Breast Milk Iodine Concentration Is a More Accurate Biomarker of Iodine Status Than Urinary Iodine Concentration in Exclusively Breastfeeding Women

Do Postpartum Maternal Iodine Status or Supplementation Affect Thyroid Function After Delivery? A Systematic Review and Meta-Analysis

The aim of this systematic review and meta-analysis was, for the first time, to explore whether postpartum maternal iodine status or supplementation is associated with thyroid function after delivery. The MEDLINE/PubMed, Web of Science, Embase, and Scopus were searched up to December 2021 to identify relevant studies. The pooled mean thyroid stimulating hormone (TSH), free thyroxine (fT4), and thyroxine (T4) concentrations and 95% confidence intervals (CIs) were estimated based on maternal urinary iodine concentration (UIC) (< 50, 50-100, 100-200, and > 200 µg/L) or breast milk iodine concentration (BMIC) (< 100 µg/L vs. ≥ 100 µg/L) during postpartum. A fixed/random effects model was used based on the absence/presence of heterogeneity, respectively. The study is registered with PROSPERO, number CRD42022336145. A total of 2175 studies were identified, of which 18 were eligible for the meta-analysis. The pooled values for TSH, fT4, and T4 concentrations in all subgroups were within the normal range; however, except for TSH, comparing the 95% CI showed no statistically significant difference among different subgroups. The pooled mean for TSH concentration in women with UIC > 200 µg/L was 2.23 mIU/L, whereas the corresponding values in women with UIC < 50, 50-100 and 100-200 µg/L were 0.56, 0.56 and 0.95 mIU/L, respectively. Thyroid hormones in women with BMIC < 100 µg/L and ≥ 100 µg/L were within the normal range. Iodine supplementation during postpartum was not associated with any differences in thyroid parameters, compared to non-supplemented women. In conclusion, iodine status or supplementation had no effect on thyroid hormones in postpartum women.

The Assessment of Iodine Concentrations in Colostrum and Breast Milk Using ICP-MS: The Impact of Delivery Type, Thyroid Function and Gestational Diabetes-A Pilot Study

Considering the spectrum of benefits of breast milk feeding, determining the essential components of an infant's only food-mother's milk-seems justified, especially in the case of those whose deficiency (e.g., iodine) may result in developmental disorders. The main aim of this study was the determination of the total iodine content of breast milk (including colostrum and mature milk). A secondary objective was to assess the influence of factors such as the type of delivery, hypothyroidism, gestational diabetes or the stage of lactation on this parameter. The study materials were colostrum and milk after 1 (n = 14), 2 and 3 months (n = 8) of lactation with a range of iodine concentrations (µg/L): 195-1648 and 170-842, 174-650 and 273-751, respectively. Iodine was determined using the inductively coupled plasma mass spectrometry (ICP-MS). Multivariate statistical analysis revealed, e.g., that delivery by caesarean section or dose of L-thyroxine taken by women to normalise thyroid hormones, had a significant effect on iodine concentrations in breast milk. Further research aimed at assessing the quality of breast milk should also include determining the factors influencing it.

Iodine Metabolism in Urine and Breast Milk among Lactating Women with Adequate Iodine

BACKGROUND

In lactating women, iodine metabolism is regulated and maintained by the kidneys and mammary glands. Limited research exists on how iodine absorbed by lactating women is distributed between the kidneys and breasts.

OBJECTIVES

This study aimed to accurately evaluate the total iodine intake (TII), urinary iodine excretion (UIE), and breast milk iodine excretion (BMIE) in lactating women and explore the relationship between TII and total iodine excretion (TIE).

METHODS

A 7-d iodine metabolism study was conducted on 41 lactating women with a mean age of 30 y in Yuncheng and Gaoqing, China, from December 2021 to August 2023. TII and TIE were calculated by measuring the iodine content in food, water, 24-h urine, feces, and breast milk. The urinary iodine excretion rate (UIER), breast milk iodine excretion rate (BMIER), and partitioning of iodine excretion between urine and breast milk were determined.

RESULTS

Iodine metabolism studies were performed for 285 d. The median TII and TIE values were 255 and 263 μg/d, respectively. With an increase in TII, UIER, and BMIER, the UIE and BMIE to TII ratio exhibited a downward trend. The median UIER, BMIER, and proportion of iodine excreted in urine and breast milk were 51.5%, 38.5%, 52%, and 37%, respectively. When the TII was <120 μg/d, the BMIER decreased with the increase of the TII (β: -0.90; 95% confidence interval: -1.08, -0.72).

CONCLUSIONS

When maternal iodine intake is low, the proportion in breast milk increases, ensuring sufficient iodine nutrition for infants. In addition, the UIE of lactating women with adequate iodine concentrations is higher than their BMIE. This study was registered at clinicaltrials.gov as NCT04492657.

Determination of the Elemental Iodine in Human Breast Milk by Inductively Coupled Plasma mass Spectrometry

Adequate iodine status in lactating women is defined by a maternal median urinary iodine concentration (UIC) ≧ 100 µg/L. However, the above-mentioned criterion does not account for the secretion of iodine into breast milk and could not truly reflect the amount of iodine delivered to the infants. Measuring breast milk median iodine concentration (BMIC) is crucial, but the method to measure BMIC has not been developed and validated in Taiwan. We adopted the ammonia dilution method without prior sample digestion to measure BMIC by inductively coupled plasma mass spectrometry (ICP-MS). Samples and iodate calibrators were prepared into an aqueous solution containing Triton X-100, 0.5% ammonia solution, and tellurium (128Te) as the internal standard. Precision, accuracy, serial dilution, and recovery tests were performed for method validation. The range of intra-assay and inter-assay coefficient of variation for the four human breast milk samples with different iodine concentrations were 3.2-4.7% and 2.3-5.5%, respectively. The standard NIST 1549 milk powder was prepared into three different concentrations of 50 µg/L, 100 µg/L, and 200 µg/L to assess the accuracy; the bias was < 5%. A recovery of 95-105% was achieved for four human breast milk samples spiked with sodium iodide solution. The serial dilution test confirmed linearity up to 0.998. The limit for detection and quantification was 0.78 µg/L and 2.34 µg/L, respectively. The results of the current study confirmed that this ICP-MS method is accurate and reliable in measuring BMIC.

The use of breast milk iodine concentration in the first week of lactation as a biomarker of iodine status in breast-feeding women

Breast milk iodine concentration (BMIC) is a promising indicator of iodine status in lactating women. However, there are limited data on its usefulness to reflect maternal iodine deficiency. Therefore, the aim of our study was to assess iodine concentration in breast milk and urine samples in exclusively breast-feeding women. Eligible pregnant women undergoing routine antenatal care in a large hospital in Shaanxi Province, China, were followed up from the third trimester of pregnancy until the first week of lactation. Urine samples (20 ml) were collected during pregnancy and lactation. Iodine concentration in samples was measured based on Sandell-Kolthoff reaction. Breast milk samples (5 ml) were provided during lactation. A receiver operating curve (ROC) was constructed to determine the diagnostic performance of BMIC. An iodine-specific FFQ was completed twice during pregnancy and lactation. A total of 200 women completed the study. The overall median BMIC was 89 μg/l, indicating iodine sufficiency (i.e. BMIC reference range between 60 and 465 μg/l). Women reported similar median urinary iodine concentration (UIC) during pregnancy and lactation (112 and 113 μg/l, respectively), but their iodine status differed - mild-to-moderate iodine deficiency during pregnancy and iodine sufficiency during lactation. The ROC for BMIC using UIC as a reference standard was 0·755 (95 % CI: 0·644, 0·866). In conclusion, this study demonstrated that women were iodine sufficient in the first week of lactation as assessed by UIC, which was consistent with BMIC. These findings suggested that BMIC is a useful biomarker to assess iodine status in lactating women.

