Context

While the consequences of severe iodine deficiency are beyond doubt, the effects of mild-to-moderate iodine deficiency in pregnancy on child neurodevelopment are less well established.

Objective

To study the association between maternal iodine status during pregnancy and child IQ and to identify vulnerable time-windows of exposure to suboptimal iodine availability.

Design

Meta-analysis of individual-participant data from three prospective population-based birth cohorts: Generation R (The Netherlands), INMA (Spain), and ALSPAC (United Kingdom); pregnant women were enrolled between 2002-2006, 2003-2008, and 1990-1992, respectively.

Setting

General community.

Participants

6180 mother-child pairs with measures of urinary iodine and creatinine concentrations in pregnancy and child IQ. Exclusion criteria were multiple pregnancy, fertility treatment, medication affecting the thyroid, and pre-existing thyroid disease.

Intervention(s)

None.

Main Outcome Measure

Child non-verbal and verbal IQ assessed at 1.5-8 years of age.

Results

There was a positive curvilinear association of the urinary iodine-to-creatinine ratio (UI/Creat) with mean verbal IQ only. UI/Creat ˂ 150 µg/g was not associated with lower non-verbal IQ [-0.6 points, 95% CI -1.7 to 0.4, P=0.246] or lower verbal IQ [-0.6, 95% CI -1.3 to 0.1, P=0.082]. Stratified analyses showed that the association of UI/Creat with verbal IQ was only present up to 14 weeks of gestation.

Conclusions

Fetal brain development is vulnerable to mild-to-moderate iodine deficiency, particularly in the first trimester. Our results show that any potential randomized, controlled trial investigating the effect of iodine supplementation in mild-to-moderate iodine deficient women on child neurodevelopment, should start with supplementation not later than the first trimester.