[Meta-analysis on the relationship between children's intelligence and factors as iodine deficiency, supplement iodine and excessive iodine].

Objective: To estimate the role and extent of iodine deficiency, iodine supplement and iodine excess on mental development of children.

Methods: Meta-analysis was applied to study 128 independent items from 63 published and non-published papers and reports. The standards of references collected included: age of sample declared by references was 5 - 15; belong to comparison study; children lived in iodine deficiency disorders (IDD) and iodine excess areas; no difference of social economic and culture development level between the study group and the control group.

Results: Sixty-seven percent and 79% of the reports mainly involved severe IDD areas respectively. Hunter test of each studies, i.e. iodine deficiency, iodine supplement and excessive iodine group had not discovered statistic difference at the level of alpha = 0.05. The weighted average ES of damage affecting on children's intelligence by iodine deficiency achieved 0.69, which was equivalent to a marked drop in 10.4 IQ points [95% confidence interval (CI): 9.9 - 10.9] when comparing with the children living under non-IDD. The weighted average ES of protective effect on children's intelligence by iodine supplement reached 0.81, which meant that the IQ of children born after correction of iodine deficiency increased 12.2 points (95% CI: 11.5 - 12.9) in comparison with those born at least one year before the correction of iodine deficiency. Most of the references about the relationship between iodine excess and intelligence were gathered from proceedings, while the others were from journals. They location were in some areas of Shandong, Hebei, Shanxi and Inner Mongolia. The mean ES of the role of iodine excess on intelligence was 0.21, which was corresponding to 3.2 IQ points (95% CI: 2.5 - 4.0).

Conclusion: 1) Iodine deficiency played a role of intermediate strength compared with other causes in delaying brain development making children to be at least 10 IQ points loss in IDD areas. 2) Effective iodine supplement plays a remarkable strengthening role in promoting brain development and can cause 12 IQ points increase for children who were born after the correction of iodine status. 3) Iodine excess has not shown significant important role in children's intelligence.