Association between thyroid hormones and diabetes in older adults aged 65 years and older in 18 longevity regions of China

Objective: To investigate the associations of the triiodothyronine (T3) to thyroxine (T4) ratio (T3/T4), thyroid-stimulating hormone (TSH), and free T3 to free T4 ratio (FT3/FT4) with the risk of diabetes among elderly individuals aged 65 years and older in longevity areas of China. Methods: Data were drawn from the 2021 cross-sectional survey of the Healthy Ageing and Biomarkers Cohort Study (HABCS), which enrolled 5 795 participants across 18 longevity areas in China. Plasma concentrations of FT3, FT4, T3, T4, and TSH were determined using chemiluminescence immunoassay, and fasting blood glucose (FBG) was measured via the hexokinase method. Diabetes mellitus was defined as an FBG≥7.0 mmol/L and/or self-reported physician-diagnosed diabetes. Multivariable logistic regression models were applied to evaluate the independent associations of FT3/FT4, T3/T4, and TSH with diabetes. Additionally, restricted cubic splines (RCS) were utilized to visualize and analyze the exposure-response relationships. Results: The age of the 5 795 study participants was (85.25±11.56) years; 56.3% were female, and the prevalence of diabetes was approximately 16.1%. The M (Q₁, Q₃) values for FT3/FT4, T3/T4, and TSH were 0.27 (0.24, 0.30), 0.020 (0.017, 0.022), and 2.12 (1.34, 3.44) mIU/L, respectively. Multivariable logistic regression analysis showed that, after adjusting for confounding factors, compared with the Q₄ group of FT3/FT4 ratio, the risk of diabetes was higher in the Q₁ (OR=2.02, 95%CI: 1.61-2.54), Q₂ (OR=1.72, 95%CI: 1.39-2.13), and Q₃ groups (OR=1.37, 95%CI: 1.11-1.70); compared with the Q₄ group of T3/T4 ratio, the risk of diabetes was higher in the Q₁ (OR=2.16, 95%CI: 1.71-2.73), Q₂ (OR=1.70, 95%CI: 1.37-2.12), and Q₃ groups (OR=1.51, 95%CI: 1.22-1.86) (all P<0.001). RCS analysis revealed that FT3/FT4 (P overall<0.001, P nonlinear=0.846) and T3/T4 (P overall<0.001, P nonlinear=0.492) exhibited linear inverse associations with diabetes. Conclusion: Lower FT3/FT4 and T3/T4 ratios are linearly and inversely associated with an increased risk of diabetes among the population aged 65 years and older across 18 longevity areas in China.