Elevated FT4 is associated with prevalent stroke in euthyroid adults: a cross‑sectional study.

Research on normal‑range thyroid function and stroke remains inconsistent. In this cross‑sectional study of 6,932 euthyroid adults from NHANES 2007‑2012, we examined associations of thyroid hormone sensitivity indices with prevalent stroke using multivariable logistic regression, subgroup, mediation, and sensitivity analyses, with independent clinical validation. After full adjustment for demographic and lifestyle factors, FT4 (OR = 1.11 per pmol/L, 95% CI 1.01‑1.22, P = 0.037) and TFQI (OR = 1.97 per unit, 95% CI 1.04‑3.74, P = 0.047) were positively associated with prevalent stroke. Mediation analysis showed that FT4's effect was not mediated by glucose, blood pressure, or cholesterol, whereas TFQI's effect was partially mediated by glucose and blood pressure. Sensitivity analyses using an alternative age cut‑off of 70 years and age‑specific TSH reference intervals for older adults confirmed robustness. An independent validation cohort confirmed FT4 remained significant (OR = 1.33, 95% CI 1.06‑1.65, P = 0.012), while TFQI did not. Elevated FT4 is a direct, robust marker for prevalent stroke in euthyroid adults. Prospective studies are needed to establish causality.