| Summary: | In animal studies, the combination of in utero fluoride exposure and low iodine has greater negative effects on offspring learning and memory than either alone, but this has not been studied in children. We evaluated whether the maternal urinary iodine concentration (MUIC) modifies the association between maternal urinary fluoride (MUF) and boys’ and girls’ intelligence. We used data from 366 mother–child dyads in the Maternal–Infant Research on Environmental Chemicals Study. We corrected trimester-specific MUF and MUIC for creatinine, and averaged them to yield our exposure variables (MUF<sub>CRE</sub>, mg/g; MUIC<sub>CRE</sub>, µg/g). We assessed children’s full-scale intelligence (FSIQ) at 3 to 4 years. Using multiple linear regression, we estimated a three-way interaction between MUF<sub>CRE</sub>, MUIC<sub>CRE</sub>, and child sex on FSIQ, controlling for covariates. The MUIC<sub>CRE</sub> by MUF<sub>CRE</sub> interaction was significant for boys (<i>p</i> = 0.042), but not girls (<i>p</i> = 0.190). For boys whose mothers had low iodine, a 0.5 mg/g increase in MUF<sub>CRE</sub> was associated with a 4.65-point lower FSIQ score (95% CI: −7.67, −1.62). For boys whose mothers had adequate iodine, a 0.5 mg/g increase in MUF<sub>CRE</sub> was associated with a 2.95-point lower FSIQ score (95% CI: −4.77, −1.13). These results suggest adequate iodine intake during pregnancy may minimize fluoride’s neurotoxicity in boys.
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