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.

The impact of replacing milk with plant-based alternatives on iodine intake: a dietary modelling study

PURPOSE

Cow's milk is the primary source of iodine in the UK, but consumption of plant-based milk alternatives (PBMA) is increasing and these products are often not fortified with iodine. We evaluated the impact that replacing current milk consumption with PBMA would have on iodine intake.

METHODS

We used data from the National Diet and Nutrition Survey (2016-2019) for children (1.5-10 years), girls 11-18 years, and women of reproductive age (WRA). We used a dietary modelling approach with scenarios using brand-level iodine-fortification data (0, 13, 22.5, 27.4 and 45 µg/100 mL). Relative to usual diet, we calculated change in iodine intake, and the proportion with intake below the Lower Reference Nutrient Intake (LRNI) or above the upper limit.

RESULTS

For all groups, replacement with PBMA, either unfortified or fortified at the lowest concentration, resulted in a meaningful decrease in iodine intake, and increased the proportion with intake < LRNI; compared to usual diet, iodine intake reduced by 58% in children 1.5-3 years (127 vs. 53 µg/day) and the proportion with intake < LRNI increased in girls (11-18 years; 20% to 48%) and WRA (13% to 33%) if an unfortified PBMA was used. Replacement of milk with PBMA fortified at 27.4 µg/100 mL had the lowest impact.

CONCLUSION

Replacing milk with commercially available PBMAs has potential to reduce population iodine intake, depending on the fortification level. PBMAs fortified with ≥ 22.5 and < 45 µg iodine/100 mL would be required to minimize the impact on iodine intake. Research is needed on the impact of total dairy replacement.

Iodine and plant-based diets: a narrative review and calculation of iodine content

An increasing number of food-based recommendations promote a plant-based diet to address health concerns and environmental sustainability in global food systems. As the main sources of iodine in many countries are fish, eggs and dairy products, it is unclear whether plant-based diets, such as the EAT-Lancet reference diet, would provide sufficient iodine. This is important as iodine, through the thyroid hormones, is required for growth and brain development; adequate iodine intake is especially important before, and during, pregnancy. In this narrative review, we evaluated the current literature and estimated iodine provision from the EAT-Lancet reference diet. There is evidence that those following a strict plant-based diet, such as vegans, cannot reach the recommended iodine intake from food alone and are reliant on iodine supplements. Using the EAT-Lancet reference diet intake recommendations in combination with iodine values from UK food tables, we calculated that the diet would provide 128 μg/d (85 % of the adult recommendation of 150 μg/d and 51–64 % of the pregnancy recommendation of 200–250 μg/d). However, if milk is replaced with unfortified plant-based alternatives, total iodine provision would be just 54 μg/d (34 % and 22–27 % of the recommendations for adults and pregnancy, respectively). Plant-based dietary recommendations might place consumers at risk of iodine deficiency in countries without a fortification programme and where animal products provide the majority of iodine intake, such as the UK and Norway. It is essential that those following a predominantly plant-based diet are given appropriate dietary advice to ensure adequate iodine intake.

Exploration of thyroglobulin as a biomarker of iodine status in iodine-sufficient and mildly iodine-deficient pregnant women

PURPOSE

Urinary iodine-to-creatinine ratio (UI/Creat) reflects recent iodine intake but has limitations for assessing habitual intake. Thyroglobulin (Tg) concentration, which increases with thyroid size, appears to be an indicator of longer-term iodine status in children and adults, however, less is known in pregnancy. This study investigated the determinants of serum-Tg in pregnancy and its use as an iodine-status biomarker in settings of iodine-sufficiency and mild-to-moderate deficiency.

METHODS

Stored blood samples and existing data from pregnant women from the Netherlands-based Generation R (iodine-sufficient) and the Spain-based INMA (mildly-to-moderately iodine-deficient) cohorts were used. Serum-Tg and iodine status (as spot-urine UI/Creat) were measured at median 13 gestational weeks. Using regression models, maternal socio-demographics, diet and iodine-supplement use were investigated as determinants of serum-Tg, as well as the association between UI/Creat and serum-Tg.

RESULTS

Median serum-Tg was 11.1 ng/ml in Generation R (n = 3548) and 11.5 ng/ml in INMA (n = 1168). When using 150 µg/g threshold for iodine deficiency, serum-Tg was higher in women with UI/Creat < 150 vs ≥ 150 µg/g (Generation R, 12.0 vs 10.4 ng/ml, P = 0.010; INMA, 12.8 vs 10.4 ng/ml, P < 0.001); after confounder adjustment, serum-Tg was still higher when UI/Creat < 150 µg/g (regression coefficients: Generation R, B = 0.111, P = 0.050; INMA, B = 0.157, P = 0.010). Iodine-supplement use and milk intake were negatively associated with serum-Tg, whereas smoking was positively associated.

CONCLUSION

The association between iodine status and serum-Tg was stronger in the iodine-deficient cohort, than in the iodine-sufficient cohort. Serum-Tg might be a complementary (to UI/Creat) biomarker of iodine status in pregnancy but further evidence is needed.

