BACKGROUND: The effects of severe iodine deficiency during critical periods of brain development are well documented. There is little known about the consequences of milder forms of iodine deficiency on neurodevelopment. OBJECTIVE: The objective was to determine whether supplementing mildly iodine-deficient children with iodine improves cognition. DESIGN: A randomized, placebo-controlled, double-blind trial was conducted in 184 children aged 10-13 y in Dunedin, New Zealand. Children were randomly assigned to receive a daily tablet containing either 150 microg I or placebo for 28 wk. Biochemical, anthropometric, and dietary data were collected from each child at baseline and after 28 wk. Cognitive performance was assessed through 4 subtests from the Wechsler Intelligence Scale for Children. RESULTS: At baseline, children were mildly iodine deficient median urinary iodine concentration (UIC): 63 microg/L; thyroglobulin concentration: 16.4 microg/L. After 28 wk, iodine status improved in the supplemented group (UIC: 145 microg/L; thyroglobulin: 8.5 microg/L), whereas the placebo group remained iodine deficient (UIC: 81 microg/L; thyroglobulin: 11.6 microg/L). Iodine supplementation significantly improved scores for 2 of the 4 cognitive subtests picture concepts (P = 0.023) and matrix reasoning (P = 0.040) but not for letter-number sequencing (P = 0.480) or symbol search (P = 0.608). The overall cognitive score of the iodine-supplemented group was 0.19 SDs higher than that of the placebo group (P = 0.011). CONCLUSIONS: Iodine supplementation improved perceptual reasoning in mildly iodine-deficient children and suggests that mild iodine deficiency could prevent children from attaining their full intellectual potential. The trial was registered with the Australia New Zealand Clinical Trials Register as ACTRN12608000222347.
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