Urinary Iodine Concentrations in Pregnant Women and Offspring Brain Morphology

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.

A Cohort Study on the Effects of Maternal High Serum Iodine Status During Pregnancy on Infants in Terms of Iodine Status and Intellectual, Motor, and Physical Development

The objective of the present study was to explore the effects of maternal iodine excess during pregnancy on infants' neurodevelopment and physical development. A total of 143 mother-child pairs were enrolled in this cohort study. Maternal blood samples were collected during the obstetric examination. A mother-child questionnaire survey was conducted, and infants' blood samples were collected during the newborn physical examination. Infants' single-spot urine samples were collected, and intellectual, motor, and physical development were assessed at 2 months of age. The median (IQR) maternal serum iodine concentrations (SICs) in the first, second, and third trimesters of pregnancy were 91.2 (74.4, 102.2) μg/L, 81.2 (70.6, 94.8) μg/L, and 82.0 (68.9, 100.3) μg/L, respectively. In the first trimester of pregnancy, infants' psychomotor developmental index (PDI), body mass index (BMI) and weight-for-length Z score (WLZ) were higher with maternal suitable SIC (40 ~ 92 μg/L) than with maternal excess SIC (P < 0.05). Infants' PDI, BMI, weight-for-age Z score (WAZ) and WLZ were negatively correlated with maternal SIC (P < 0.05). Maternal excess SIC had a slightly negative effect on infants' MDI (OR = 1.304, P = 0.035, 95% CI = 1.019 ~ 1.668), PDI (OR = 1.124, P = 0.001, 95% CI = 1.052 ~ 1.200) and BMI (OR = 0.790, P = 0.005, 95% CI = 0.669 ~ 0.933). In the third trimester, infants' length-for-age Z score (LAZ) was higher with maternal high SIC (> 92 μg/L) (P = 0.015), and maternal SIC was positively correlated with infants' urine iodine concentration (UIC) (P = 0.026). Maternal iodine excess in the first trimester had a slightly negative effect on infants' intellectual, motor, and physical development. In the third trimester, maternal iodine excess only may have a positive impact on infants' height. Additionally, maternal iodine status was closely related to infants' iodine status.

Effect of Iodine Nutrition Status on Thyroid Function and Pregnancy Outcomes

Iodine is essential for normal thyroid function, supporting healthy fetal and child development. The relevance between maternal iodine nutrition status and pregnancy outcomes remains controversial. The aim was to explore whether urinary iodine concentrations (UIC)/urinary creatinine (UCr) was associated with thyroid function and adverse pregnancy outcomes. This study was performed in the Department of Endocrinology and Metabolism of the First Affiliated Hospital of Nanjing Medical University. A total of 212 pregnant women were enrolled from May 2018 to November 2021, from the first visit until postpartum. Maternal serum samples were obtained in the second half of pregnancy, and then thyroid-stimulating hormone (TSH), free thyroxine (FT4), free triiodothyronine (FT3), UIC, and UCr were tested. The correlation of UIC/UCr, which represented maternal iodine nutrition status, with TSH, FT4, and FT3 was studied using linear regression. And we assessed associations between UIC/UCr and pregnancy outcomes. Notably, we explored consistency between UIC/UCr and the incidence of low birth weight (LBW) by application of logistic regression analysis. A total of 212 women were divided into 3 groups according to the upper and lower quartiles of UIC/UCr. There were 53 women in group 1 (UIC/UCr < 106.96 ug/g), 106 women in group 2 (UIC/UCr 106.96-259.08 ug/g), and 53 women in group 3 (UIC/UCr > 259.08 ug/g). The level of UIC/UCr had a negative correlation with FT4 (r =  - 0.139, p = 0.043) but a positive correlation with TSH (r = 0.096, p = 0.162 > 0.05). There was a significant difference in the incidence of LBW among the 3 groups (p = 0.007). Logistic regression analysis found that the level of UIC/UCr was an independent factor for LBW (p = 0.048, OR = 0.991, 95%CI (0.982, 0.999)). The receiver operating characteristic (ROC) curve showed that the area under the curve (AUC) for UIC/UCr predicting the incidence of low birth weight was 0.687 (p = 0.013, 95%CI 0.575, 0.799). Lower UIC/UCr during pregnancy was associated with higher FT4 and lower TSH. And iodine deficiency during pregnancy is a risk factor for low birth weight. Our findings indicated that more attention should be paid to the appropriate iodine nutrition status in pregnant women, which can help prevent suffering from adverse pregnancy outcomes.

