Iodine deficiency and adverse health outcomes in children and adolescents

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A meta-analysis on studies of children in moderately and severely iodine-deficient areas carried out by Bleichrodt and Born showed that a deficiency in iodine results in a 13.5-point reduction in IQ on average. Qian et al calculated that there was 10 IQ points difference between moderate to severely iodine-deficient and iodine-sufficient populations in a meta-analysis of Chinese studies. Similar results showing a lower IQ were found in Spanish school children who were mildly iodine deficient in a cross-sectional study carried out by Santiago-Fernandez et al.  All of these studies are limited by the fact that they are observational, and other factors that impact cognitive ability such as genetics, socioeconomic status and environmental factors were not considered.

A cross-sectional analytical study of Mexican schoolchildren demonstrated that the risk of a low IQ was 4.26 times higher in those with moderate iodine deficiency.  A limitation of this study is that while they measured the iodine levels in drinking water, they failed to measure for other goitrogens that are likely to have been present in the studied environment, including heavy metals in the drinking water, specifically in this location mercury, arsenic and lithium.

In Dunedin, New Zealand, a randomized, placebo-controlled, double-blind trial including 184 children aged between 10–13  showed improved perceptual reasoning in mildly iodine deficient children once they were administered iodine supplementation. This again suggests that even mild deficiency of iodine can be of determent to a child’s cognitive development. The small sample size and single location of the trial however limit the usefulness of the data when compared to the entire diverse Australian population.

While all the papers mentioned so far correlate a link between iodine deficiency and low IQ scores, there is not a straightforward correspondence between IQ scores and urinary iodine values. Urinary iodine reflects short term iodine intake, whereas IQ scores are impacted over a long period and by multiple variables.

The negative effects on IQ are unlikely to pose a problem in the Australian population according to the NNPAS survey results, as deficiency levels, even at a ‘usual intake’ level were less than 1% of the population for all under 18’s except girls aged 14-18.  

13% of males and 6% of females in the age 2-3 group in NNPAS had intakes that exceed the recommended UL. The Chinese meta-analysis shows that excess iodine is unlikely to impact mental development and therefore does not pose a risk to the Australian population from this perspective. However, excess iodine is also associated with hypothyroidism.

Recent studies have raised concerns over iodine excess in populations caused by fortification. 5 years of excessive iodine consumption in the Brazilian population was correlated to an increased prevalence of autoimmune hypothyroidism in genetically predisposed individuals.  A Danish study also demonstrated that even minimal salt iodisation was associated with increases in the incidence rate of hypothyroidism.  An Italian study also showed that increases in UIE that were measured 15 years after the introduction of iodine fortification were correlated with increases in hypothyroidism.  Kotwal et al  also showed that iodine excess can trigger and exacerbate autoimmune thyroiditis, leading to an increased incidence of hypothyroidism. This research was a case-control study with a reasonable sample size of 792 in India. Case control studies are not high on the evidence hierarchy and also, the correlation of high UIE and increased hypothyroidism does not implicitly indicate causation.

Comparing results between different epidemiological studies is challenging as studies may use different biochemical and epidemiological methods and the cohorts studied usually vary in age and sex. Genetic and environmental factors may also influence the results that may not be present in the Australian population. However, the level of iodine excess seen in this research (e.g. urinary iodine >300 μg/L) was not found in the under 18 age groups in NNPAS. The highest recorded in these groups was 176.9 μg/L, which is still within the WHO ranges of adequate levels.  The risk of hypothyroidism caused by iodine excess is therefore not likely in the Australian under 18 population. 

Recommendations to improve intake levels of iodine

A systematic review of 89 studies  showed a 72–76% reduction in risk for low IQ and an 8·2–10·5 point increase in IQ when iodised salt was added to the diet.  This suggests that the intake of iodised salt should be encouraged, however, I feel it would be irresponsible to encourage increasing salt intake given the relationship between high dietary salt intake and cardiovascular disease.

Predictive modelling of interventions to improve iodine intake in New Zealand showed the manufacture of bread with iodised salt was the most likely intervention to decrease deficiency.  Since October 2009, all bread made in Australia must use iodised salt (except organic and home baking mixes).  The possibility of increasing the iodisation of salt or further fortification of bread and other products is an option that would increase iodine consumption, however care should be taken not to induce iodine excess states correlated with hypothyroidism.

I don’t believe fortification of weaning foods is needed as 13% of males and 6% of females aged 2-3 (the target of these foods), have usual intakes exceeding the UL for iodine.

Since only small adjustments are required to ensure the majority of the under 18 population in Australia get sufficient iodine I would suggest that the best way to do this is to make parents and girls age 14-18 with the highest percentage of iodine deficiency aware of the importance of including iodine containing foods, and what these foods are.

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