Iodine status of pregnant women from the Republic of Cyprus

Iodine supply is crucial during pregnancy to ensure that the proper thyroid function of mother and baby support fetal brain development. Little is known about iodine status or its dietary determinants in pregnant women in the Republic of Cyprus. We therefore recruited 128 pregnant women at their first-trimester ultrasound scan to a cross-sectional study. We collected spot-urine samples for the measurement of urinary iodine concentration (UIC, µg/l) and creatinine concentration (Creat, g/l), the latter of which allows us to correct for urine dilution and to compute the iodine-to-creatinine ratio (UI/Creat). Women completed a FFQ and a general questionnaire. We used a General Linear model to explore associations between maternal and dietary characteristics with UI/Creat. The median UIC (105 µg/l) indicated iodine deficiency according to the WHO criterion (threshold for adequacy = 150 µg/l), and the UI/Creat was also low at 107 µg/g. Only 32 % (n 45) of women reported the use of iodine-containing supplements; users had a higher UI/Creat than non-users (131 µg/g v. 118 µg/g), though this difference was NS in the adjusted analysis (P = 0·37). Of the dietary components, only egg intake was significantly associated with a higher UI/Creat in adjusted analyses (P = 0·018); there was no significant association with milk, dairy products or fish intake. Our results suggest that pregnant women in Cyprus have inadequate iodine status and are at risk of mild-to-moderate iodine deficiency. Further research on dietary sources in this population is required.

Dairy as a Source of Iodine and Protein in the UK: Implications for Human Health Across the Life Course, and Future Policy and Research

This narrative review summarizes key concepts in dairy nutrition for supporting human health throughout the life course. Milk and dairy products have been a staple component of our diet for thousands of years and provide a wide range of important nutrients that are otherwise difficult to obtain from dairy-free diets. In this review, we provide a broad perspective on the nutritional roles of iodine and dairy protein in supporting human health during pregnancy and early life, childhood and adolescence, mid- and later-life. New methodologies to identify biomarkers of dairy intake via high-throughput mass spectrometry are discussed, and new concepts such as the role of the food matrix in dairy nutrition are introduced. Finally, future policy and research related to the consumption of dairy and non-dairy alternatives for health are discussed with a view to improving nutritional status across the lifespan.

OUP accepted manuscript

Context

Adequate iodine intake is essential throughout life. Key dietary sources are iodized salt and animal products, but dietary patterns in Europe are changing, for example toward lower salt intake and a more plant-based diet.

Objective

To review iodine intake (not status) in European populations (adults, children, and pregnant women) to identify at-risk groups and dietary sources.

Data sources

PubMed, Embase, and Cochrane databases, as well as European national nutrition surveys were searched for data on had iodine intake (from dietary assessment) and sources of iodine, collected after 2006.

Data selection

In total, 57 studies were included, comprising 22 national surveys and 35 sub-national studies. Iodine intake data were available from national surveys of children aged <10 years (n = 11), 11–17 years (n = 12), and adults (n = 15), but data from pregnancy were only available from sub-national studies.

Results

Iodine intake data are lacking—only 17 of 45 (38%) European countries had iodine-intake data from national surveys. Iodine intake reported from national surveys was below recommendations for: (1) children aged <10 years in 2 surveys (18%), (2) boys and girls aged 11–17 years in 6 (50%) and 8 (68%) surveys, respectively, and (3) adult men and women in 7 (47%) and 12 (80%) surveys, respectively. In pregnant women, intake was below recommendations except where women were taking iodine-containing supplements. Just 32% of national surveys (n = 7) included iodized salt when estimating iodine intake. Milk, dairy products, fish, and eggs were important contributors to intake in many countries, suggesting limited sources in plant-based diets.

Conclusion

Results are limited by the challenges of dietary assessment for measuring iodine intake. Future national surveys should include iodine intake. Policy makers should consider dietary sources alongside any iodized salt policies when considering methods for improving population iodine intake.

Systematic Review Registration

PROSPERO 2017 CRD42017075422.

Selenium status in a Northern Irish pregnant cohort with iodine deficiency

Selenium and iodine are trace elements that are maximally concentrated in the thyroid. Iodine is a substrate for thyroid hormone synthesis, while the selenoproteins protect the thyroid from the oxidative stress incurred. We measured plasma selenium concentration in 241 pregnant women in 1st trimester, previously reported to have iodine deficiency. Mean age was 30.3 years (SD 5.4), BMI 26.2 kg/m² (SD 4.9) and 53% reported taking supplements. Median urinary iodine concentration was 73 μg/L (IQR 37-122) (WHO recommendation, ≥150 μg/L). Mean plasma selenium concentration was 75 µg/L (SD 7.7) which is below the 80-125 µg/L reported to be optimal. Four-day food diaries revealed a selenium intake of 43 µg/day (SD 15.9), also below the 55-70 µg/day reported to be optimal. This is the first report of selenium status in pregnancy on the island of Ireland. The possible combined effects of iodine and selenium deficiencies in pregnancy merit further investigation.

Systematic review and meta-analysis of the effects of iodine supplementation on thyroid function and child neurodevelopment in mildly-to-moderately iodine-deficient pregnant women

BACKGROUND

Mild-to-moderate iodine deficiency, particularly in pregnancy, is prevalent; this is of concern because observational studies have shown negative associations with child neurodevelopment. Although neither the benefits nor the safety of iodine supplementation in pregnancy in areas of mild-to-moderate deficiency are well researched, such supplementation is increasingly being recommended by health authorities in a number of countries.

OBJECTIVES

By reviewing the most recent published data on the effects of iodine supplementation in mildly-to-moderately deficient pregnant women on maternal and infant thyroid function and child cognition, we aimed to determine whether the evidence was sufficient to support recommendations in these areas.

METHODS

A systematic review of randomized controlled trials (RCTs), non-RCT interventions, and observational studies was conducted. To identify relevant articles, we searched the PubMed and Embase databases. We defined mild-to-moderate iodine deficiency as a baseline median urinary iodine concentration (UIC) of 50-149 µg/L. Eligible studies were included in meta-analyses.