Iodine Concentration in the Breast Milk and Urine as Biomarkers of Iodine Nutritional Status of Lactating Women and Breastfed Infants in Taiwan

Breast milk iodine concentration (BMIC) can be different when median urinary iodine concentration (UIC) is similar. The BMIC, UIC/creatinine (Cr), estimated 24-h urinary iodine excretion (24-h UIE) of lactating women in Taiwan is unknown. This study enrolled lactating women from Taipei Veterans General Hospital (August 2021-February 2023). Each participant provided a random spot urine sample, two breast milk samples, a blood sample, and completed a food frequency questionnaire on the same day. Iodine measurement was performed by inductively coupled plasma mass spectrometry. The median UIC of the enrolled 71 women was 91.1 μg/L, indicating insufficient iodine status; however, the median BMIC was 166.6 μg/L and this suggested that the amount of iodine delivered through breast milk was adequate for the breastfed infants. BMIC was correlated with UIC/Cr and 24-h UIE (both rs = 0.49) but not with UIC (rs = 0.18) or thyroid stimulating hormone (rs = 0.07). Women who did not consume dairy products (adjusted odds ratio: 24.41, 95% confidence interval: 1.26-471.2) and multivitamins (adjusted odds ratio: 8.26, 95% confidence interval: 1.76-38.79) were at increased odds for having lower BMIC. The results suggest that measuring maternal UIC alone may not be sufficient, as BMIC, UIC/Cr, and 24-h UIE are all important biomarkers. Ingestion of dairy products and multivitamins were independently associated with BMIC.

Relationship between iodine knowledge and dietary iodine intake in pregnant and lactating women: a cross-sectional study

OBJECTIVE

This study assessed the iodine knowledge of pregnant and lactating women and the relationship to dietary iodine intake and iodine status. The factors influencing iodine intake were analysed.

DESIGN

Basic information and iodine knowledge were collected via a questionnaire. A FFQ assessed dietary iodine intake. The urinary iodine concentration (UIC) was measured using the arsenic-cerium catalytic spectrophotometric determination of iodine in urine (WS/T 107 -2016).

SETTING

A cross-sectional study involving pregnant and lactating women in Xinjiang, China was conducted.

PARTICIPANTS

A total of 1181 pregnant women and 504 lactating women were enrolled in the study.

RESULTS

The median UIC for pregnant and lactating women was 179·27 and 192·81 µg/l, respectively, and the dietary iodine intake was 407·16 and 356·89 µg/d, respectively. Of the pregnant and lactating women, 73·4 % and 82·5 % had medium iodine knowledge, respectively. In pregnant women, iodine knowledge and dietary iodine intake were positively correlated. High iodine knowledge and iodine education were shown to be protective factors for excessive iodine intake in pregnant women.

CONCLUSION

This study demonstrated that the iodine nutritional status of women in Xinjiang was appropriate, and iodine knowledge was at a medium level, but there was confusion about iodine nutrition. Public education is needed to improve iodine knowledge and active iodine supplementation awareness among these populations of women.

Iodine Metabolism and Thyroid Function During the Perinatal Period: Maternal-Neonatal Correlation and Effects of Topical Povidone-Iodine Skin Disinfectants

An adequate maternal iodine intake during pregnancy and lactation is essential for growth and mental development in fetuses and newborns. There are limited data on perinatal iodine metabolism in mothers and infants, as well as the effect of povidone-iodine (PVP-I) antiseptics used in cesarean delivery. The urinary iodine concentration (UIC), serum iodine, thyrotropin (TSH), free thyroxine (FT4), and breast milk iodine concentration (BMIC) were measured consecutively in a total of 327 mothers and 249 term-infants in two prospective studies. The maternal median UIC was 164 μg/L in the third trimester, increased to 256 μg/L at 44 h after birth, and then decreased to 116 μg/L 1 month later. The BMIC on the 4th and 32th postpartum days was 17.6 and 13.5 μg/100 g, respectively. In neonatal infants born to the mothers unexposed to PVP-I, the median UIC was 131 μg/L in the first voiding urine and increased to 272 μg/L on day 4 and then slightly decreased to 265 μg/L on day 28 suggesting sufficient iodine reserve at birth. PVP-I antiseptics containing 1 g of iodine for skin preparation at cesarean delivery transiently increased maternal serum iodine concentration (1.9-fold), UIC (7.8-fold) at 41 h after surgery and BMIC, while it had little effect on maternal TSH, FT4, and neonatal UIC, TSH, or FT4. The iodine status of pregnant women and their infants was adequate in this population; however, the UIC in lactating mothers at one postpartum month was low enough to suggest iodine deficiency or near iodine deficiency. Further studies are necessary.

Estimation of appropriate dietary intake of iodine among lactating women in China based on iodine loss in breast milk

OBJECTIVE

Data on iodine loss in breast milk, which are critical for establishing the appropriate dietary iodine intake for lactating women, is currently limited. A study was conducted to assess iodine loss in breast milk among Chinese lactating women to estimate the appropriate dietary intake of iodine.

METHODS

A total of 54 pairs of healthy, lactating women and their infants aged 0-6 months were recruited from Tianjin and Luoyang cities in China. A 4 days infant weighing study was conducted to assess iodine loss in the breast milk of lactating women. Mothers were required to weigh and record their infants' body weights before and after each feeding for a 24 h period from 8:00 am to 8:00 am. During the weighing study, 2812 breast milk samples and 216 24-h urine samples were collected from each lactating mother for four consecutive days. In addition, a 3 days 24 h dietary record, including salt weighing and drinking water samples collecting, was performed by each lactating mother to determine dietary iodine intake during the weighing study.

RESULTS

The average dietary iodine intake of lactating women was 323 ± 80 μg/d. The median breast milk iodine concentration and 24 h urinary iodine concentration of lactating women were 154 (122-181) and 135 (104-172) μg/L, respectively. The mean volume of breast milk and the mean iodine loss in the breast milk of lactating women were 711 ± 157 mL/d and 112 ± 47 μg/d, respectively. The appropriate dietary intake of iodine among lactating Chinese women is approximately 260 µg/d.

CONCLUSIONS

Based on the iodine loss in breast milk (110 μg/d) found in this study, and the estimated average requirement of iodine for adults, the appropriate dietary intake of iodine among lactating Chinese women is 260 µg/d, which is higher than the 240 μg/d recommended by the China Nutrition Science Congress in 2013.

Variations in Breast Milk Iodine Concentration over 24 h among Lactating Women in Northern China

BACKGROUND

Adequate breast milk iodine concentration (BMIC) is essential for the growth and cognitive development of exclusively breastfed infants; however, data on variations in BMIC over 24 h are limited.

OBJECTIVE

We aimed to explore in lactating women the variation in 24-h BMIC.

METHODS

Thirty pairs of mothers and breastfed infants aged 0-6 mo were recruited from the cities of Tianjin and Luoyang, China. A 3-d 24-h dietary record, including salt intake, was performed to assess the dietary iodine intake of lactating women. Breast milk samples before and after each feeding for 24 h and 24-h urine samples were collected from the women for 3 d to estimate iodine excretion. A multivariate linear regression model was used to analyze the factors influencing BMIC. A total of 2658 breast milk samples and 90 24-h urine samples were collected.