An update: maternal iodine supplementation, thyroid function tests, and child neurodevelopmental outcomes

PURPOSE OF REVIEW

The impact of maternal iodine supplementation (MIS) during pregnancy on thyroid function and child neurodevelopmental outcomes in areas of mild-to-moderate iodine deficiency (MMID) remains unclear.

RECENT FINDINGS

Despite growing success of salt iodization programs, a 2022 meta-analysis found that 53% of pregnant patients worldwide continue to have insufficient iodine intake during pregnancy. A 2021 randomized controlled trial (RCT) found that MIS in women with mild iodine deficiency led to iodine sufficiency and positive effects on maternal thyroglobulin. A 2021 cohort study of MIS initiated prior to pregnancy was associated with lower thyroid-stimulating hormone (TSH), higher FT3, and FT4. Other cohort studies, however, found that neither salt iodization nor MIS were adequate to meet pregnancy iodine needs. Data have been mixed regarding maternal iodine status and pregnancy outcomes in patients of MMID. Meta-analyses have not shown any clear benefit on infant neurocognitive outcomes with MIS of MMID patients. A 2023 meta-analysis found that the prevalence of excess iodine intake in pregnancy was 52%.

SUMMARY

MMID continues to exist during pregnancy. Salt iodization alone may be insufficient to ensure adequate iodine status during pregnancy. There is an absence of high-quality data to support routine MIS in areas of MMID. However, patients with specialized diets (vegan, nondairy, no seafood, noniodized salt, and so on) may be at risk for inadequate iodine status in pregnancy. Excess iodine intake can be detrimental to the fetus and should be avoided during pregnancy.

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.

Iodine Deficiency, Maternal Hypothyroxinemia and Endocrine Disrupters Affecting Fetal Brain Development: A Scoping Review

This scoping review critically discusses the publications of the last 30 years on the impact of mild to moderate iodine deficiency and the additional impact of endocrine disrupters during pregnancy on embryonal/fetal brain development. An asymptomatic mild to moderate iodine deficiency and/or isolated maternal hypothyroxinemia might affect the development of the embryonal/fetal brain. There is sufficient evidence underlining the importance of an adequate iodine supply for all women of childbearing age in order to prevent negative mental and social consequences for their children. An additional threat to the thyroid hormone system is the ubiquitous exposure to endocrine disrupters, which might exacerbate the effects of iodine deficiency in pregnant women on the neurocognitive development of their offspring. Ensuring adequate iodine intake is therefore essential not only for healthy fetal and neonatal development in general, but it might also extenuate the effects of endocrine disruptors. Individual iodine supplementation of women of childbearing age living in areas with mild to moderate iodine deficiency is mandatory as long as worldwide universal salt iodization does not guarantee an adequate iodine supply. There is an urgent need for detailed strategies to identify and reduce exposure to endocrine disrupters according to the "precautional principle".

Iodine status during child development and hearing ability: a systematic review

Iodine, through the thyroid hormones, is required for the development of the auditory cortex and cochlea (the sensory organ for hearing). Deafness is a well-documented feature of endemic cretinism resulting from severe iodine deficiency. However, the range of effects of suboptimal iodine intake during auditory development on the hearing ability of children is less clear. We therefore aimed to systematically review the evidence for the association between iodine exposure (i.e. intake/status/supplementation) during development (i.e. pregnancy and/or childhood) and hearing outcomes in children. We searched PubMed and Embase and identified 330 studies, of which thirteen were included in this review. Only three of the thirteen studies were of low risk of bias or of good quality, this therefore limited our ability to draw firm conclusions. Nine of the studies (69 %) were in children (one RCT, two non-RCT interventions and six cross-sectional studies) and four (31 %) were in pregnant women (one RCT, one cohort study and two case reports). The RCT of iodine supplementation in mildly iodine-deficient pregnant women found no effect on offspring hearing thresholds. However, hearing was a secondary outcome of the trial and not all women were from an iodine-deficient area. Iodine supplementation of severely iodine-deficient children (in both non-RCT interventions) resulted in improved hearing thresholds. Five of six cross-sectional studies (83 %) found that higher iodine status in children was associated with better hearing. The current evidence base for the association between iodine status and hearing outcomes is limited and further good-quality research on this topic is needed.