RESULTS

In total, 37 publications were included-10 RCTs, 4 non-RCT interventions, and 23 observational studies. Most studies showed no effect of iodine supplementation on maternal or infant thyroid-stimulating hormone and free thyroxine. Most RCTs found that supplementation reduced maternal thyroglobulin and in 3 RCTs, it prevented or diminished the increase in maternal thyroid volume during pregnancy. Three RCTs addressed child neurodevelopment; only 1 was adequately powered. Meta-analyses of 2 RCTs showed no effect on child cognitive [mean difference (MD): -0.18; 95% CI: -1.22, 0.87], language (MD: 1.28; 95% CI: -0.28, 2.83), or motor scores (MD: 0.28; 95% CI: -1.10, 1.66).

CONCLUSIONS

There is insufficient good-quality evidence to support current recommendations for iodine supplementation in pregnancy in areas of mild-to-moderate deficiency. Well-designed RCTs, with child cognitive outcomes, are needed in pregnant women who are moderately deficient (median UIC < 100 µg/L). Maternal intrathyroidal iodine stores should be considered in future trials by including appropriate measures of preconceptional iodine intake.This review was registered at www.crd.york.ac.uk/prospero as CRD42018100277.

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

BACKGROUND

Severe iodine deficiency during pregnancy can cause intellectual disability, presumably through inadequate placental transfer of maternal thyroid hormone to the fetus. The association between mild-to-moderate iodine deficiency and child neurodevelopmental problems is not well understood.

OBJECTIVES

We investigated the association of maternal iodine status during pregnancy with child attention-deficit hyperactivity disorder (ADHD) and autistic traits.

METHODS

This was a collaborative study of 3 population-based birth cohorts: Generation R (n = 1634), INfancia y Medio Ambiente (n = 1293), and the Avon Longitudinal Study of Parents and Children (n = 2619). Exclusion criteria were multiple fetuses, fertility treatment, thyroid-interfering medication use, and pre-existing thyroid disease. The mean age of assessment in the cohorts was between 4.4 and 7.7 y for ADHD symptoms and 4.5 and 7.6 y for autistic traits. We studied the association of the urinary iodine-to-creatinine ratio (UI/Creat) <150 μg/g-in all mother-child pairs, and in those with a urinary-iodine measurement at ≤18 weeks and ≤14 weeks of gestation-with the risk of ADHD or a high autistic-trait score (≥93rd percentile cutoff), using logistic regression. The cohort-specific effect estimates were combined by random-effects meta-analyses. We also investigated whether UI/Creat modified the associations of maternal free thyroxine (FT4) or thyroid-stimulating hormone concentrations with ADHD or autistic traits.

RESULTS

UI/Creat <150 μg/g was not associated with ADHD (OR: 1.2; 95% CI: 0.7, 2.2; P = 0.56) or with a high autistic-trait score (OR: 0.8; 95% CI: 0.6, 1.1; P = 0.22). UI/Creat <150 μg/g in early pregnancy (i.e., ≤18 weeks or ≤14 weeks of gestation) was not associated with a higher risk of behavioral problems. The association between a higher FT4 and a greater risk of ADHD (OR: 1.3; 95% CI: 1.0, 1.6; P = 0.017) was not modified by iodine status.

CONCLUSIONS

There is no consistent evidence to support an association of mild-to-moderate iodine deficiency during pregnancy with child ADHD or autistic traits.

Similarities and differences of dietary and other determinants of iodine status in pregnant women from three European birth cohorts

PURPOSE

As a component of thyroid hormones, adequate iodine intake is essential during pregnancy for fetal neurodevelopment. Across Europe, iodine deficiency is common in pregnancy, but data are lacking on the predictors of iodine status at this life stage. We, therefore, aimed to explore determinants of iodine status during pregnancy in three European populations of differing iodine status.

METHODS

Data were from 6566 pregnant women from three prospective population-based birth cohorts from the United Kingdom (ALSPAC, n = 2852), Spain (INMA, n = 1460), and The Netherlands (Generation R, n = 2254). Urinary iodine-to-creatinine ratio (UI/Creat, µg/g) was measured in spot-urine samples in pregnancy (≤ 18-weeks gestation). Maternal dietary intake, categorised by food groups (g/day), was estimated from food-frequency questionnaires (FFQs). Multivariable regression models used dietary variables (energy-adjusted) and maternal characteristics as predictors of iodine status.

RESULTS

Median UI/Creat in pregnant women of ALSPAC, INMA, and Generation R was 121, 151, and 210 µg/g, respectively. Maternal age was positively associated with UI/Creat in all cohorts (P < 0.001), while UI/Creat varied by ethnicity only in Generation R (P < 0.05). Of the dietary predictors, intake of milk and dairy products (per 100 g/day) was positively associated with UI/Creat in all cohorts [ALSPAC (B = 3.73, P < 0.0001); INMA (B = 6.92, P = 0.002); Generation R (B = 2.34, P = 0.001)]. Cohort-specific dietary determinants positively associated with UI/Creat included fish and shellfish in ALSPAC and INMA, and eggs and cereal/cereal products in Generation R.

CONCLUSIONS

The cohort-specific dietary determinants probably reflect not only dietary habits but iodine-fortification policies; hence, public-health interventions to improve iodine intake in pregnancy need to be country-specific.

Association of Maternal Iodine Status With Child IQ: A Meta-Analysis of Individual Participant Data

CONTEXT

Although the consequences of severe iodine deficiency are beyond doubt, the effects of mild to moderate iodine deficiency in pregnancy on child neurodevelopment are less well established.

