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Maternal iodine status may affect childhood IQ
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A mother’s nutritional iodine status may be more influential on a child’s neurointellectual outcome than maternal thyroid function, according to recent study findings.
The findings also suggest that prenatal, mild to moderate iodine deficiency negatively affects cognitive development later in life, according to the researchers.
Francesco Vermiglio, MD, of the University of Messina in Italy, and colleagues evaluated 60 mother-child pairs to determine the effects on childhood IQ of iodine supplementation, with or without levothyroxine, before and during pregnancy. Participants were divided into the following groups based on iodized-salt consumption and levothyroxine treatment before and during pregnancy: iodine, no iodine, iodine with levothyroxine, and no iodine without levothyroxine (each included 15 mother-child pairs).
The Wechsler Intelligence Scale for Children-3rd edition (WISC-III), full-scale IQ, verbal IQ and performance IQ were administered to children at ages 6 to 12 years.
Similar full-scale, performance and verbal IQ scores were found between children in the iodine and iodine plus levothyroxine groups; these scores were higher than those of the no iodine and no iodine without levothyroxine groups.
Nine children were diagnosed with mild mental retardation (three in the iodine with levothyroxine group; six in the no iodine plus levothyroxine grou[); three children from the iodine plus levothyroxine group and 14 from the no iodine plus levothyroxine group had borderline intellectual functioning. Children of mothers not supplemented with iodine had a nearly threefold higher prevalence of borderline or defective cognitive function compared with those whose mothers who received iodine supplementation (P = .0001).
“There is evidence that brain development is vulnerable to thyroid hormone deficiency,” the researchers wrote. “In addition to affecting maternal thyroid function, a less than adequate maternal iodine intake during pregnancy potentially leads to insufficient fetal thyroid hormone output due to reduced iodine storage in the fetal gland. Our findings provide evidence that mild to moderate iodine deficiency during fetal development, even with the lack of maternal thyroid insufficiency, may result in cognitive impairment later in life, with a seemingly greater impact on verbal abilities.”
The researchers recommend close monitoring of maternal iodine intake before and during pregnancy. – by Amber Cox
Disclosure: The researchers report no relevant financial disclosures.
Perspective
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Angela M. Leung, MD, MSc
This was a prospective cohort study of mother-child pairs in northern Sicily, a region of mild to moderate iodine deficiency, grouped by use of iodized salt and/or levothyroxine use (total four groups) during preconception and pregnancy. The primary outcomes were IQ tests performed in the children at 6 to 12 years of age. Regardless of maternal levothyroxine use, children born to mothers who had used iodized salt either prior to or during pregnancy had higher IQ scores (whether verbal, performance or full-scale IQ results) than those whose mothers had not, thus demonstrating the crucial importance of adequate maternal iodine nutrition in pregnancy for fetal and infant neurodevelopment. In fact, the prevalence of decreased neurocognitive test outcomes was threefold higher among children of noniodine supplemented mothers than children whose mothers had received iodine supplementation (76.9% vs 23.1%; P =.0001).
This study underscores the critical role of sufficient maternal iodine nutritional status, required for thyroid hormone synthesis particularly during early pregnancy, on neurocognitive measures among their offspring even up to 12 years later. Strengths of the study include its use of a sample population that had endemic mild to moderate iodine deficiency. The findings in part support the conclusions reported by the Avon Longitudinal Study of Parents and Children in the United Kingdom, which showed that iodine-deficient women during pregnancy were more likely to have children with lower verbal IQ and reading scores at 8 to 9 years of age, compared with iodine-sufficient women. In another study by Taylor and colleagues, hypothyroid or hypothyroxinemic pregnant women in the Controlled Antenatal Thyroid Study who were exposed to the highest decile of levels of perchlorate, an environmental toxicant that can inhibit iodine uptake into the thyroid, were more likely to have children with IQ in the lowest decile, regardless of whether or not mothers thyroid dysfunction was corrected with levothyroxine supplementation, thus suggesting that there may be potential mechanisms of neurodevelopment that are independent of maternal thyroid function.
This report by Moleti and colleagues was a pilot study of small group sizes, but groups were well-matched by epidemiologic, socioeconomic and clinical parameters. However, the classification of maternal iodized-salt users was by self-report and likely captured a wide range of the amount and frequency of iodized- salt use. The period of study inclusion was also quite heterogeneous, since it included both women contemplating pregnancy and across all trimesters of pregnancy, making the studys findings less generalizable. The studys findings are intriguing, but further larger studies are needed to help understand the relative strength of contribution and importance of timing that maternal iodine status and thyroid function have on neurodevelopment and childhood cognition.
References:
Bath SC, et al. Lancet. 2013;doi:10.1016/S0140-6736(13)60436-5.
Taylor PN, et al. J Clin Endocrinol Metab. 2014;doi:10.1210/jc.2014-1901.
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