Objectives To explore the effect of iodide excess on children's intelligence quotient(IQ) and monoamine neurotransmitters as well as investigate the relationship between monoamine neurotransmitters and IQ.

Methods Normal iodide region (10 μg/L ≤ water iodide < 150 μg/L), No.1 iodide-excess region (150 μg/L ≤ water iodide < 300 μg/L), and No.2 iodide-excess region (iodide in water ≥ 300 μg/L) were selected from Binhai New Area and Jinghai District in Tianjin City according to the concentrations of iodine in drinking water. Via cluster random sampling, 399 children aged from (7~12) years were chosen. Intelligence quotient of children was measured using the Combined Raven's Test-The Rural in China (CRT-RC). The levels of monoamine neurotransmitters, including dopamine (DA), 5-hydroxytryptamine (5-HT), norepinephrine (NE), and epinephrine (E), were measured using ELISA test.

Results Children's IQ in No.2 iodide-excess region was 109.59±14.24, which was significantly lower than that in normal iodide region and No.1 iodide-excess region, and the difference was statistically significant (all P < 0.05). The level of DA of children in No.2 iodide-excess region (321.69±104.95) ng/mL was higher than that of DA in normal iodide region and No.1 iodide-excess region, while the level of 5-HT of children in No.2 iodide-excess region (142.62±71.64) ng/mL was lower than that of 5-HT in other two regions, and the differences were statistically significant (all P < 0.05). While there was no significant difference in the levels of NE and E among the three regions (all P>0.05). Threshold analysis showed that there was a significant nonlinear relationship between iodide exposure and DA. The level of plasma DA increased significantly with the increase of urinary iodide in children with urinary iodide levels below 492.75 μg/L. There is also a significant nonlinear relationship between DA and IQ. Every 1 ng/mL increment in the level of DA was associated with a raise of 0.1 in the IQ score when DA level was below 80 ng/mL in children. In contrast, every 1 ng/mL increment in the level of DA was associated with a decline of 0.1 in the IQ score when DA level was in the range of 80.0 to 359.8 ng/mL. There were no significant linear relationship or threshold effect for the effect of the urinary iodide on 5-HT, NE, and E and the relationship between IQ and 5-HT, NE and E.

Conclusions The metabolic dysfunction of DA in children exposed to excessive iodide may be related to the decrease of intelligence quotient, and a threshold effect exists between DA and intelligence quotient.