OBJECTIVE

To study the association between maternal iodine status during pregnancy and child IQ and identify vulnerable time windows of exposure to suboptimal iodine availability.

DESIGN

Meta-analysis of individual participant data from three prospective population-based birth cohorts: Generation R (Netherlands), INMA (Spain), and ALSPAC (United Kingdom); pregnant women were enrolled between 2002 and 2006, 2003 and 2008, and 1990 and 1992, respectively.

SETTING

General community.

PARTICIPANTS

6180 mother-child pairs with measures of urinary iodine and creatinine concentrations in pregnancy and child IQ. Exclusion criteria were multiple pregnancies, fertility treatment, medication affecting the thyroid, and preexisting thyroid disease.

MAIN OUTCOME MEASURE

Child nonverbal and verbal IQ assessed at 1.5 to 8 years of age.

RESULTS

There was a positive curvilinear association of urinary iodine/creatinine ratio (UI/Creat) with mean verbal IQ only. UI/Creat <150 µg/g was not associated with lower nonverbal IQ (-0.6 point; 95% CI: -1.7 to 0.4 points; P = 0.246) or lower verbal IQ (-0.6 point; 95% CI: -1.3 to 0.1 points; P = 0.082). Stratified analyses showed that the association of UI/Creat with verbal IQ was only present up to 14 weeks of gestation.

CONCLUSIONS

Fetal brain development is vulnerable to mild to moderate iodine deficiency, particularly in the first trimester. Our results show that potential randomized controlled trials investigating the effect of iodine supplementation in women with mild to moderate iodine deficiency on child neurodevelopment should begin supplementation not later than the first trimester.

Maternal Thyroid Function in Early Pregnancy and Child Attention-Deficit Hyperactivity Disorder: An Individual-Participant Meta-Analysis

Background: Thyroid hormone is essential for optimal fetal brain development. Evidence suggests that both low and high maternal thyroid hormone availability may have adverse effects on child neurodevelopmental outcomes, but the effect on behavioral problems remains unclear. We studied the association of maternal thyrotropin (TSH) and free thyroxine (fT4) concentrations during the first 18 weeks of pregnancy with child attention-deficit hyperactivity disorder (ADHD). Methods: A total of 7669 mother-child pairs with data on maternal thyroid function and child ADHD were selected from three prospective population-based birth cohorts: INfancia y Medio Ambiente (INMA; N = 1073, Spain), Generation R (N = 3812, The Netherlands), and Avon Longitudinal Study of Parents and Children (ALSPAC; N = 2784, United Kingdom). Exclusion criteria were multiple pregnancy, fertility treatment, usage of medication affecting the thyroid, and pre-existing thyroid disease. We used logistic regression models to study the association of maternal thyroid function with the primary outcome, ADHD, assessed via the Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM-IV) criteria by parents and/or teachers at a median child age of 4.5 to 7.6 years, and with the secondary outcome, an ADHD symptom score above the 90th percentile. Effect modification by gestational age and sex was tested with interaction terms and stratified analyses. Results: Overall, 233 (3%) children met the criteria for ADHD. When analyzed continuously, neither fT4 nor TSH was associated with a higher risk of ADHD (odds ratio [OR] 1.1, 95% confidence interval [CI 1.0-1.3], p = 0.060 and OR 0.9 [CI 0.9-1.1], p = 0.385, respectively) or with high symptom scores. When investigating effect modification by gestational age, a higher fT4 was associated with symptoms above the 90th percentile but only in the first trimester (for fT4 per 1 SD: OR 1.2 [CI 1.0-1.4], p = 0.027). However, these differential effects by gestational age were not consistent. No significant effect modification by sex was observed. Conclusions: We found no clear evidence of an association between maternal thyroid function and child ADHD.

Iodine status of teenage girls on the island of Ireland

PURPOSE

The trace element iodine is a vital constituent of thyroid hormones. Iodine requirements increase during pregnancy, when even mild deficiency may affect the neurocognitive development of the offspring. Urinary iodine concentration (UIC) is the means of assessing iodine status in population surveys; a median UIC of 100-199 µg/L is deemed sufficient in a non-pregnant population. Milk is the main dietary source of iodine in the UK and Ireland.

METHODS

We surveyed the iodine status of 903 girls aged 14-15 years in seven sites across the island of Ireland. Urine iodine concentration was measured in spot-urine samples collected between March 2014 and October 2015. Food group intake was estimated from iodine-specific food-frequency questionnaire. Milk-iodine concentration was measured at each site in summer and winter.

RESULTS

The median UIC overall was 111 µg/L. Galway was the only site in the deficient range (median UIC 98 µg/L). All five of the Republic of Ireland sites had UIC ≤ 105 µg/L. In the two sites surveyed twice, UIC was lower in summer vs winter months [117 µg/L (IQR 76-165) vs 130 µg/L (IQR 91-194) (p < 0.01)]. Milk samples collected from Galway and Roscommon had a lower mean iodine concentration than those from Derry/Londonderry (p < 0.05). Milk intake was positively associated with UIC (p < 0.001).

CONCLUSIONS

This is the largest survey of its kind on the island of Ireland, which currently has no iodine-fortification programme. Overall, the results suggest that this young female population sits at the low end of sufficiency, which has implications if, in future, they enter pregnancy with borderline status.

Thyroid Function in Early Pregnancy, Child IQ, and Autistic Traits: A Meta-Analysis of Individual Participant Data

CONTEXT

Low maternal free T4 (FT4) has been associated with poor child neurodevelopment in some single-center studies. Evidence remains scarce for the potential adverse effects of high FT4 and whether associations differ in countries with different iodine status.

