Maternal Iodine Status During Pregnancy Is Not Consistently Associated with Attention-Deficit Hyperactivity Disorder or Autistic Traits in Children

Resurgence of Iodine Deficiency in the United States During Pregnancy: Potential Implications for Cognitive Development in Children

Iodine is an essential trace mineral for thyroid hormone synthesis, metabolism, and neurological development, particularly during pregnancy and early childhood. Although salt iodization in the 1920s significantly reduced iodine deficiency, recent studies suggest a resurgence in the United States, particularly among pregnant women. This review examines the resurgence of iodine deficiency among women in the United States while highlighting factors influencing intake, focusing on maternal iodine status during pregnancy, followed by its implications on cognitive health of children globally. Articles published from 2010 to 2024 were identified using PubMed, EMBASE, and Cochrane databases following PRISMA 2020 guidelines. Search terms included "iodine deficiency in women" and "maternal iodine deficiency and neurodevelopment." Studies on urinary iodine concentration (UIC) in nonpregnant, pregnant, and lactating women in the United States, and maternal iodine deficiency impact on offspring neurodevelopment globally, were included. Nine articles were reviewed addressing current iodine status among women in the United States, 4 of which used the National Health and Nutrition Examination Survey (NHANES). NHANES data (2011-2020) indicated declining median UIC (mUIC) among women of reproductive age and mUICs in pregnant women were below the World Health Organization recommendation (<150 µg/L). Prevalence of inadequate iodine status ranged from 23% to 59% in pregnant women. The primary contributing factor to declining iodine status is dietary changes, specifically reduced milk consumption. Eleven additional articles examined maternal UIC and cognition in global populations, highlighting the association between low UIC during pregnancy, particularly in the first and second trimesters, and poor neurodevelopmental outcomes such as reduced IQ scores in offspring outside of the United States. Offspring born to mothers with iodine deficiency are at greater risk of poor cognitive health. Comprehensive nutrition policies are needed to raise awareness of adequate iodine intake during critical developmental periods and to support food-system changes, ensuring sufficient intake.

Urinary Iodine Concentration and Thyroid Hormone Metabolism in Pregnant Women and Neurodevelopment in Their Children: A Longitudinal Canadian Birth Cohort

Background/Objectives: Iodine is essential for thyroid hormone (TH) synthesis, and THs in pregnant women are critical for fetal brain development. It is unclear whether urinary iodine concentrations (UICs) are associated with thyroid parameters in pregnant women and neurodevelopment in their 3-4-year-old children. Methods: In the Canadian Maternal-Infant Research on Environmental Chemicals (MIREC) cohort, we categorized UIC adjusted for urinary creatinine (UIC/Cr) in the first two trimesters as <150, 150-500, or ≥500 µg/g. We used multivariable regression to quantify associations between UIC/Cr and thyroid parameters in maternal plasma (n = 1501), including thyroid stimulating hormone (TSH), total T4 (tT4), free T4 (fT4), thyroglobulin (Tg) and Tg antibodies (TgAb), and thyroid peroxidase antibodies (TPOAb). We defined positive thyroid autoantibodies as TgAb ≥ 4.11 or TPOAb ≥ 5.61 IU/mL. We also examined the associations between UIC/Cr with the Wechsler Preschool and Primary Scale of Intelligence (n = 503), Behavior Assessment System for Children (n = 751), and the Social Responsiveness Scale (n = 498). Results: Twenty-two percent of women had UIC/Cr < 150 and 17% ≥ 500 µg/g. UIC/Cr was not associated with TSH, tT4, or fT4. After excluding women with positive thyroid autoantibodies, those with UIC/Cr < 150 µg/g had higher tT4 compared to those with 150-500 µg/g. Compared to women with UIC/Cr 150-500 µg/g, those with UIC/Cr < 150 had higher Tg and, those with UIC/Cr ≥ 500 had less frequent positive thyroid autoantibodies. Neurodevelopmental outcomes were not associated with maternal Tg, nor did they differ for maternal UIC/Cr < 150 and ≥500 compared to 150-500 µg/g. Conclusions: In this cohort, Tg and tT4 were higher in women with UIC/Cr < 150 µg/g compared to those with UIC/Cr 150-500 µg/g. Urinary iodine in pregnant women was not associated with neurodevelopment in their 3-4-year-old children.

