How are FT3 and longevity related?

The relationship between FT3 (free triiodothyronine) and longevity is complex and not fully understood. Some studies in centenarians show lower FT3 levels compared to younger controls, suggesting a reduced metabolic rate and possibly lower oxidative damage. However, low FT3 in older adults is also often linked to illness, malnutrition, or the 'euthyroid sick syndrome,' complicating interpretation. High FT3 (hyperthyroidism) is clearly associated with increased cardiovascular mortality and shortened lifespan. Evidence comes mostly from observational studies; no RCTs prove a causal link. Thus, the predictive power for individual health decisions is limited. No optimal FT3 range for longevity has been defined. Thyroid hormones should only be taken under medical supervision, as both over- and under-function carry risks.