OBJECTIVE

To assess the association of maternal thyroid function in early pregnancy with child neurodevelopment in countries with a different iodine status.

DESIGN, SETTING, AND PARTICIPANTS

Meta-analysis of individual participant data from 9036 mother-child pairs from three prospective population-based birth cohorts: INMA [Infancia y Medio Ambiente (Environment and Childhood project) (Spain)], Generation R (Netherlands), and ALSPAC (Avon Longitudinal Study of Parents and Children, United Kingdom). The exclusion criteria were multiple pregnancies, fertility treatments, thyroid-interfering medication usage, and known thyroid disease.

MAIN OUTCOMES

Child nonverbal IQ at 5 to 8 years of age, verbal IQ at 1.5 to 8 years of age, and autistic traits within the clinical range at 5 to 8 years of age.

RESULTS

FT4 <2.5th percentile was associated with a 3.9-point (95% CI, -5.7 to -2.2) lower nonverbal IQ and a 2.1-point (95% CI, -4.0 to -0.1) lower verbal IQ. A suggestive association of hypothyroxinemia with a greater risk of autistic traits was observed. FT4 >97.5th percentile was associated with a 1.9-fold (95% CI, 1.0 to 3.4) greater risk of autistic traits. No independent associations were found with TSH.

CONCLUSIONS

Low maternal FT4 was consistently associated with a lower IQ across the cohorts. Further studies are needed to replicate the findings of autistic traits and investigate the potential modifying role of maternal iodine status. FT4 seems a reliable marker of fetal thyroid state in early pregnancy, regardless of the type of immunoassay.

Iodine Status during Pregnancy in a Region of Mild-to-Moderate Iodine Deficiency is not Associated with Adverse Obstetric Outcomes; Results from the Avon Longitudinal Study of Parents and Children (ALSPAC)

Severe iodine deficiency during pregnancy has been associated with pregnancy/neonatal loss, and adverse pregnancy outcomes; however, the impact of mild–to–moderate iodine insufficiency, though prevalent in pregnancy, is not well-documented. We assessed whether mild iodine deficiency during pregnancy was associated with pregnancy/infant loss, or with other adverse pregnancy outcomes. We used samples and data from the Avon Longitudinal Study of Parents and Children (ALSPAC), from 3140 singleton pregnancies and from a further 42 women with pregnancy/infant loss. The group was classified as mildly-to-moderately iodine deficient with a median urinary iodine concentration of 95.3 µg/L (IQR 57.0–153.0; median urinary iodine-to-creatinine ratio (UI/Creat) 124 µg/g, IQR 82–198). The likelihood of pregnancy/infant loss was not different across four UI/Creat groups (<50, 50–149, 150–250, >250 µg/g). The incidence of pre-eclampsia, non-proteinuric gestational hypertension, gestational diabetes, glycosuria, anaemia, post-partum haemorrhage, preterm delivery, mode of delivery, being small for gestational age, and large for gestational age did not differ significantly among UI/Creat groups, nor were there any significant differences in the median UI/Creat. We conclude that maternal iodine status was not associated with adverse pregnancy outcomes in a mildly-to-moderately iodine-deficient pregnant population. However, in view of the low number of women with pregnancy/infant loss in our study, further research is required.

Cow Milk Consumption Increases Iodine Status in Women of Childbearing Age in a Randomized Controlled Trial

Background

Recent evidence has highlighted the prevalence of mild-to-moderate iodine deficiency in women of childbearing age and pregnant women, with important public health ramifications due to the role of iodine, which is required for thyroid hormone production, in neurodevelopment. Cow milk contributes the greatest amount to iodine intakes in several countries.

Objective

The objective of this study was to investigate the effect of increased cow milk consumption on iodine status, thyroid hormone concentrations, and selenium status.

Methods

A 12-wk randomized controlled trial was conducted in 78 low-moderate milk-consuming (<250 mL/d) healthy women (aged 18-45 y). The intervention group was asked to consume 3 L semiskimmed milk/wk, whereas the control group continued their usual milk consumption (baseline median: 140 mL/d; IQR: 40-240 mL/d). At baseline and weeks 6 and 12, participants provided a spot urine sample [urinary iodine concentration (UIC), creatinine] and a fasting blood sample (thyroid hormone concentrations, serum total selenium, selenoprotein P).

Results

At baseline, the median (IQR) UIC of all participants was 78.5 µg/L (39.1-126.1 µg/L). Changes in the median UIC from baseline to week 6 (35.4 compared with 0.6 µg/L; P = 0.014) and week 12 (51.6 compared with -3.8 µg/L; P = 0.045) were significantly greater in the intervention group compared with the control group. However, despite being higher within the intervention group at weeks 6 and 12, the change in the iodine:creatinine ratio from baseline was not significantly different between groups at either week 6 (P = 0.637) or week 12 (P = 0.178). There were no significant differences in thyroid hormone concentrations or selenium status between groups at any time point.

Conclusions

The present study shows that the consumption of additional cow milk can significantly increase UIC in women of childbearing age. These results suggest that cow milk is a potentially important dietary source of iodine in this population group. This trial was registered at www.clinicaltrials.gov as NCT02767167.

