Too Much Too Little: Clarifying the Relationship Between Maternal Iodine Intake and Neurodevelopmental Outcomes

Background: In 2009, the Australian government mandated the fortification of bread salt with iodine. In 2010, pregnant and lactating women were also advised to take an iodine-containing supplement. Our assessment of this policy in an iodine-sufficient population showed that children whose mothers were in the highest and lowest quartiles of iodine intake performed more poorly on early childhood tests of cognition and language than those in the second quartile. However, we did not quantify the iodine intake associated with optimal neurodevelopment. Objectives: The aim was to establish the iodine intake range in pregnancy associated with optimal child neurodevelopment. Methods: A prospective cohort study of pregnant women and their young children (n = 699). Iodine intake was assessed by a validated food frequency questionnaire at 16 and 28 wk of gestation. Child neurodevelopment at 18 mo of age was measured using the Bayley Scales of Infant and Toddler Development, Third Edition (Bayley-III). The relationship between average iodine intake during pregnancy and child neurodevelopment was assessed using linear regression with fractional polynomials and adjustment for confounders. Results: Mean (SD) iodine intake was similar at study entry and 28 wk, 308 (120) mu g/d, with 82% of women taking iodine supplements at study entry. The relationship between iodine intake during pregnancy and Bayley-III cognitive and language scores was curvilinear (P = 0.001 and P = 0.004, respectively), with the lowest Bayley-III scores observed at lower and higher iodine intakes. The inflection point that drove the association between lower iodine intake in pregnancy and poorer child neurodevelopment scores was around 185 mu g/d; for the higher pregnancy iodine intakes, language and cognitive scores were negatively affected from similar to 350 mu g/d to 370 mu g/d, respectively. Higher iodine intakes were being driven by supplement use. Conclusions: Targeted, not blanket, iodine supplementation may be needed for pregnant women with low-iodine intake from food.