RESULTS

The median BMIC and 24-h urine iodine concentration (UIC) of lactating women for a mean of 3.6 ± 1.48 mo were 158 μg/L and 137 μg/L, respectively. The interindividual variability of BMIC (35.1%) was higher than that observed within individuals (11.8%). The variation in BMIC showed a "V" shaped curve over 24 h. The median BMIC at 08:00-12:00 (137 μg/L) was significantly lower than that at 20:00-24:00 (163 μg/L) and 00:00-04:00 (164 μg/L). A progressively increasing curve was obtained for BMIC until it peaked at 20:00 and plateaued at a higher concentration from 20:00 to 04:00 than at 08:00-12:00 (all P < 0.05). BMIC was associated with dietary iodine intake (β: 0.366; 95% CI: 0.004, 0.018) and infant age (β: -0.432; 95% CI: -1.07, -0.322).

CONCLUSIONS

Our study shows that the BMIC presents a "V" shaped curve over 24 h. We recommend that breast milk samples be collected between 08:00 and 12:00 for evaluation of the iodine status of lactating women.

Prevalence and Predictors of Low Breast Milk Iodine Concentration in Women Following Vegan, Vegetarian, and Omnivore Diets

Purpose: Breast milk iodine concentration (BMIC) from vegan and vegetarian lactating mothers has not previously been evaluated. The goal of this study was to assess BMIC from vegans, vegetarians, and omnivores and to assess intake of iodine by breastfed infants. Materials and Methods: Breast milk samples from vegans (n = 12), vegetarians (n = 6), and omnivores (n = 12) living in the United States were analyzed. BMIC was determined at the mass-to-charge ratio (m/z) 127 by inductively coupled plasma mass spectrometry (ICP-MS) using an Agilent 8800 ICP-MS/MS (Agilent Technologies). Results: There was a significant difference in mean BMIC between participants following a plant-based diet (vegan and vegetarian, n = 18) compared with omnivores [4.42 versus 5.02 Ln(BMIC), respectively; p = 0.0405]. In linear regression to predict BMIC, vegan diet was a negative predictor (standardized β = -0.409) and use of multi- or prenatal supplements was a positive predictor (standardized β = 0.319). There were differences in the percentage of inadequate BMIC per maternal diet (75% vegan, 67% vegetarian, omnivore 58%) but this did not reach statistical significance. In 67% of the samples (20/30) BMIC was lower than the National Academy of Medicine's adequate intake (AI), assuming infant milk consumption of 0.78 L/day. Conclusions: Most samples from vegans and vegetarians contained a lower BMIC than AI for infants 0-6 months. Counseling of pregnant vegans and vegetarians should highlight importance of iodine supplementation during lactation. The findings are based on a small number of samples, especially for vegetarians, and thus, they need to be confirmed by larger studies.

Characteristics and predictors of breast milk iodine in exclusively breastfed infants: Results from a repeated-measures study of iodine metabolism

Background

The iodine supply of exclusively breastfed infants entirely depends upon breast milk. Changes in breast milk iodine affect infants’ iodine nutritional status. This study aimed to comprehensively assess the characteristics and predictors of breast milk iodine concentration (BMIC).

Materials and methods

This 7-day iodine metabolism experiment was conducted in 25 exclusively breastfed mother-infant pairs. The duplicate portion method was used to measure the mother’s daily iodine intake from foods and water, and maternal 24-h urine excretion was assessed. We recorded the number of breastfeeds per mother per day and collected breast milk samples before and after each feeding.

Results

The median [quartile (Q)1–Q3 range] of BMIC was 115 (86.7, 172) μg/L. The BMIC before breastfeeding was generally higher than that after breastfeeding. Time-sequential analysis found that morning BMIC was most highly correlated with the prior day’s iodine intake. Breast milk samples taken in the afternoon or after midnight are closer to the median level of BMIC throughout the day. The number of breast milk samples needed to estimate the iodine level with 95% CI within precision ranges of ± 20% was 83 for a population, 9 for an individual, and 2 for an individual’s single day. Maternal total iodine intake (TII) and urine iodine were significantly associated with BMIC. 24-h urinary iodine excretion (24-h UIE) was found to be the best predictive indicator for the BMIC (β = 0.71, 95% CI: 0.64, 0.79).

Conclusion

BMIC is a constantly changing indicator and trended downward during each breastfeeding. Breast milk samples taken in the afternoon or after midnight are most representative. BMIC was significantly associated with recent iodine intake. Maternal 24-h UIE was the best predictor of BMIC.

Assessment of Iodine Concentration in Human Milk from Donors: Implications for Preterm Infants

Preterm infants are particularly vulnerable to developing iodine deficiency. Donor human milk (DHM) is the preferred feeding option if the mother’s own milk (MOM) is not available, but information on DHM iodine concentration (DHMIC) is lacking. Hence, we aimed to assess DHMIC to further evaluate the adequacy of iodine provision in preterm infants. Finally, associations that might influence DHMIC were studied. In 113 donors, we measured iodine intake by evaluating dietary records for five consecutive days with the DIAL® Software. From the second day of dietary record, donors provided human milk samples (at least one per day) for four consecutive days. Daily human milk samples were analyzed for DHMIC. A DHMIC ≥ 200 µg/L was considered an adequate iodine content for preterm infants. DHMIC and urine iodine concentration (UIC) were determined using ICP-MS. In our study, 83.2% of donors had a full-term infant. Breastfeeding time range was 1.5−49.4 months. During the dietary record, 55.8% took iodine-containing supplements, providing 40−200 µg/day of iodine. The medians (p25, p75) UIC and DHMIC were 112.4 (75.8, 160.1) and 148.5 (97.6, 206.1) µg/L, respectively. In this iodine-sufficient population, 70% had a DHMIC of <200 µg/L. Donors’ intake of iodine-containing supplements was associated with higher DHMIC.

Mechanisms of Sodium/Iodide Symporter-Mediated Mammary Gland Iodine Compensation during Lactation

This research aimed to investigate the compensation mechanism of iodine deficiency and excess in the mammary gland during lactation. Female rats were divided into the low iodine group (LI), the normal iodine group (NI), the 10-fold high iodine group (10HI) and the 50-fold high iodine group (50HI). We measured the iodine levels in the urine, blood, milk, and mammary gland. The protein expression of sodium/iodide symporter (NIS), DPAGT1, and valosin-containing protein (VCP) in the mammary gland was also studied. The 24-hour urinary iodine concentration, serum total iodine concentration, serum non-protein-bound iodine concentration, breast milk iodine concentration, and mammary gland iodine content in the 50HI group were significantly higher than those in the NI group (p < 0.05). Compared with the NI group, NIS expression in the 50HI group significantly decreased (p < 0.05). DAPGT1 expression was significantly higher in the LI group than in the NI group (p < 0.05). The expression level of VCP was significantly increased in the 10HI and 50HI groups. In conclusion, milk iodine concentration is positively correlated with iodine intake, and the lactating mammary gland regulates the glycosylation and degradation of NIS by regulating DPAGT1 and VCP, thus regulating milk iodine level. However, the mammary gland has a limited role in compensating for iodine deficiency and excess.