Iodine concentration of milk-alternative drinks available in the UK in comparison with cows' milk

Iodine deficiency is present in certain groups of the UK population, notably in pregnant women; this is of concern as iodine is required for fetal brain development. UK milk is rich in iodine and is the principal dietary iodine source. UK sales of milk-alternative drinks are increasing but data are lacking on their iodine content. As consumers may replace iodine-rich milk with milk-alternative drinks, we aimed to measure the iodine concentration of those available in the UK. Using inductively coupled plasma-MS, we determined the iodine concentration of seven types of milk-alternative drink (soya, almond, coconut, oat, rice, hazelnut and hemp) by analysing forty-seven products purchased in November/December 2015. For comparison, winter samples of conventional (n 5) and organic (n 5) cows' milk were included. The median iodine concentration of all of the unfortified milk-alternative drinks (n 44) was low, at 7·3 μg/kg, just 1·7 % of our value for winter conventional cows' milk (median 438 μg/kg). One brand (not the market leader), fortified its soya, oat and rice drinks with iodine and those drinks had a higher iodine concentration than unfortified drinks, at 280, 287 and 266 μg/kg, respectively. The iodine concentration of organic milk (median 324 μg/kg) was lower than that of conventional milk. Although many milk-alternative drinks are fortified with Ca, at the time of this study, just three of forty-seven drinks were fortified with iodine. Individuals who consume milk-alternative drinks that are not fortified with iodine in place of cows' milk may be at risk of iodine deficiency unless they consume alternative dietary iodine sources.

Thyroglobulin as a Functional Biomarker of Iodine Status in a Cohort Study of Pregnant Women in the United Kingdom

BACKGROUND

Though iodine deficiency in pregnancy is a matter of public-health concern, a functional measure of iodine status is lacking. The thyroid-specific protein thyroglobulin (Tg), which reflects thyroid size, has shown promise as a functional measure in studies of children and adults, but data in pregnancy are sparse. In a cohort of mildly to moderately iodine-deficient pregnant women, this study aimed to explore whether serum Tg is a sensitive functional biomarker of iodine status and to examine longitudinal change in Tg with gestational age.

METHOD

A total of 230 pregnant women were recruited at an antenatal clinic at 12 weeks of gestation to the Selenium in PRegnancy INTervention study, in Oxford, United Kingdom. Repeated measures of urinary iodine-to-creatinine ratio, serum thyrotropin (TSH), and Tg at 12, 20, and 35 weeks of gestation were made. Women were dichotomized by their iodine-to-creatinine ratio (<150 or ≥150 μg/g) to group them broadly as iodine deficient or iodine sufficient. Women with thyroid antibodies were excluded; data and samples were available for 191 women.

RESULTS

Median Tg concentrations were 21, 19, and 23 μg/L in the first, second, and third trimesters, respectively. In a linear mixed model, controlling for confounders, Tg was higher in the <150 μg/g group than it was in the ≥150 μg/g group (p < 0.001) but there was no difference in TSH (p = 0.27). Gestational week modified the effect of iodine status on TSH (p = 0.01) and Tg (p = 0.012); Tg did not increase with gestational week in the ≥150 μg/g group, but it did in the <150 μg/g group, and TSH increased more steeply in the <150 μg/g group.

CONCLUSIONS

Low iodine status (<150 μg/g) in pregnancy is associated with higher serum Tg, suggesting that the thyroid is hyperstimulated by iodine deficiency, which causes it to enlarge. Tg is a more sensitive biomarker of iodine status in pregnancy than is TSH.

A multi-centre pilot study of iodine status in UK schoolchildren, aged 8-10 years

PURPOSE

Iodine, as an essential constituent of thyroid hormones, is required for brain development. Iodine status is low in some UK population groups, notably in teenage girls, women of childbearing age and pregnant women. We aimed to assess iodine status of UK schoolchildren as there are no data on children below 14 years of age.

METHODS

Children (boys and girls) aged 8-10 years were recruited to a cross-sectional study from schools in three areas of the UK (Omagh, Northern Ireland; Glasgow, Scotland, and Guildford, South-East England). Spot urine samples, for measurement of urinary iodine concentration, were collected in the winter months (November 2012 to March 2013) and in the summer, in Omagh only (September 2013). A food frequency questionnaire was completed.

RESULTS

A total of 168 schoolchildren provided 165 urine samples. The median urinary iodine concentration was 161 µg/L in winter samples (n = 134) and 127 µg/L in summer samples (n = 31). The median urinary iodine concentration for the whole group was 144 µg/L, weighted to account for the unequal proportion of samples from the two seasons. The children are classified as iodine-sufficient by WHO criteria (100-199 µg/L), even in the summer. Milk intake was positively associated with iodine status.

CONCLUSIONS

This pilot study suggests that iodine deficiency is unlikely to be a problem in UK children aged 8-10 years. This could be a result of higher intake of milk, the principal UK dietary iodine source, in this age group than in teenagers and adults. Further assessment of iodine status in a representative sample of UK schoolchildren is required.

The new emergence of iodine deficiency in the UK: consequences for child neurodevelopment

Adequate iodine intake is important during pregnancy as it is a component of the thyroid hormones that are crucial for fetal brain and neurological development. While randomized controlled trials in severe iodine deficiency have shown that iodine deficiency in pregnancy causes impaired offspring cognition, less is known of the effects in regions of mild/mild-to-moderate deficiency. The United Kingdom is now classified as mildly iodine deficient by the World Health Organization, based on a 2011 national study of 14-15-year-old schoolgirls. As pregnancy is the most critical time for brain development, we evaluated iodine status in pregnant women in Surrey (n = 100) and Oxford (n = 230). The median urinary iodine concentration was 85.3 μg/L in Surrey women, considerably lower than the WHO/United Nations Children's Fund/International Council for the Control of Iodine Deficiency Disorders cut-off of 150 μg/L. Oxford women had similarly low status. We investigated whether that level of iodine deficiency was associated with adverse child cognitive effects using stored samples and data from the Avon Longitudinal Study of Parents and Children cohort. In adjusted analyses, we found a significant association between low maternal iodine status in early pregnancy (urinary iodine-to-creatinine ratio <150 μg/g) such that children had an approximately 60% greater risk of being in the bottom quartile of scores for verbal intelligence quotient, reading accuracy and comprehension. UK women who might become pregnant should ensure they have adequate iodine status to avoid compromising their children's brain development.

