Association of sensitivity to thyroid hormones with all-cause mortality in euthyroid US adults: a nationwide cohort study

Abstract

Background: This study aimed to examine the associations of thyroid hormone sensitivity indices, including free triiodothyronine-to-free thyroxine (FT3/FT4) ratio, thyroid feedback quantile-based index by FT4 (TFQIFT4), thyroid-stimulating hormone index (TSHI), and thyrotrophic thyroxine resistance index (TT4RI) with all-cause mortality in euthyroid adults. Methods: The study included 6243 euthyroid adults from the National Health and Nutrition Examination Survey (NHANES) 2007–2012. FT3/FT4 ratio, TFQIFT4, TSHI, and TT4RI were calculated. The multivariable Cox proportional hazard regression, restricted cubic spline (RCS), and subgroup analysis were conducted. Results: Individuals in fourth quartile (Q4) had lower all-cause mortality than those in first quartile (Q1) of FT3/FT4 ratio (hazard ratio (HR): 0.70, 95% CI: 0.51, 0.94). Regarding TFQIFT4, individuals in Q4 of TFQIFT4 had a 43% higher all-cause mortality than those in Q1 (HR: 1.43, 95% CI: 1.05, 1.96) (P < 0.05, all). Compared with participants in Q1, no associations of TSHI and TT4RI with mortality were found. TFQIFT4 was linearly and positively associated with mortality. However, the FT3/FT4 ratio showed a U-shaped association with mortality. Conclusions: Increased risk for all-cause mortality was positively associated with TFQIFT4, suggesting that increased risk for all-cause mortality was associated with decreased central sensitivity to thyroid hormones. Furthermore, the FT3/ FT4 ratio showed a U-shaped association with mortality, with an inflection point at 0.5. However, more cohort studies are needed to validate the conclusions.