The Role of Iodine for Thyroid Function in Lactating Women and Infants

Iodine is a micronutrient needed for the production of thyroid hormones, which regulate metabolism, growth, and development. Iodine deficiency or excess may alter the thyroid hormone synthesis. The potential effects on infant development depend on the degree, timing, and duration of exposure. The iodine requirement is particularly high during infancy because of elevated thyroid hormone turnover. Breastfed infants rely on iodine provided by human milk, but the iodine concentration in breast milk is determined by the maternal iodine intake. Diets in many countries cannot provide sufficient iodine, and deficiency is prevented by iodine fortification of salt. However, the coverage of iodized salt varies between countries. Epidemiological data suggest large differences in the iodine intake in lactating women, infants, and toddlers worldwide, ranging from deficient to excessive intake. In this review, we provide an overview of the current knowledge and recent advances in the understanding of iodine nutrition and its association with thyroid function in lactating women, infants, and toddlers. We discuss risk factors for iodine malnutrition and the impact of targeted intervention strategies on these vulnerable population groups. We highlight the importance of appropriate definitions of optimal iodine nutrition and the need for more data assessing the risk of mild iodine deficiency for thyroid disorders during the first 2 years in life.

Breast Milk Iodine Concentration (BMIC) as a Biomarker of Iodine Status in Lactating Women and Children <2 Years of Age: A Systematic Review

Background: Iodine is needed for the production of thyroid hormones, which are essential for infant growth and development. Given that there are wide variations in breast milk iodine concentration (BMIC) and urinary iodine concentration (UIC), it is unclear if BMIC is associated with UIC in populations residing in iodine sufficient or deficient areas. Aim: To investigate if BMIC can be used as a biomarker for iodine status in lactating women and children <2 years of age. Methods: Electronic databases; PubMed, Web of Science and Scopus were searched until year 2021, for studies investigating the relationship between BMIC and UIC. Studies were reviewed for eligibility, according to inclusion and exclusion criteria, followed by data extraction, according to the PRISMA guidelines. Results: Overall, 51 studies met the criteria for inclusion in the systematic review. BMIC ranged from 18 to 1153 µg/L. In iodine-deficient and iodine-sufficient lactating women, BMIC ranged from 26 to 185 µg/L and 15 to 1006 µg/L, respectively. In most studies, the categorisation of iodine status assessed by median UIC was consistent with the categorisation of iodine status assessed by median BMIC cut off of ≥100 µg/L, to determine iodine sufficiency in lactating women and children <2 years of age. Conclusions: The systematic review indicated that BMIC is a promising biomarker of iodine status in lactating women and children <2 years of age. However, these data need to be interpreted cautiously, given the study limitations in the included studies. Future studies should consider investigating the optimal median BMIC, as there is a lack of high-quality observational and intervention studies in lactating women and infants.

Iodine Status of Mother-Infant Dyads from Montréal, Canada: Secondary Analyses of a Vitamin D Supplementation Trial in Breastfed Infants

BACKGROUND

Most pregnant or lactating women in Canada will not meet iodine requirements without iodine supplementation.

OBJECTIVES

To assess the iodine status of 132 mother-infant pairs based on secondary analyses of a vitamin D supplementation trial in breastfed infants from Montréal, Canada.

METHODS

Maternal iodine status was assessed using the breastmilk iodine concentration (BMIC). Singleton, term-born infants were studied from 1-36 months of age. Usual (adjusted for within-person variation) iodine intakes were estimated from urinary iodine and creatinine concentrations. Iodine status was assessed using median urinary iodine concentrations (UICs) and by estimating inadequate intakes by the cut-point method using a proposed Estimated Average Requirement for infants 0-6 months of age (72 μg/d).

RESULTS

At 1, 3, and 6 months of age, 70%, 63%, and 3% of infants, respectively, were exclusively breastfed. From 1-36 months of age (n = 82-129), the median UICs were ≥100 μg/L (range, 246-403 μg/L), which is the cutoff for adequate intakes set by the WHO for children <2 years. Almost all (98%-99%) infants at 1 and 2 months, 2 and 3 months, and 3 and 6 months of age had usual creatinine-adjusted iodine intakes ≥ 72 μg/d. The median BMIC was higher (P < 0.001) at 1 month compared to 6 months of lactation [1 month, 198 μg/kg (IQR, 124-274; n = 105) and 6 months, 109 μg/kg (IQR, 67-168; n = 78)]. At 1 and 6 months, 96% and 79% of mothers, respectively, had a BMIC ≥ 60 μg/kg, the lower limit of a normal reference range. The percentages of mothers that used a multivitamin-mineral (MVM) supplement containing iodine were 90% in pregnancy and 79% and 59% at 1 and 6 months of lactation, respectively.

CONCLUSIONS

The iodine status of infants was adequate throughout infancy. These results support a recommendation that all women who could become pregnant, who are pregnant, or who are breastfeeding take a daily MVM supplement containing iodine.

Improving Iodine Status in Lactating Women: What Works?

PURPOSE OF REVIEW

Iodine deficiency is a global concern, and in recent years, there has been a significant improvement in the number of countries identified as being iodine-sufficient. This review considers the best strategies to ensure iodine sufficiency among breastfeeding women and their infants.

RECENT FINDINGS

Fortification strategies to improve iodine intake have been adequate for school-age children (SAC); however, often, iodine deficiency remains for breastfeeding women and their infants. Daily supplementation with iodine is not an ideal strategy to overcome deficiency. Countries defined as iodine-sufficient, but where pregnant and breastfeeding women have inadequate intake, should consider increasing salt iodine concentration, such that the median urinary iodine concentration of SAC can be up to 299 µg/L. This will ensure adequate intake for mothers and infants, without SAC being at risk thyroid dysfunction. Consensus is required for thresholds for iodine adequacy for breastfeeding women and infants.

Infant Iodine and Selenium Status in Relation to Maternal Status and Diet During Pregnancy and Lactation

Iodine and selenium are essential trace elements. Recent studies indicate that pregnant and lactating women often have insufficient intake of iodine and selenium, but the impact on fetal and infant status is unclear. Here, we assessed iodine and selenium status of infants in relation to maternal intake and status of these trace elements in the birth cohort NICE, conducted in northern Sweden (n = 604). Iodine was measured in urine (UIC) in gestational week 29, and in breast milk and infant urine 4 months postpartum, while selenium was measured in maternal plasma and erythrocytes in gestational week 29, and in breast milk and infant erythrocytes 4 months postpartum, in both cases using ICP-MS. Maternal intake was assessed with semi-quantitative food frequency questionnaires in gestational week 34 and at 4 months postpartum. The median intake of iodine and selenium during pregnancy (98 and 40 μg/d, respectively) and lactation (108 and 39 μg/d, respectively) was below recommended intakes, reflected in insufficient status (median UIC of 113 μg/L, median plasma selenium of 65 μg/L). Also, breast milk concentrations (median iodine 77 μg/L, median selenium 9 μg/L) were unlikely to meet infant requirements. Median UIC of the infants was 114 μg/L and median erythrocyte selenium 96 μg/kg, both similar to the maternal concentrations. Infant UIC correlated strongly with breast milk levels (rho = 0.64, p < 0.001). Their erythrocyte selenium correlated with maternal erythrocyte selenium in pregnancy (rho = 0.38, p < 0.001), but not with breast milk selenium, suggesting formation of prenatal reserves. Our results indicate that the transport of iodine and selenium to the fetus and infant is prioritized. Still, it is uncertain whether most infants had sufficient intakes. Further, the results might indicate an involvement of iodine in asthma development during the first year of life, which is essential to follow up. The low maternal and infant dietary intake of both iodine and selenium, especially when the mothers did not use supplements or iodized table salt, suggest a need for a general screening of women and young children.

