Abstract
Background: Severe iodine deficiency during pregnancy can cause intellectual disability, presumably through inadequate placental transfer of maternal thyroid hormone to the fetus. The association between mild-to-moderate iodine deficiency and child neurodevelopmental problems is not well understood. Objective: We investigated the association of maternal iodine status during pregnancy with child attention-deficit hyperactivity disorder (ADHD) and autistic traits. Methods: Collaborative study of three population-based birth cohorts: Generation R (N=1634), INMA (N=1293), and ALSPAC (N=2619). Exclusion criteria were multiple fetuses, fertility treatment, thyroid-interfering medication use, and pre-existing thyroid disease. The mean age of assessment in the cohorts was between 4.4 – 7.7 years for ADHD symptoms and 4.5 – 7.6 years for autistic traits. We studied the association of the urinary iodine-to-creatinine ratio (UI/Creat) < 150 μg/g – in all mother-child pairs, and in those with a urinary-iodine measurement at ≤ 18 weeks and ≤ 14 weeks of gestation – with the risk of ADHD or a high autistic-trait score (≥ 93rd percentile cut-off), using logistic regression. The cohort-specific effect estimates were combined by random effects meta-analyses. We also investigated whether UI/Creat modified the association of maternal free thyroxine (FT4) or thyroid stimulating hormone (TSH) concentrations with ADHD or autistic traits. Results: UI/Creat <150 μg/g was not associated with ADHD [odds ratio (OR): 1.2; 95% CI: 0.7, 19 2.2; P=0.56] or with a high autistic-trait score (OR: 0.8; 95% CI: 0.6-1.1; P=0.22). UI/Creat <150 20 μg/g in early pregnancy (i.e., ≤ 18 weeks or ≤ 14 weeks of gestation) was not associated with a higher risk of behavioral problems. The association between a higher FT4 and a greater risk of ADHD (OR: 1.3; 95% CI: 1.0, 1.6; P=0.017) was not modified by iodine status. Conclusion: There is no consistent evidence to support an association of mild-to-moderate iodine deficiency during pregnancy with child ADHD or autistic traits.