Association between thyroid hormone levels and body mass index, waist circumference and body composition in older adults aged 65 years and above in 18 longevity areas in China

Objective: To understand the association of thyroid hormone level with body mass index (BMI), waist circumference and body composition in older adults aged ≥65 years in 18 longevity areas in China. Methods: Based on cross-sectional data from the 2021 China Healthy Ageing and Biomarkers Cohort Study, 4 566 older adults aged ≥65 years were included in this study. Multiple linear regression and restricted cubic spline models was used to analyze the associations of thyroid hormone level with BMI, waist circumference, muscle mass, and fat mass. Stratified analyses were performed according to gender, age, and fish intake of the older adults. Results: The mean age of the older adults was (82.6±10.5) years, 47.5% of them were men, the free triiodothyronine (FT3) and free thyroxine (FT4) levels were (4.2±0.6) pmol/L and (15.4±2.2) pmol/L respectively. After controlling confounding factors, the results of the multiple linear regression model showed that for each increment of 1 pmol/L in FT3 level, the mean levels β (95%CI) of change in BMI, waist circumference, muscle mass, and fat mass was 0.64(0.44-0.83), 1.21(0.66-1.76), 0.22(0.03-0.41), and 1.44(1.05-1.84) respectively. The mean change in muscle mass for each increment of 1 pmol/L in FT4 level was -0.15 (-0.19--0.10). The restricted cubic spline analysis showed that positive nonlinear associations of FT3 with BMI, waist circumference, muscle mass, and fat mass (all P for nonlinear <0.05) and negative linear association between FT4 and muscle mass (P for linear <0.001). Conclusions: In the older adults aged ≥65 years in 18 longevity areas in China, FT3 level was positively associated with the changes of BMI, waist circumference and body composition. FT4 level was negatively correlated with muscle mass.