Urinary Iodine and Maternal Thyroid Hormones Across Pregnancy Trimesters: a Stratified and Nonlinear Analysis from...

Iodine is essential for the synthesis of thyroid hormones, which are vital to prenatal growth and neurological development. The nonlinear relationship between the urine iodine concentration and thyroid indicators in pregnant women is unclear, although both minimal and excessive iodine consumption may affect thyroid function. This repeated cross-sectional analysis study was conducted between 2020 and 2024 and involved 649 pregnant women from Beijing, utilizing data from the National Endemic Disease Surveillance System. The obtained data included urine iodine concentrations (UICs), household salt iodine levels, and serum thyroid hormone levels (thyroid-stimulating hormone (TSH), free thyroxine (FT)₄, and triiodothyronine (FT₃)). UICs were classified into quartiles. Kendall's nonparametric correlation, restricted cubic spline regression, multivariate linear regression, and logistic regression were used to explore the associations between UICs and thyroid function. Sensitivity analysis stratified by trimester was conducted to evaluate differences in gestational stages. The median UIC remained within the adequate range; however, spline regression indicated a nonlinear (U-shaped) correlation between UICs and TSH levels. No substantial monotonic relationships were identified between UICs and TSH, FT₄, or FT₃ according to Kendall's test (all P > 0.5). No significant relationships were detected between UIC quartiles and TSH, FT4, or FT₃ levels in multivariate models adjusted for age, gestational age, salt iodine, or year. In the third trimester subgroup, higher UIC quartiles (Q₂ and Q₃) were significantly correlated with increased TSH levels (Q₂: β = 0.76, P = 0.009; Q₃: β = 0.60, P = 0.041). Logistic analysis indicated that pregnant women in the highest UIC quartile had an increased, albeit nonsignificant, risk of TSH abnormalities (odds ratio = 0.62, 95% confidence interval: 0.26-1.21). Our data indicate a nonlinear relationship between iodine intake and thyroid function during pregnancy, characterized by trimester-specific effects. Ensuring adequate iodine levels while preventing both iodine deficiency and excess is essential for maternal thyroid health. These findings highlight the need for gestational stage-specific iodine monitoring and provide evidence to guide public health policies on maternal iodine supplementation.