Association of fluoride exposure with thyroid morphology and function in children from iodine-adequate areas: a cross-sectional study in Tianjin, China

Epidemiological evidence has shown directionally consistent associations between fluoride exposure and thyroid dysfunction in children. However, the dose-response characteristics, combined effects of cumulative and recent total exposure, and multidimensional thyroid outcomes remain incompletely characterized. This study investigated associations of fluoride exposure with thyroid morphology and function in children from Tianjin, China. A cross-sectional study was conducted in iodine-adequate areas of Tianjin, China, including 751 children aged 8–10 years. Urinary fluoride (a biomarker of recent total intake) and urinary iodine were measured. Dental fluorosis (a marker of cumulative exposure during tooth development) was clinically assessed. Thyroid volume was measured by ultrasound, and serum free triiodothyronine (FT 3 ), free thyroxine (FT 4 ), and thyroid-stimulating hormone (TSH) were measured and classified as low, normal, or elevated. Multiple linear and logistic regression models evaluated associations between fluoride exposure and thyroid outcomes, adjusting for 33 confounders. Restricted cubic spline (RCS) models analyzed nonlinear dose-response relationships. In adjusted models, both dental fluorosis and urinary fluoride were associated with reduced thyroid volume ( β = −0.57, 95% CI : −0.75, − 0.39; and β = −0.18, 95% CI : −0.25, − 0.10, respectively) and lower goiter risk ( OR = 0.14, 95% CI : 0.05, 0.39; and OR = 0.70, 95% CI : 0.55, 0.90, respectively). Urinary fluoride was associated with lower FT 4 ( β = −0.20, 95% CI : −0.33, − 0.06), reduced risk of elevated TSH ( OR = 0.71, 95% CI : 0.54, 0.92), and increased risk of low FT 3 ( OR = 1.63, 95% CI : 1.21, 2.20). RCS analyses showed thyroid volume declined with increasing urinary fluoride, plateauing at approximately 7.45 mg/L. FT 4 decreased rapidly at lower levels and plateaued at approximately 2.87 mg/L. For categorical outcomes, goiter and elevated TSH risks decreased monotonically, crossing OR = 1.0 at approximately 1.1 mg/L, whereas low FT 3 risk increased progressively, also crossing OR = 1.0 at approximately 1.1 mg/L. Combined classification of dental fluorosis and urinary fluoride showed that children with both indicators positive had the most pronounced reductions in FT 4 and goiter risk. Sensitivity analyses confirmed robust continuous findings (thyroid volume and FT 4 ) after excluding children with extreme iodine nutrition. Childhood fluoride exposure in iodine-adequate areas is associated with altered thyroid morphology and function, including reduced thyroid volume, lower goiter risk, lower FT 4 , reduced risk of elevated TSH, and increased risk of low FT 3 . Evaluating fluoride health risks requires considering multiple thyroid indicators and complementary exposure biomarkers.