A review of the iodine status of UK pregnant women and its implications for the offspring

Iodine, as a component of the thyroid hormones, is crucial for brain development and is therefore especially important during pregnancy when the brain is developing most rapidly. While randomised controlled trials of pregnant women in regions of severe iodine deficiency have shown that prenatal iodine deficiency causes impaired cognition, less is known of the effects in regions of mild deficiency. This is relevant to the UK as the World Health Organisation now classifies the UK as mildly iodine deficient, based on a national study of 14-15 year old schoolgirls in 2011. We have previously published a study using samples and data from the UK-based Avon Longitudinal Study of Parents and Children (ALSPAC) that found an association between low iodine status in early pregnancy (urinary iodine-to-creatinine ratio <150 μg/g) and lower verbal IQ and reading scores in the offspring. Though the women in ALSPAC were recruited in the early 1990s, the results of the study are still relevant as their iodine status was similar to that reported in recent studies of UK pregnant women. This review discusses the evidence that mild-to-moderate iodine deficiency during pregnancy has deleterious effects on child neurodevelopment and relates that evidence to the data on iodine status in the UK. It has highlighted a need for nationwide data on iodine status of pregnant women and that a randomised controlled trial of iodine supplementation in pregnant women in a region of mild-to-moderate iodine deficiency with child outcomes as the primary endpoint is required.

Gestational changes in iodine status in a cohort study of pregnant women from the United Kingdom: season as an effect modifier

BACKGROUND

Iodine is required throughout pregnancy for thyroid hormone production, which is essential for fetal brain development. Studies of iodine status in pregnant women from the United Kingdom (UK) have focused on early gestation (<16 wk). Data on the effect of advancing gestation on urinary iodine excretion are conflicting, with suggestions of both an increase and a decrease.

OBJECTIVES

The aims were to evaluate iodine status in a cohort of UK pregnant women and to explore how it changes throughout gestation.

DESIGN

We used samples and data from 230 UK pregnant women who were recruited to the Selenium in PRegnancy INTervention study. Iodine concentration was measured in spot-urine samples that were collected at ∼12, 20, and 35 wk of gestation; creatinine concentration was also measured to correct for urine dilution. A linear mixed model was used to explore the effect of gestational week on iodine-to-creatinine ratio, with change in season, body mass index, daily milk intake, and maternal age controlled for.

RESULTS

The median urinary iodine concentration from urine samples collected at all time points (n = 662) was 56.8 μg/L, and the iodine-to-creatinine ratio was 116 μg/g, thus classifying this cohort as mildly-to-moderately iodine deficient. The median iodine-to-creatinine ratios at 12, 20, and 35 wk were 102.5, 120.0, and 126.0 μg/g, respectively. Only 3% of women were taking iodine-containing prenatal supplements. The iodine-to-creatinine ratio increased with advancing gestation, and there was a significant interaction between gestational week and season (P = 0.026). For a 1-wk increase in gestation, the iodine-to-creatinine ratio increased by a factor of 1.05 (95% CI: 1.02, 1.08) in winter and by a factor of 1.04 (95% CI: 1.00, 1.08) in summer.

CONCLUSIONS

This group of UK pregnant women was mildly-to-moderately iodine deficient at all trimesters, which is of public health concern. The finding that the iodine-to-creatinine ratio increased over the course of gestation may not be generalizable to populations with different iodine status from ours and merits further investigation. This trial was registered at www.isrctn.com as ISRCTN37927591.

Selenium status in U.K. pregnant women and its relationship with hypertensive conditions of pregnancy

Dietary intake/status of the trace mineral Se may affect the risk of developing hypertensive conditions of pregnancy, i.e. pre-eclampsia and pregnancy-induced hypertension (PE/PIH). In the present study, we evaluated Se status in U.K. pregnant women to establish whether pre-pregnant Se status or Se supplementation affected the risk of developing PE/PIH. The samples originated from the SPRINT (Selenium in PRegnancy INTervention) study that randomised 230 U.K. primiparous women to treatment with Se (60 μg/d) or placebo from 12 weeks of gestation. Whole-blood Se concentration was measured at 12 and 35 weeks, toenail Se concentration at 16 weeks, plasma selenoprotein P (SEPP1) concentration at 35 weeks and plasma glutathione peroxidase (GPx3) activity at 12, 20 and 35 weeks. Demographic data were collected at baseline. Participants completed a FFQ. U.K. pregnant women had whole-blood Se concentration lower than the mid-range of other populations, toenail Se concentration considerably lower than U.S. women, GPx3 activity considerably lower than U.S. and Australian pregnant women, and low baseline SEPP1 concentration (median 3.00, range 0.90-5.80 mg/l). Maternal age, education and social class were positively associated with Se status. After adjustment, whole-blood Se concentration was higher in women consuming Brazil nuts (P= 0.040) and in those consuming more than two seafood portions per week (P= 0.054). A stepwise logistic regression model revealed that among the Se-related risk factors, only toenail Se (OR 0.38, 95% CI 0.17, 0.87, P= 0.021) significantly affected the OR for PE/PIH. On excluding non-compliers with Se treatment, Se supplementation also significantly reduced the OR for PE/PIH (OR 0.30, 95% CI 0.09, 1.00, P= 0.049). In conclusion, U.K. women have low Se status that increases their risk of developing PE/PIH. Therefore, U.K. women of childbearing age need to improve their Se status.

