direct studies on FT4 and longevity, so the evidence is mixed/unclear. DNA tests for thyroid-related SNPs (e.g., DIO1, DIO2) have very weak predictive power for lifespan. Clinical thyroid function should always be interpreted by a physician.