Thyroid function and iodine intake: global recommendations and relevant dietary trends

Iodine is a micronutrient that is essential for thyroid hormone production. Adequate iodine intake is especially important during pregnancy and early life, when brain development is dependent on thyroid hormones. Iodine intake recommendations vary around the world, but most recommendations generally reflect the increased requirements during pregnancy and lactation, although adequate iodine intake before pregnancy is also important. Tremendous progress has been made in improving iodine intake across the world over the past 30 years, mainly through salt-iodization programmes. However, in countries without strong iodine fortification programmes, and with shifts in dietary patterns, a need has arisen for health organizations, governments and clinicians to ensure that adequate iodine is consumed by everyone in the population. For example, in countries in which adequate iodine intake depends on individual food choice, particularly of iodine-rich milk and dairy products, intake can be highly variable and is also vulnerable to changing dietary patterns. In this Review, iodine is considered in the wider context of the increasing prevalence of overweight and obesity, the dietary trends for salt restriction for cardiovascular health and the increasing uptake of plant-based diets.

Maternal Thyroid Dysfunction During Pregnancy as an Etiologic Factor in Autism Spectrum Disorder: Challenges and Opportunities for Research

Background: Autism spectrum disorder (ASD) is a neurodevelopmental condition with unknown etiology. Both genetic and environmental factors have been associated with ASD. Environmental exposures during the prenatal period may play an important role in ASD development. This narrative review critically examines the evidence for a relationship between maternal thyroid dysfunction during pregnancy and ASD in the child. Summary: Studies that assessed the associations of hypothyroidism, hyperthyroidism, hypothyroxinemia, thyroid hormone concentrations, or autoimmune thyroid disease with ASD outcomes were included. Most research focused on the relationship between hypothyroidism and ASD. Multiple population-based studies found that maternal hypothyroidism was associated with higher likelihood of an ASD diagnosis in offspring. Associations with other forms of maternal thyroid dysfunction were less consistent. Findings may have been affected by misclassification bias, survival bias, or publication bias. Studies using medical records may have misclassified subclinical thyroid dysfunction as euthyroidism. Two studies that assessed children at early ages may have misclassified those with ASD as typically developing. Most studies adjusted for maternal body mass index (BMI) and/or mental illness, but not interpregnancy interval or pesticide exposure, all factors associated with fetal survival and ASD. Most studies reported a combination of null and statistically significant findings, although publication bias is still possible. Conclusions: Overall, evidence supported a positive association between maternal thyroid dysfunction during pregnancy and ASD outcomes in the child, especially for hypothyroidism. Future studies could reduce misclassification bias by using laboratory measures instead of medical records to ascertain thyroid dysfunction and evaluating children for ASD at an age when it can be reliably detected. Survival bias could be further mitigated by adjusting models for more factors associated with fetal survival and ASD. Additional research is needed to comprehensively understand the roles of maternal levothyroxine treatment, iodine deficiency, or exposure to thyroid-disrupting compounds in the relationship between maternal thyroid dysfunction and child ASD outcomes.

