This paper aims to evaluate the effects of thyroid disruption on the mental development of children. A total of 258 three-year-old children in Guiyu (e-waste-exposed group) and Nanao (reference group), China were examined. FT₃, FT₄, TSH, lead (BPb) and cadmium (BCd) in blood were determined, and cognitive and language scores of children were assessed based on the Bayley Scales of Infant Development III. Stepwise multiple regression was used to estimate the relationship between heavy metals and cognitive and language scores; mediation analysis was performed to determine whether thyroid disruption was mechanistically involved. Medians of BPb and BCd in Guiyu were higher than that of Nanao (11.30±5.38 vs. 5.77±2.51 μg/dL BPb; 1.22±0.55 vs. 0.72±0.37 μg/L BCd, both p<0.001). Means of FT₄ and TSH in Guiyu were also higher than those in Nanao (16.65±1.83 vs.16.06±1.66 pmol/L FT₄, p=0.007; 2.79±1.30 vs. 2.21±1.43 mIU/L TSH, p=0.001). Guiyu children had lower cognitive scores (100.00±25.00 vs. 120.00±20.00, p<0.001) and lower language scores (99.87±7.52 vs. 111.39±7.02, p<0.001). Mediation analysis showed that Pb negatively correlated with both cognitive and language scores (both p<0.001). However, FT₃, FT₄ and TSH did not significantly mediate the relationship between Pb and mental development of children (all p>0.05). In contrast, Cd correlated with neither cognitive nor language scores (both p>0.05). Results suggest exposure to heavy metal (Pb) reduces cognitive and language skills, and affects thyroid function, but fail to confirm that thyroid disruption is involved in the neurotoxicity induced by Pb-Cd co-exposure.