TSH levels within the normal range and risk of cardiovascular and all-cause mortality among individuals with diabetes

Background Evidence regarding thyroid-stimulating hormone (TSH) levels within the normal range and mortality in adults with diabetes is scarce. This study aimed to identify the association between TSH levels and cardiovascular disease (CVD) and all-cause mortality among euthyroid patients with diabetes. Methods This prospective cohort study included 1830 adults with diabetes from the Third National Health and Nutrition Examination Survey III. Mortality outcomes were ascertained by linkage to National Death Index records through December 31, 2019. Participants were categorized by tertiles of TSH levels (low-normal, 0.39–1.30 mIU/L; medium-normal, 1.30–2.09 mIU/L; high-normal, 2.09–4.60 mIU/L). Multivariable Cox proportional hazards models were used to explore the association between TSH levels within the normal range and overall and CVD mortality. Furthermore, restricted cubic spline analyses were used to determine the nonlinear relationship between TSH levels and mortality. Results During a median follow-up of 17.1 years, 1324 all-cause deaths occurred, including 525 deaths from CVD. After multivariate adjustment, a U-shaped relationship was observed between TSH levels in euthyroid status and all-cause or CVD mortality among patients with diabetes (both P < 0.05 for nonlinearity). Compared with participants with medium-normal TSH levels, those with high-normal TSH levels had a significantly higher risk of all-cause (hazard ratio, 1.31; 95% confidence interval, 1.07–1.61) and CVD (1.52; 1.08–2.12) mortality. Similarly, low-normal TSH levels also increased all-cause (1.39; 1.12–1.73) and CVD (1.69; 1.17–2.44) mortality risk. In stratum-specific analyses, we found that high-normal TSH levels were associated with higher mortality risk in younger (< 60 years) patients with diabetes but not in older (≥ 60 years) participants. Conclusion Low- and high-normal serum TSH levels were associated with increased all-cause and CVD mortality in euthyroid adults with diabetes. Further studies are needed to confirm the present observation in a wider population.