Iodine and mental development of children 5 years old and under: a systematic review and meta-analysis

23 studies across four design strata - 2 RCTs of maternal supplementation, 8 non-randomized trials, 9 cohorts stratified by maternal iodine status, 4 cohorts stratified by newborn iodine status

Children aged 5 and under, and their mothers during pregnancy; predominantly iodine-deficient populations.

Medline search 1980-November 2011, with studies grouped by design rather than pooled into one estimate. The bulk of the evidence is prenatal maternal supplementation or observational stratification by iodine status, not supplementation of school-age children. The authors themselves name weak study designs, omitted confounders, small samples, absent cluster analyses and unseparated verbal/non-verbal subtests as limitations.

Average effect sizes were 0.68 (2 RCTs of maternal supplementation), 0.46 (8 non-randomized trials), 0.52 (9 cohorts stratified by maternal iodine status) and 0.54 (4 cohorts stratified by newborn iodine status), translating to 6.9-10.2 IQ points lower in iodine-deficient than iodine-replete children. The convergence across design strata is notable but the observational strata carry heavy confounding.

Medium to high for the cohort strata - maternal iodine status correlates with maternal diet, education and SES, all heritability-laden. Low for the two RCTs, which are also the smallest part of the evidence base.