Iodine intake and status of UK women of childbearing age recruited at the University of Surrey in the winter

As intra-thyroidal iodine stores should be maximised before conception to facilitate the increased thyroid hormone production during pregnancy, women who are planning to become pregnant should ideally consume 150 μg iodine/d (US RDA). As few UK data exist for this population group, a cross-sectional study was carried out at the University of Surrey to assess the iodine intake and status of women of childbearing age. Total iodine excretion was measured from 24 h urine samples in fifty-seven women; iodine intake was estimated by assuming that 90 % of ingested iodine was excreted. The average iodine intake was also estimated from 48 h food diaries that the participants completed. The median urinary iodine concentration value (63·1 μg/l) indicated the group to be mildly iodine deficient by WHO criteria. By contrast, the median 24 h urinary iodine excretion value (149·8 μg/24 h) indicated a relatively low risk of iodine deficiency. The median estimated iodine intake, extrapolated from urinary excretion, was 167 μg/d, whereas it was lower, at 123 μg/d, when estimated from the 48 h food diaries. Iodine intake estimated from the food diaries and 24 h urinary iodine excretion were strongly correlated (r 0·75, P< 0·001). The intake of milk, eggs and dairy products was positively associated with iodine status. The iodine status of this UK cohort is probably a best-case scenario as the women were mostly nutrition students and were recruited in the winter when milk-iodine content is at its highest; further study in more representative cohorts of UK women is required. The present study highlights the need for revised cut-off values for iodine deficiency that are method- and age group-specific.

Availability of iodised table salt in the UK - is it likely to influence population iodine intake?

OBJECTIVE

Iodine deficiency has recently been found in UK young and pregnant women, which is of concern given the importance of adequate iodine intake in pregnancy for fetal brain development. The WHO recommends that iodine deficiency in a population should be corrected through salt iodisation but there is a lack of UK data on iodised-salt availability, a situation that the present study aimed to address.

DESIGN

Availability of iodised salt for household use was determined by a shelf survey in five supermarket chains in each of sixteen UK areas (in Southern England, Wales and Northern Ireland) encompassing a total of seventy-seven supermarkets. All branches of a sixth supermarket chain that had 2·3% of the market share sold exclusively iodised salt. Weighted iodised-salt availability was calculated taking the market share of supermarkets into account.

SETTING

The UK.

SUBJECTS

Not applicable.

RESULTS

Iodised salt was available in thirty-two of the seventy-seven supermarkets (41·6%). After accounting for market share and including all six UK supermarket chains, the weighted availability of iodised salt was 21·5%. The iodine concentration of the major UK brand of iodised salt is low, at 11·5 mg/kg.

CONCLUSIONS

In contrast to other countries, iodised household table salt is unlikely to contribute meaningful amounts to UK iodine intake as (i) availability is low, (ii) table salt is only a small percentage of total UK salt intake and (iii) UK public-health campaigns have encouraged reduced salt consumption. As iodine intake in the UK is dependent entirely on food choices, regular monitoring of iodine status is essential.

Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC)

BACKGROUND

As a component of thyroid hormones, iodine is essential for fetal brain development. Although the UK has long been considered iodine replete, increasing evidence suggests that it might now be mildly iodine deficient. We assessed whether mild iodine deficiency during early pregnancy was associated with an adverse effect on child cognitive development.

METHODS

We analysed mother-child pairs from the Avon Longitudinal Study of Parents and Children (ALSPAC) cohort by measuring urinary iodine concentration (and creatinine to correct for urine volume) in stored samples from 1040 first-trimester pregnant women. We selected women on the basis of a singleton pregnancy and availability of both a urine sample from the first trimester (defined as ≤13 weeks' gestation; median 10 weeks [IQR 9-12]) and a measure of intelligence quotient (IQ) in the offspring at age 8 years. Women's results for iodine-to-creatinine ratio were dichotomised to less than 150 μg/g or 150 μg/g or more on the basis of WHO criteria for iodine deficiency or sufficiency in pregnancy. We assessed the association between maternal iodine status and child IQ at age 8 years and reading ability at age 9 years. We included 21 socioeconomic, parental, and child factors as confounders.

FINDINGS

The group was classified as having mild-to-moderate iodine deficiency on the basis of a median urinary iodine concentration of 91·1 μg/L (IQR 53·8-143; iodine-to-creatinine ratio 110 μg/g, IQR 74-170). After adjustment for confounders, children of women with an iodine-to-creatinine ratio of less than 150 μg/g were more likely to have scores in the lowest quartile for verbal IQ (odds ratio 1·58, 95% CI 1·09-2·30; p=0·02), reading accuracy (1·69, 1·15-2·49; p=0·007), and reading comprehension (1·54, 1·06-2·23; p=0·02) than were those of mothers with ratios of 150 μg/g or more. When the less than 150 μg/g group was subdivided, scores worsened ongoing from 150 μg/g or more, to 50-150 μg/g, to less than 50 μg/g.

INTERPRETATION

Our results show the importance of adequate iodine status during early gestation and emphasise the risk that iodine deficiency can pose to the developing infant, even in a country classified as only mildly iodine deficient. Iodine deficiency in pregnant women in the UK should be treated as an important public health issue that needs attention.

FUNDING

None.