Prevalence of thyroid dysfunction in postpartum women with suboptimal iodine and selenium and adequate iron status

OBJECTIVE

Postpartum women experience thyroid dysfunction at twice the prevalence of the general population. Adequate biosynthesis of thyroid hormones depends on three trace elements: iodine, selenium and iron. This study aimed to investigate thyroid dysfunction within a cohort of women at six months postpartum in relation to iodine, selenium and iron status.

DESIGN

This cross-sectional study was part of an observational longitudinal cohort Mother and Infant Nutrition Investigation; data obtained at six months postpartum are reported.

SUBJECTS

Mother-infant pairs (n = 87) were recruited at three months postpartum and followed up at six months postpartum (n = 78).

MEASUREMENTS

Thyroid hormones (free triiodothyronine, free thyroxine, thyroid-stimulating hormone) and thyroid peroxidase antibodies were measured. Urinary iodine concentration, breast milk iodine concentration, serum thyroglobulin, plasma selenium, serum ferritin and serum soluble transferrin receptors were determined. Nonparametric data were expressed as median (25th, 75th percentile).

RESULTS

Thyroid dysfunction was found in 18% of women, and 4% of women had iron deficiency. Median urinary iodine concentration was 85 (43, 134) µg/L, median breast milk iodine concentration was 59 (39, 109) µg/L, and median serum thyroglobulin at 11.4 (8.6, 18.6) µg/L, indicating iodine deficiency. Median plasma selenium concentration was 105.8 (95.6, 115.3) µg/L. Women with marginally lower plasma selenium concentration were 1.12% times more likely to have abnormal TSH concentrations (p = .001).

CONCLUSIONS

There was a high prevalence of thyroid dysfunction. Plasma selenium concentration was the only significant predictor of the likelihood that women had thyroid dysfunction within this cohort, who were iodine deficient and mostly had adequate iron status. Strategies are required to improve both iodine and selenium status to better support maternal thyroid function.

Use of Iodine Supplements by Breastfeeding Mothers Is Associated with Better Maternal and Infant Iodine Status

Adequate iodine status during conception, pregnancy and lactation is essential for supporting infant neurodevelopment. Iodine status in adults and children was improved after two New Zealand government initiatives, but the status of breastfeeding women is unknown. This study aimed to investigate the iodine intake and status of lactating mother-infant pairs at 3 months postpartum and to assess maternal iodine knowledge and practice. Iodine intake was estimated by a weighed 4-day diet diary (4DDD). Maternal urinary iodine concentrations (UIC) in spot urine, breast milk iodine concentrations (BMIC) and infant UIC were measured. Questions about iodine-specific knowledge and practice were asked. In 87 breastfeeding mother-infant pairs, maternal iodine intake was 151 (99, 285) μg/day, and 58% had an intake below the estimated average requirement (EAR) of 190 μg/day. Maternal median UIC (MUIC) was 82 (46, 157) μg/L indicating iodine deficiency (i.e., < 100 μg/L). Women who used iodine-containing supplements had a significantly higher MUIC (111 vs 68 μg/L, P = 0.023) and BMIC (84 vs 62 μg/L, P < 0.001) than non-users. Infants fed by women using iodine-containing supplements had a higher MUIC (150 vs 86 μg/L, P = 0.036) than those of non-users. A total of 66% (57/87) of women had no or low iodine knowledge. The iodine knowledge score was a statistically significant predictor of consuming iodine-containing supplements [(beta = 1.321, P = 0.008)]. Despite a decade of initiatives to increase iodine intakes in New Zealand, iodine knowledge was low; iodine intake and status of these lactating women were suboptimal, but women who used iodine-containing supplement were more likely to achieve adequate status.Study Registration Number (Australia and New Zealand Clinical Trials Registry): ACTRN12615001028594.

Characterization of Genetic Variants in the SLC5A5 Gene and Associations With Breast Milk Iodine Concentration in Lactating Women of African Descent: The NUPED Study

Background: The sodium iodide symporter is responsible for the transfer of iodine into breast milk and is encoded for by the SLC5A5 gene. The role of genetic variants in the SLC5A5 gene locus in relation to the transfer of iodine from plasma into breast milk in healthy lactating individuals has, to our knowledge, not been explored. Objective: To identify and characterize possible genetic variants of the SLC5A5 gene in women of African descent living in urban South Africa, and to study associations with breast milk iodine concentrations (BMIC) in lactating women. Methods: This study is affiliated to the Nutrition during Pregnancy and Early Development (NuPED) cohort study (n = 250 enrolled pregnant women). In a randomly selected sub-sample of 32 women, the SLC5A5 gene was sequenced to identify known and novel variants. Of the identified variants, genotyping of selected variants was performed in all pregnant women who gave consent for genetic analyses (n = 246), to determine the frequency of the variants in the study sample. Urinary iodine concentration (UIC) in spot urine samples and BMIC were measured to determine iodine status. Associations of SLC5A5 genetic variants with BMIC were studied in lactating women (n = 55). Results: We identified 27 variants from sequencing of gene exomes and 10 variants were selected for further study. There was a significant difference in BMIC between the genotypes of the rs775249401 variant (P = 0.042), with the homozygous GG group having lower BMIC [86.8 (54.9-167.9) μg/L] compared to the (A) allele carriers rs775249401(AG+AA) [143.9 (122.4-169.3) μg/L] (P = 0.042). Of the rs775249401(GG), 49% had UIC <100 μg/L and 61% had BMIC <100 μg/L. On the other hand, 60% of the rs775249401(AG+AA) carriers had UIC <100 μg/L, and none had a BMIC <100 μg/L. Conclusion: Our results suggest that A-allele carriers of rs775249401(AG+AA) are likely to have higher iodine transfer into breast milk compared to the homozygous GG counterparts. Thus, genetic variations in the SLC5A5 gene may play an important role in the transfer of iodine from plasma into breast milk and may partially explain inter-individual variability in BMIC.

Iodine Nutrition and Iodine Supplement Initiation in Association with Thyroid Function in Mildly-to-Moderately Iodine-Deficient Pregnant and Postpartum Women

BACKGROUND

Whereas the adverse effects of severe iodine deficiency during pregnancy are well documented, the effects of mild-to-moderate deficiency are not well established.

OBJECTIVES

We aimed to explore whether iodine nutrition and timing of iodine supplement initiation are associated with thyroid function in pregnant and postpartum women.

METHODS

In this cohort study, 137 pregnant women were enrolled and followed up at gestational weeks (GWs) 18 and 36, and 3 and 6 mo postpartum. Thyroid function tests [thyroid-stimulating hormone (TSH), free triiodothyronine (fT3), and free thyroxine (fT4)], urinary iodine and creatinine concentration (UIC:Cr), and iodine intake (including iodine supplement use) were measured at each time point. The associations between thyroid hormone concentrations and UIC:Cr, iodine intakes, and iodine supplement use were estimated using multiple generalized estimating equation models.

RESULTS

The median UIC at GW18 was 94 μg/L, indicating mild-to-moderate iodine deficiency. UIC:Cr (β; 95% CI) per 100 μg/g was negatively associated with fT3 (-0.191; -0.331, -0.051) and fT4 (-0.756; -1.372, -0.141) concentrations. Iodine intake (β; 95% CI) per 100 μg/d was positively associated with TSH (0.099; 0.022, 0.177), and negatively associated with fT3 (-0.084; -0.0141, -0.027) and fT4 (-0.390; -0.599, -0.182) concentrations. Compared with no use of supplement, those initiating an iodine-containing supplement prepregnancy and continuing through pregnancy had lower TSH (estimated means) (1.35 compared with 1.68 mIU/L, P = 0.021), and higher fT3 (4.48 compared with 4.28 pmol/L, P = 0.035) and fT4 (15.2 compared with 14.4 pmol/L, P = 0.024) concentrations.

