Association between childhood obesity and thyroid function: a cross-sectional study in a Chinese population

Although both obesity and thyroid dysfunction adversely affect children’s health, research on their relationship remains limited and primarily focuses on adult and pregnant populations, with inconsistent findings. This cross-sectional study aimed to investigate the prevalence of thyroid dysfunctions, and their association with obesity among children in Tianjin, a major city in northern China. A cross-sectional study was conducted from 2016 to 2024 across 16 districts in Tianjin, China. A total of 1333 children were included as study subjects. Height and weight were measured, and household salt iodine, urinary iodine, and thyroid hormone levels were detected. Restricted cubic spline models, logistic regression models, and multiple linear regression models were conducted to analyze the association between obesity and thyroid dysfunction as well as thyroid hormone levels in children. The detection rates of hyperthyroidism, hypothyroidism, subclinical hyperthyroidism, subclinical hypothyroidism, TgAb positivity, and TPOAb positivity among children in Tianjin were 0.75%, 0.53%, 2.48%, 4.88%, 4.05%, and 3.75%, respectively. Multivariate analysis revealed a significant association between childhood obesity and subclinical hypothyroidism (OR = 2.37, 95% CI: 1.40, 4.01). Multiple linear regression indicated that BMI-z-score was positively correlated with log10-transformed TSH (β = 0.012, 95% CI: 0.004, 0.019) and FT3 (β = 0.063, 95% CI: 0.036, 0.089). Restricted cubic spline models revealed positive associations of higher BMI-z-scores with both increased odds of subclinical hypothyroidism and elevated FT3 levels (P for overall association: 0.0049 and < 0.001; P for nonlinear association: 0.5665 and 0.8404, respectively). BMI-z-score exhibited a U-shaped nonlinear correlation with log10-transformed TSH (P for overall association < 0.001, P for nonlinear association = 0.0083). Notably, TSH levels increased significantly with rising BMI-z-score above the nadir of the curve (BMI-z-score > -0.36), while variations in TSH within the lower BMI-z-score range remained within normal reference limits. Childhood obesity is associated with subclinical hypothyroidism. BMI-z-score is correlated with TSH and FT3 levels. Further studies are needed to better understand the causal association between changes in thyroid function and obesity.