Thyroid Function,Intelligence,and Low-moderate Fluoride Exposure Among Chinese School-age Children

Objective:We conducted a cross-sectional study to examine the associations between low-moderate fluoride exposure and thyroid function in relation to children's intelligence.Methods:We selected 571 children aged 9 to 13 from the permanent residents of Baodi District,Tianjin,using multi-stage random sampling by region in 2015.The basic information about the children and their parents was collected by face-to-face interviews and questionnaires.Children's IQ scores were assessed using the Combined Raven's Test-The Rural in China?CRT-RC2?.We collected children's morning urine and early morning fasting venous blood for the determination of urinary fluoride content and serum thyroid hormones content,respectively.Thyroid function was evaluated through the measurements of basal THs[(total triiodothyronine?TT₃?,total thyronine?TT₄?,free triiodothyronine?FT₃?,free thyronine?FT₄?]and thyroid-stimulating hormone?TSH?levels in serum.Multivariable linear and logistical regression models were used to assess associations among fluoride exposure,thyroid function and IQ scores.Interaction and mediation effects models were used to analyze whether thyroid hormones play a role in the relationship between fluoride and IQ.Results:The range of water fluoride exposure in this population was 0.20³.90 mg/L,the mean?±standard deviation?was 1.39?±1.01?mg/L,the median was 1 mg/L;The range of urine fluoride was 0.01⁵.54 mg/L,the mean?±standard deviation?was 1.28?±1.30?mg/L,with a median of 0.40 mg/L.In adjusted models,every 1 mg/L increment of water fluoride was associated with 0.127 uIU/mL increase in TSH??=0.127,95%CI:0.014,0.241?.Every 1 mg/L increment of urinary fluoride was associated with 0.09?g/dL decrease in TT₄??=-0.090,95%CI:-0.164,-0.016?,0.009 ng/dL decrease in FT₄??=-0.009,95%CI:-0.017,-0.002?and 0.11 uIU/mL increase in TSH??=0.110,95%CI:0.024,0.196?.There was a dose-response between water fluoride and TT₄,FT₄ and TSH,respectively?P for trend were 0.036,0.009,0.019?.There was a dose-response between urinary fluoride and TT₄,FT₄,FT₃ and TSH,respectively?P for trend were 0.021,0.027,0.014,0.019?.Fluoride exposure was inversely related to IQ scores??=-1.587,95%CI:-2.607,-0.568 for water fluoride and?=-1.214,95%CI:-1.987,-0.442 for urinary fluoride?.There was a dose-response between water fluoride,urinary fluoride and IQ,respectively?P for trend were all<0.01?.Higher TT₃,FT₃ were related to the increased odds of children having high normal intelligence?OR=3.407,95%CI:1.044,11.120 for TT₃;OR=3.277,95%CI:1.621,6.623 for FT₃?.We detected a modification effect by TSH on the association between urinary fluoride and IQ scores,without mediation by THs.Conclusions:Our study suggests low-moderate fluoride exposure was associated with alterations in childhood thyroid function and loss in children's intelligence.Higher TT₃,FT₃ were related to the increased odds of children having high normal intelligence.Moreover,TSH may modify the association between fluoride and children's intelligence.