CONCLUSIONS

Lower iodine availability during pregnancy and postpartum was associated with lower TSH, and higher fT3 and fT4 concentrations. The use of an iodine-containing supplement that was initiated prepregnancy and continuing through pregnancy was associated with lower TSH, and higher fT3 and fT4 concentrations, which may suggest improved thyroid function. These findings support the notion that optimization of iodine intake should start before pregnancy.This trial was registered at clinicaltrials.gov as NCT02610959.

Lower educational status interferes with maternal iodine intake during both pregnancy and lactation

OBJECTIVE

To investigate factors affecting conscious iodine intake among pregnant and lactating women in a rural area in Hungary.

METHODS

Pregnant women were studied and followed during lactation. Urinary and breast milk iodine concentration (UIC and MIC) were measured by inductively coupled plasma mass spectrometry. Potential interfering factors, including age, educational status and smoking were assessed.

RESULTS

During pregnancy and lactation, mild iodine deficiency was observed; median UIC were 66 and 49 µg/L, respectively. Educational status was found to be a strong determinant of both iodine nutrition and smoking status during pregnancy (P < 0.01 and P < 0.001) and lactation (P < 0.001 and P < 0.01). While smoking and non-smoking lactating mothers had similar concentrations of urinary iodine (median UIC: 47 and 51 µg/L, P = 0.95), the breast milk of smoking mothers contained less iodine (median MIC: 150 and 203 µg/L, P = 0.03).

CONCLUSIONS

Both low iodine intake and smoking contribute to the higher risk of iodine deficiency in women with lower educational status. In smokers, MIC is often low in spite of normal UIC, presumably due to the iodine transport blocking effect of the cigarette smoke towards breast milk; normal UIC may be misinterpreted as sufficient iodine supply towards the child. Antenatal health promotion strategies should focus on young women with lower educational status, even in regions where sufficient iodine intake has been achieved in non-pregnant adults.

GLOBAL ENDOCRINOLOGY: Global perspectives in endocrinology: coverage of iodized salt programs and iodine status in 2020

Iodine deficiency has multiple adverse effects on growth and development. Diets in many countries cannot provide adequate iodine without iodine fortification of salt. In 2020, 124 countries have legislation for mandatory salt iodization and 21 have legislation allowing voluntary iodization. As a result, 88% of the global population uses iodized salt. For population surveys, the urinary iodine concentration (UIC) should be measured and expressed as the median, in μg/L. The quality of available survey data is high: UIC surveys have been done in 152 out of 194 countries in the past 15 years; in 132 countries, the studies were nationally representative. The number of countries with adequate iodine intake has nearly doubled from 67 in 2003 to 118 in 2020. However, 21 countries remain deficient, while 13 countries have excessive intakes, either due to excess groundwater iodine, or over-iodized salt. Iodine programs are reaching the poorest of the poor: of the 15 poorest countries in the world, 10 are iodine sufficient and only 3 (Burundi, Mozambique and Madagascar) remain mild-to-moderately deficient. Nigeria and India have unstable food systems and millions of malnourished children, but both are iodine-sufficient and population coverage with iodized salt is a remarkable 93% in both. Once entrenched, iodine programs are often surprisingly durable even during national crises, for example, war-torn Afghanistan and Yemen are iodine-sufficient. However, the equity of iodized salt programs within countries remains an important issue. In summary, continued support of iodine programs is needed to sustain these remarkable global achievements, and to reach the remaining iodine-deficient countries.

IODINE STATUS AND THYROID FUNCTION IN LACTATING WOMEN AND INFANTS - A SURVEY IN THE ZAGREB AREA, CROATIA

Lactating women (LW) and infants have high dietary iodine requirements and are at risk of iodine deficiency. The aim of the study was to assess iodine status and thyroid function in LW and their breastfed infants in Zagreb, Croatia. The study included 133 LW and breastfed infant pairs. Urinary iodine concentration (UIC) and thyroid function parameters were measured in all subjects. In LW, breast milk iodine concentration (BMIC) was measured and iodine and salt rich food frequency questionnaire data were collected. Results of analysis indicated that 99.2% of the LW used iodized salt in household and 20.4% used iodine-containing vitamin and mineral supplements. Median (IQR) UIC was 75 µg/L (19.0-180.5 µg/L) in LW and 234 µg/L (151.0-367.5 µg/L) in infants, whereas BMIC was 121 µg/kg (87.8-170.8 µg/kg). Multivariate regression analysis revealed BMIC to be a significant predictor of infant UIC (p<0.001). Positive correlation was recorded between LW and infant thyroid function. This was the first study in Croatia demonstrating BMIC to be a reliable biomarker of iodine status during lactation and predicting iodine intake in breastfed infants. The study confirmed that mandatory salt iodization in Croatia ensured sufficient dietary iodine for LW and optimal iodine intake for breastfed infants via breast milk.

Adequate Urinary Iodine Concentration among Infants in the Inland Area of Norway

Considering the importance of iodine to support optimal growth and neurological development of the brain and central nervous system, this study aimed to assess and evaluate iodine status in Norwegian infants. We collected data on dietary intake of iodine, iodine knowledge in mothers, and assessed iodine concentration in mother’s breast milk and in infant’s urine in a cross-sectional study at two public healthcare clinics in the inland area of Norway. In the 130 mother–infant pairs, the estimated infant 24-h median iodine intake was 50 (IQR 31, 78) µg/day. The median infant urinary iodine concentration (UIC) was 146 (IQR 93, 250) µg/L and within the recommended median defined by the World Health Organization for this age group. Weaned infants had a higher UIC [210 (IQR 130, 330) µg/L] than exclusively breastfed infants [130 (IQR 78, 210) µg/L] and partially breastfed infants [135 (IQR 89, 250) µg/L], which suggest that the dietary data obtained in this study did not capture the accurate iodine intake of the included infants. The iodine status of infants in the inland area of Norway seemed adequate. Weaned infants had higher UIC compared to breastfed infants, suggesting early access and consumption of other sources of iodine in addition to breast milk.

Does maternal iodine supplementation during the lactation have a positive impact on neurodevelopment of children? Three-year follow up of a randomized controlled trial

PURPOSE

The aim of this study was to examine, for the first time, the neurodevelopmental outcomes in children whose mothers received different doses of iodine supplements during lactation.

METHODS

We conducted a follow-up study on children whose mothers participated in a randomized clinical trial to receive placebo, 150 µg/day or 300 µg/day of iodine until 12 months postpartum. Child neurocognitive development was assessed at 36 months of age using the Bayley Scales of Infant and Toddler Development Third Edition. Linear mixed-model analysis was preformed to assess iodine supplement dose effects on child cognitive, language, and motor functions.

RESULTS

A total of 122 children provided neurodevelopmental data as follows: 300 µg/d iodine group: 45; 150 µg/d iodine group: 35; and placebo group: 42. Cognitive scores were higher in children whose mothers received 150 µg iodine/d compared to children whose mothers received placebo [102.8 (SD 13.2) vs. 99.2 (SD 10.5); β = 4.43, P = 0.032]. However, supplementation with 150 µg iodine/d had no effect on language or motor development. No significant differences were observed in cognitive, language, or motor functions between children whose mothers received 300 µg iodine/d and those whose mothers received 150 µg iodine/d or placebo.