A Scoping Review of Iodine and Fluoride in Pregnancy in Relation to Maternal Thyroid Function and Offspring Neurodevelopment

Iodine (I), an essential nutrient, is important for thyroid function and therefore growth and development. Fluoride (F), also an essential nutrient, strengthens bones and teeth, and prevents childhood dental caries. Both severe and mild-to-moderate I deficiency and high F exposure during development are associated to decreased intelligence quotient with recent reports associating high levels of F exposure during pregnancy and infancy to low intelligence quotient. Both F and I are halogens, and it has been suggested that F may interfere with the role of I in thyroid function. We provide a scoping review of the literature on I and F exposure dur pregnancy and their individual effects on thyroid function and offspring neurodevelopment. We first discuss I intake and status in pregnancy and the relationship to thyroid function and offspring neurodevelopment. We follow with the F in pregnancy and offspring neurodevelopment. We then review the interaction between I and F on thyroid function. We searched for, and found only one study that assessed both I and F in pregnancy. We conclude more studies are needed.

Study on the Effect of Different Iodine Intake on Hippocampal Metabolism in Offspring Rats

Iodine is an essential trace element in the human body. Severe maternal iodine deficiency during pregnancy leads to obvious intellectual disability in the offspring. The effects of iodine deficiency on brain development have been demonstrated, but there is no clear evidence of the effects of iodine excess on brain development. To clarify the effects of iodine excess on the brain development of offspring and to provide clues to the mechanisms underlying the effects of iodine deficiency and iodine excess on the brain development of offspring. In this study, animal models with different iodine intakes were constructed using potassium iodate (KIO₃). The models included four experimental groups (low-iodine group one (LI, 0μg/L iodine), low-iodine group two (LII, 5μg/L iodine), high-iodine group one (HI, 3000μg/L iodine), and high-iodine group two (HII, 10000μg/L iodine)) and one control group (NI, 100μg/L iodine). There were 20 female rats in each group, and 8 offspring were chosen from each group following birth to assess metabolic alterations. The metabolites of subsets of brain hippocampal tissue were profiled by ultra-performance liquid chromatography-linked electrospray ionization quadrupole time-of-flight mass spectrometry (UPLC-ESI-QTOFMS) and the results were subjected to multivariate data analysis. Differential substances were screened by t test (p<0.05), principal component analysis (PCA), and partial least squares analysis (PLS-DA, VIP>1). The thyroid function of the female rats in the experimental group was abnormally changed. Metabolic analysis showed that the five groups were separated which revealed significant differences in hippocampal tissue metabolism among the five groups of offspring. A total of 12 potential metabolites were identified, with the majority of them being related to amino acid and energy metabolism. These metabolites are involved in various metabolic pathways, are interrelated, and may play a function in brain development. Our study highlights changes in metabolites and metabolic pathways in the brain hippocampus of offspring rats with different iodine intakes compared to controls, revealing new insights into hippocampal metabolism in offspring rats and new relevant targets.

Pregnant Dutch Women Have Inadequate Iodine Status and Selenium Intake

Iodine and selenium are essential for thyroid hormone synthesis. Iodine and selenium interact. Pregnancy increases the maternal iodine requirement. We previously reported inadequate iodine status in pregnant Dutch women. Since little is known about their selenium intake, we investigated the iodine status and selenium intake in relation to iodine and selenium supplement use during pregnancy. Iodine status was established in 201 apparently healthy pregnant women as 24 h iodine excretion (24H-UIE; sufficient if median ≥225 µg), iodine concentration (24H-UIC; ≥150 µg/L) and iodine/creatinine ratio (24H-UICR; ≥150 µg/g). Selenium intake was calculated from 24 h selenium excretion. Iodine status in pregnancy proved insufficient (medians: 24H-UIE 185 µg; 24H-UIC 95 µg/L; 24H-UICR 141 µg/g). Only women taking 150 µg iodine/day were sufficient (median 24H-UIE 244 µg). Selenium intake was below the Estimated Average Requirement (EAR; 49 µg/day) in 53.8%, below the Recommended Dietary Allowance (RDA; 60 µg/day) in 77.4% and below the Adequate Intake (AI; 70 µg/day) in 88.7%. Combined inadequate iodine status and selenium intake <RDA was found in 61%. Women who want to become pregnant should, consistently with WHO and ETA recommendations, be advised to use a 150 µg iodine-containing supplement. Concomitant selenium supplementation should be added to this advice, at least in The Netherlands.