An Evaluation of Food and Nutrient Intake among Pregnant Women in The Netherlands: A Systematic Review

Nutritional deficiencies during pregnancy can have serious consequences for the health of the (unborn) child. This systematic review provides an updated overview of the available food and nutrient intake data for pregnant women in The Netherlands and an evaluation based on the current recommendations. Embase, MEDLINE, and national institute databases were used. Articles were selected if they had been published since 2008 and contained data on food consumption, nutrient intake, or the status of healthy pregnant women. A qualitative comparison was made with the 2021 Dutch Health Council recommendations and reference values. A total of 218 reports were included, representing 54 individual studies. Dietary assessments were primarily performed via food frequency questionnaires. Protein, vitamin A, thiamin, riboflavin, vitamin B₆, folate, vitamin B₁₂, vitamin C, iron, calcium, and magnesium intakes seemed to be adequate. For folate and vitamin D, supplements were needed to reach the recommended intake. The reasons for concern are the low intakes of fruits, vegetables, and (fatty) fish, and the intakes of alcohol, sugary drinks, and salt. For several foods and nutrients, no or limited intake data were found. High-quality, representative, and recent data are needed to evaluate the nutrient intake of pregnant women in order to make accurate assessments and evaluations, supporting scientific-based advice and national nutritional policies.

Invited review: Iodine level in dairy products-A feed-to-fork overview

The theme of iodine in the dairy sector is of particular interest due to the involvement and the interconnection of several stakeholders along the dairy food chain. Iodine plays a fundamental role in animal nutrition and physiology, and in cattle it is an essential micronutrient during lactation and for fetal development and the calf's growth. Its correct use in food supplementation is crucial to guarantee the animal's recommended daily requirement to avoid excess intake and long-term toxicity. Milk iodine is fundamental for public health, being one of the major sources of iodine in Mediterranean and Western diets. Public authorities and the scientific community have made great efforts to address how and to what extent different drivers may affect milk iodine concentration. The scientific literature concurs that the amount of iodine administered through animal feed and mineral supplements is the most important factor affecting its concentration in milk of most common dairy species. Additionally, farming practices related to milking (e.g., use of iodized teat sanitizers), herd management (e.g., pasture vs. confinement), and other environmental factors (e.g., seasonality) have been identified as sources of variation of milk iodine concentration. Overall, the aim of this review is to provide a multilevel overview on the mechanisms that contribute to the iodine concentration of milk and dairy products.

Evaluation of iodine nutritional status during pregnancy by estimated 24-h urinary iodine excretion: population variation range and individual accuracy

Objective:

To explore the accuracy of estimated 24-h urinary iodine excretion (24-h UIE_est) in assessing iodine nutritional status.

Design:

Fasting venous blood, 24-h and spot urine samples were collected during the day. The urinary iodine concentration (UIC) and urinary creatinine concentration (UCrC) were measured, and the urinary iodine-to-creatinine ratio (UI/Cr), 24-h UIE_est, and 24-h urinary iodine excretion (24-h UIE) were calculated. At the population level, correlation and consistency between UIC, UI/Cr, 24-h UIE_est and 24-h UIE were assessed using correlation analysis and Bland–Altman plots. At the individual level, receiver operating characteristic (ROC) curves were used to analyse the accuracy of the above indicators for evaluating insufficient and excessive iodine intake. The reference interval of 24-h UIE_est was established based on percentile values.

Setting:

Indicator can accurately evaluate individual iodine nutrition during pregnancy remains controversial.

Participants:

Pregnant women (n 788).

Results:

Using 24-h UIE as standard, the correlation coefficients of 24-h UIE_est from different periods of the day ranged from 0·409 to 0·531, and the relative average differences ranged from 4·4 % to 10·9 %. For diagnosis of insufficient iodine intake, the area under the ROC curve of 24-h UIE_est was 0·754, sensitivity and specificity were 79·6 % and 65·4 %, respectively. For diagnosis of excessive iodine intake, the area of 24-h UIE_est was 0·771, sensitivity and specificity were 66·7 % and 82·0 %, respectively. The reference interval of 24-h UIE_est was 58·43–597·65 μg.

Conclusions:

Twenty-four-hour UIE_est can better indicate iodine nutritional status at a relatively large sample size in a given population of pregnant women. It can be used for early screening at the individual level to obtain more lead time for pregnant women.