CONCLUSION

Maternal iodine supplementation with 150 µg/d during lactation may have a beneficial effect on child cognitive development; however, we found no evidence of either improved or delayed neurodevelopmental outcomes in children whose mothers received iodine supplements at doses higher than recommended. Further randomized controlled trials with larger sample sizes are needed to confirm these results.

CLINICAL TRIAL REGISTRY

IRCT201303164794N8; registration date: 2013-05-20.

Iodine status of postpartum women and their infants aged 3, 6 and 12 months: Mother and Infant Nutrition Investigation (MINI)

To alleviate the re-emergence of iodine deficiency in New Zealand, two strategies, the mandatory fortification of bread with iodised salt (2009) and a government-subsidised iodine supplement for breast-feeding women (2010), were introduced. Few studies have investigated mother and infant iodine status during the first postpartum year; this study aimed to describe iodine status of mothers and infants at 3, 6 and 12 months postpartum (3MPP, 6MPP and 12MPP, respectively). Partitioning of iodine excretion between urine and breast milk of exclusive breast-feeding (EBF) women at 3MPP was determined. In total, eighty-seven mother-infant pairs participated in the study. Maternal and infant spot urinary iodine concentration (UIC) and breast milk iodine concentration (BMIC) were determined. The percentage of women who took iodine-containing supplements decreased from 46 % at 3MPP to 6 % at 12MPP. Maternal median UIC (MUIC) at 3MPP (82 (46, 157) µg/l), 6MPP (85 (43, 134) µg/l) and 12MPP (95 (51, 169) µg/l) were <100 µg/l. The use of iodine-containing supplements increased MUIC and BMIC only at 3MPP. Median BMIC at all time points were below 75 µg/l. Infant MUIC at 3MPP (115 (69, 182) µg/l) and 6MPP (120 (60, 196) µg/l) were below 125 µg/l. Among EBF women at 3MPP, an increased partitioning of iodine into breast milk (highest proportion 60 %) was shown at lower iodine intakes, along with a reduced fractional iodine excretion in urine (lowest proportion 40 %), indicating a protective mechanism for breastfed infants' iodine status. In conclusion, this cohort of postpartum women was iodine-deficient. Iodine status of their breastfed infants was suboptimal. Lactating women who do not consume iodine-rich foods and those who become pregnant again should take iodine-containing supplements.

Iodine nutritional status of lactating women in northern Taiwan in 2019

BACKGROUND

Pregnant and lactating women are vulnerable to iodine deficiency. This study was conducted to evaluate the iodine nutritional status of lactating women in northern Taiwan.

METHODS

Women recruited from Taipei Veterans General Hospital (TVGH) in 2019 provided a spot urine sample and completed a Food Frequency Questionnaire. The urinary iodine concentration (UIC) was measured by inductively coupled plasma mass spectrometry.

RESULTS

The overall median UIC in 198 women was 120.4 μg/L, indicating a sufficient iodine status. Univariate analysis revealed a lower median UIC in women of younger age (p = 0.004), who were not taking multivitamins (p = 0.004), not on a postpartum nourishment diet (p = 0.04), and whose infant received more breast milk (p = 0.004). The median UIC was <100 μg/L in the group aged 20 to 29 years (UIC: 74.4 μg/L) and in women whose infants' diet was composed of >50% breast milk (UIC: 86.1 μg/L). A postpartum nourishment diet was followed by 73.7% (n = 146) of the women. Nevertheless, a significant decrease in the intake frequency of iodine-containing foods, including seaweeds (p < 0.001), seafood (p < 0.001), dairy products (p = 0.009), and multivitamins (p < 0.001) was observed compared with the intake noted in a previous survey of pregnant women in TVGH. Following multivariate analysis, only younger age (20-29 vs ≥30 years; odds ratio [OR]: 3.38; 95% confidence interval [CI]: 1.49-7.65), no use of multivitamin (OR: 1.89; 95% CI: 1.03-3.48), and infant diet composition (>50% breast milk vs <50% breast milk; OR: 2.93; 95% CI: 1.37-6.25) were independently associated with UIC < 100 μg/L.

CONCLUSION

The results suggest that the iodine status in lactating women in northern Taiwan is adequate. However, iodine deficiency may continue to be present in certain subgroups, such as women of younger age and those who do not take multivitamins.

Iodine nutrition: Disorders, monitoring and policies

Iodine is an essential mineral nutrient and an integral component of thyroid hormones. Iodine deficiency is typically associated with goiter, but can have more serious health implications. Adequate iodine status is important for normal brain development. Iodine deficiency in utero or in early life can cause severe neurological and cognitive impairment. Over the last three decades, global efforts have reduced the prevalence of iodine deficiency disorders (IDD) in many areas of the world with implementation of nutrition policies and programs such as "salt" iodization. However, in a number of areas iodine deficiency is still widespread. Iodine deficiency in remote regions with high poverty will be more difficult to eradicate. Efforts to eliminate IDD in affected areas and sustaining successful iodine programs will be a priority given the substantial public health and economic benefits. A key component will be periodic monitoring of population iodine status to ensure sufficient intakes and the absence of excessive intakes. Median urinary iodine concentration (UIC), a validated biomarker for assessing population iodine status, will facilitate monitoring. Research validating "usual" UIC for use in combination with the Estimated Average Requirement cut-point method will expand its utility and allow accurate determination of the prevalence of inadequate intakes in populations. Further research on the development of biomarkers for assessment of individual iodine status for routine patient care will be important.

Iodine status during pregnancy and at 6 weeks, 6, 12 and 18 months post-partum

Iodine deficiency during pregnancy and in the post-partum period may lead to impaired child development. Our aim is to describe iodine status longitudinally in women from pregnancy until 18 months post-partum. Furthermore, we explore whether iodine status is associated with dietary intake, iodine-containing supplement use and breastfeeding status from pregnancy until 18 months post-partum. We also assess the correlation between maternal iodine status 18 months post-partum and child iodine status at 18 months of age. Iodine status was measured by urinary iodine concentration (UIC) during pregnancy (n = 1,004), 6 weeks post-partum (n = 915), 6 months post-partum (n = 849), 12 months post-partum (n = 733) and 18 months post-partum (n = 714). The toddlers' UIC was assessed at 18 months of age (n = 416). Demographic variables and dietary data (food frequency questionnaire) were collected during pregnancy, and dietary data and breastfeeding practices were collected at all time points post-partum. We found that iodine status was insufficient in both pregnant and post-partum women. The UIC was at its lowermost 6 weeks post-partum and gradually improved with increasing time post-partum. Intake of milk and use of iodine-containing supplements significantly increased the odds of having a UIC above 100 μg/L. Neither the mothers' UIC, vegetarian practice, nor exclusion of milk and dairy products were associated with the toddlers UIC 18 months post-partum. Women who exclude milk and dairy products from their diets and/or do not use iodine-containing supplements may be at risk of iodine deficiency. The women possibly also have an increased risk of thyroid dysfunction and for conceiving children with nonoptimal developmental status.

Is breast milk iodine concentration an influential factor in growth- and obesity-related hormones and infants' growth parameters?

