Association between Elevated Iodine Intake and IQ among School Children in Portugal

Irene P. Carvalho, Bruno Peixoto, José Carlos Caldas, Ana Costa, Sofia Silva, Bárbara Moreira, Agostinho Almeida, André Moreira-Rosário, António Guerra, Cristina Delerue-Matos, Diana Sintra, Diogo Pestana, Edgar Pinto, Francisca Castro Mendes, Inês Martins, João Costa Leite, Manuel Fontoura, Maria Luz Maia, Pedro Queirós, Roxana MoreiraSandra Leal, Sónia Norberto, Vera Costa, Virgínia Cruz Fernandes, Elisa Keating, Luís Azevedo, Conceição Calhau

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The goal of this work was to examine whether elevated iodine intake was associated with adverse effects on IQ among school-age children in Portugal. In a representative sample of children from the north of the country, IQ percentiles by age (assessed with Raven’s Colored Progressive Matrices) were dichotomized to <50 (“below-average” IQs) and ≥50. Morning urine iodine concentrations, corrected for creatinine, were dichotomized to <250 µg/g and ≥250 µg/g, according to the European Commission/Scientific Committee on Food’s tolerable upper level of daily iodine intake for young children. Data were examined with Chi-square tests, logistic regression, and GLM univariate analysis. The sample (N = 1965) was classified as generally iodine-adequate (median urinary iodine concentration = 129 µg/L; median iodine-to-creatinine ratio = 126 µg/g) according to the WHO’s criteria. A greater proportion of children in the ≥250 µg/g group had below-average IQs, compared to children with less than 250 µg/g (p = 0.037), despite a sizable (though non-significant) proportion of children in the less-than-250 µg/g group also presenting below-average IQs, at the bottom of the iodine distribution (<50 µg/g). The proportion of below-average IQs increased with increasingly elevated iodine concentrations (p = 0.047). The association remained significant after the adjustment for confounders, with the elevated iodine group showing increased odds of having below-average IQs when compared with the non-elevated iodine group (OR 1.55; 95% CI 1.11–2.17; p = 0.011). Consistently, the former group presented a lower mean IQ than the latter (p = 0.006). High iodine intake was associated with lower IQs even in a population classified as iodine-adequate. These results bear on child cognition and on initiatives involving iodine supplementation.

Original languageEnglish
Article number4493
Issue number21
Publication statusPublished - Nov 2022


  • cognition
  • excessive iodine
  • iodine-adequate population
  • representative sample
  • school ages


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