Associations between maternal urinary iodine assessment, dietary iodine intakes and neurodevelopmental outcomes in the child: a systematic review

OBJECTIVE

Mild to moderate iodine deficiency during pregnancy has been associated with adverse neurodevelopmental outcomes in offspring. Few research studies to date combine assessment of urinary iodine (UIC and/or ICr), biomarkers that best reflect dietary intake, with reported dietary intake of iodine rich foods in their assessment of iodine deficiency. Thus, a systematic review was conducted to incorporate both these important measures.

DESIGN

Using PRISMA guidelines, a comprehensive search was conducted in three electronic databases (EMBASE®, MedLine® and Web of Science®) from January 1970-March 2021. Quality assessment was undertaken using the Newcastle Ottawa Scale. Eligible studies included reported assessment of iodine status through urinary iodine (UIC and/or ICr) and/or dietary intake measures in pregnancy alongside neurodevelopmental outcomes measured in the children. Data extracted included study author, design, sample size, country, gestational age, child age at testing, cognitive tests, urinary iodine assessment (UIC in μg/L and/or ICr in μg/g), dietary iodine intake assessment and results of associations for the assessed cognitive outcomes.

RESULTS

Twelve studies were included with nine reporting women as mild-moderately iodine deficient based on World Health Organization (WHO) cut-offs for urinary iodine measurements < 150 μg/l, as the median UIC value in pregnant women. Only four of the nine studies reported a negative association with child cognitive outcomes based on deficient urinary iodine measurements. Five studies reported urinary iodine measurements and dietary intakes with four of these studies reporting a negative association of lower urinary iodine measurements and dietary iodine intakes with adverse offspring neurodevelopment. Milk was identified as the main dietary source of iodine in these studies.

CONCLUSION

The majority of studies classified pregnant women to be mild-moderately iodine deficient based on urinary iodine assessment (UIC and/or ICr) and/or dietary intakes, with subsequent offspring neurodevelopment implications identified. Although a considerable number of studies did not report an adverse association with neurodevelopmental outcomes, these findings are still supportive of ensuring adequate dietary iodine intakes and urinary iodine monitoring throughout pregnancy due to the important role iodine plays within foetal neurodevelopment. This review suggests that dietary intake data may indicate a stronger association with cognitive outcomes than urinary iodine measurements alone. The strength of this review distinguishes results based on cognitive outcome per urinary iodine assessment strategy (UIC and/or ICr) with dietary data. Future work is needed respecting the usefulness of urinary iodine assessment (UIC and/or ICr) as an indicator of deficiency whilst also taking account of dietary intakes.

Maternal iodine status in a multi-ethnic UK birth cohort: associations with autism spectrum disorder

Background

Maternal iodine requirements increase during pregnancy to supply thyroid hormones essential for fetal brain development. Maternal iodine deficiency can lead to hypothyroxinemia, a reduced fetal supply of thyroid hormones which, in the first trimester, has been linked to an increased risk of autism spectrum disorder (ASD) in the child. No study to date has explored the direct link between maternal iodine deficiency and diagnosis of ASD in offspring.

Methods

Urinary iodine concentrations (UIC) and iodine/creatinine ratios (I:Cr) were measured in 6955 mothers at 26–28 weeks gestation participating in the Born in Bradford (BiB) cohort. Maternal iodine status was examined in relation to the probability of a Read (CTV3) code for autism being present in a child’s primary care records through a series of logistic regression models with restricted cubic splines.

Results

Median (inter-quartile range) UIC was 76 μg/L (46, 120) and I:Cr was 83 μg/g (59, 121) indicating a deficient population according to WHO guidelines. Ninety two children (1·3%) in our cohort had received a diagnosis of ASD by the census date. Overall, there was no evidence to support an association between I:Cr or UIC and ASD risk in children aged 8–12 years (p = 0·3).

Conclusions

There was no evidence of an increased clinical ASD risk in children born to mothers with mild-to-moderate iodine deficiency at 26 weeks gestation. Alternative functional biomarkers of exposure and a wider range of conditions may provide further insight.