Iodine, a key constituent of thyroid hormones, plays an indirect role in prenatal and postnatal growth. This study aimed to investigate whether breast milk iodine concentration (BMIC) is associated with growth- and obesity-related hormones and subsequently the infants' anthropometric measures. In present study conducted in Tehran (Iran), 94 lactating mothers and healthy infants who were exclusively breastfed were included. Concentrations of iodine, insulin-like growth factor-1 (IGF-1), adiponectin (AD) and leptin (LP) were measured in breast milk samples collected during 3- to 5-day postpartum. Anthropometric measures of infants were assessed at 6 months of life, and age- and sex-specific z-score values were calculated using the World Health Organization growth standards. The median (interquartile range) iodine, IGF-1, AD and LP concentrations were 232.5 (157.5-296.0) μg L-1 , 15.7 (11.9-21.1) ng ml-1 , 13.2 (5.1-29.8) mg L-1 and 1.16 (0.86-1.70) ng ml-1 in breast milk, respectively. No significant correlations were found between BMIC and IGF-1, AD and LP concentrations during the first few days postpartum. In adjusted regression model, BMIC was positively associated with weight-for-length z score of infants. In the presence of IGF-1, AD or LP, the coefficients of BMIC for weight-for-length z score of infants were β = .003 (P = .021), β = .002 (P = .028) or β = .003 (P = .013), respectively. No other anthropometric measurements were associated with iodine or growth- and obesity-related hormones in breast milk. Our findings indicate that BMIC is a potential contributor to infants' growth status, independent of IGF-1, AD or LP concentrations in breast milk. The underlying mechanisms remain to be elucidated.

Inadequate iodine intake in lactating women in Sweden: A pilot 1-year, prospective, observational study

INTRODUCTION

Breastfed infants depend on breast-milk iodine for growth and brain development, as iodine is a trace element important for thyroid hormone production. Iodine need is higher during lactation; hence, mothers and children are at risk of iodine deficiency. We aimed to explore maternal iodine and thyroidal status during lactation.

MATERIAL AND METHODS

Pregnant women were recruited in Gothenburg, southwest Sweden. Maternal urine and serum were collected at pregnancy week 35-37 (n = 84) and 0.5, 4, and 12 months postpartum. Seventy mothers provided breast milk at 0.5 months.

RESULTS

Median (interquartile range) breast-milk iodine concentration was 90 (66-116) μg/L. About 58% had breast-milk iodine concentration <100 μg/L. Iodine supplement users (n = 13) had higher breast-milk iodine concentration than non-users (n = 49) (140 μg/L vs 71 μg/L, P = .001). Exclusively breastfeeding women at 4 months postpartum (n = 57) had lower median urinary iodine concentration (85 μg/L vs 133 μg/L, P = .004) and higher thyroglobulin serum concentration (22.3 μg/L vs 11.8 μg/L, P = .032) than non-exclusively breastfeeding women (n = 25). Concentrations of thyroid hormones were unaffected.

CONCLUSIONS

This pilot study suggests that lactating women in southwest Sweden present mildly inadequate iodine intake, mainly among non-iodine supplement users and exclusively breastfeeding mothers. Studies on the coverage of the iodine fortification program in breastfeeding women are warranted.

Effects of an Iodine-Containing Prenatal Multiple Micronutrient on Maternal and Infant Iodine Status and Thyroid Function: A Randomized Trial in The Gambia

Background: Iodine supplementation is recommended to pregnant women in iodine-deficient populations, but the impact in moderate iodine deficiency is uncertain. We assessed the effect of an iodine-containing prenatal multiple micronutrient (MMN) supplement in a rural Gambian population at risk of moderate iodine deficiency. Materials and Methods: This study uses data and samples collected as a part of the randomized controlled trial Early Nutrition and Immune Development (ENID; ISRCTN49285450) conducted in Keneba, The Gambia. Pregnant women (<20 weeks gestation) were randomized to either a daily supplement of MMNs containing 300 μg of iodine or an iron and folic acid (FeFol) supplement. Randomization was double blinded (participants and investigators). The coprimary outcomes were maternal urinary iodine concentration (UIC) and serum thyroglobulin (Tg), assessed at baseline and at 30 weeks' gestation. Secondary outcomes were maternal serum thyrotropin (TSH), total triiodothyronine (TT3), total thyroxine (TT4) (assessed at baseline and at 30 weeks' gestation), breast milk iodine concentration (BMIC) (assessed at 8, 12, and 24 weeks postpartum), infant serum Tg (assessed at birth [cord], 12, and 24 weeks postpartum), and serum TSH (assessed at birth [cord]). The effect of supplementation was evaluated using mixed effects models. Results: A total of 875 pregnant women were enrolled between April 2010 and February 2015. In this secondary analysis, we included women from the MMN (n = 219) and FeFol (n = 219) arm of the ENID trial. At baseline, median (interquartile range or IQR) maternal UIC and Tg was 51 μg/L (33-82) and 22 μg/L (12-39), respectively, indicating moderate iodine deficiency. Maternal MMN supplement increased maternal UIC (p < 0.001), decreased maternal Tg (p < 0.001), and cord blood Tg (p < 0.001) compared with FeFol. Maternal thyroid function tests (TSH, TT3, TT4, and TT3/TT4 ratio) and BMIC did not differ according to maternal supplement group over the course of the study. Median (IQR) BMIC, maternal UIC, and infant Tg in the MMN group were 51 μg/L (35-72), 39 μg/L (25-64), and 87 μg/L (59-127), respectively, at 12 weeks postpartum, and did not differ between supplement groups. Conclusions: Supplementing moderately iodine-deficient women during pregnancy improved maternal iodine status and reduced Tg concentration. However, the effects were not attained postpartum and maternal and infant iodine nutrition remained inadequate during the first six months after birth. Consideration should be given to ensuring adequate maternal status through pregnancy and lactation in populations with moderate deficiency.

Current iodine nutrition status in Poland (2017): is the Polish model of obligatory iodine prophylaxis able to eliminate iodine deficiency in the population?

OBJECTIVE

The monitoring of the populations' iodine status is an essential part of successful programmes of iodine deficiency elimination. The current study aimed at the evaluation of current iodine nutrition in school children, pregnant and lactating women as a marker of the effectiveness and sustainability of mandatory iodine prophylaxis in Poland.

DESIGN

The following iodine nutrition indicators were used: urinary iodine concentration (UIC) (all participants) and serum thyroglobulin (pregnant and lactating women).

SETTING

The study was conducted in 2017 within the National Health Programme in five regions of Poland.

PARTICIPANTS

The research included 300 pregnant women, 100 lactating women and 1000 school children (aged 6-12 years).

RESULTS

In pregnant women, median UIC was 111·6 µg/l; there was no significant difference in median UIC according to the region of residence. In 8 % of pregnant women, thyroglobulin level was >40 ng/ml (median thyroglobulin 13·3 ng/ml). In lactating women, median UIC was 68·0 µg/l. A significant inter-regional difference was noted (P = 0·0143). In 18 % of breastfeeding women, thyroglobulin level was >40 ng/ml (median thyroglobulin 18·5 ng/ml). According to the WHO criteria, the investigated sample of pregnant and lactating women was iodine-deficient. Median UIC in school children was 119·8 µg/l (with significant inter-regional variation; P = 0·0000), which is consistent with iodine sufficiency. Ninety-four children (9·4 %) had UIC < 50 µg/l.

CONCLUSIONS

Mandatory iodisation of household salt in Poland has led to a sustainable optimisation of iodine status in the general population. However, it has failed to assure adequate iodine nutrition during pregnancy and lactation.

Rapid inductively coupled plasma mass spectrometry method to determine iodine in amniotic fluid, breast milk and cerebrospinal fluid

BACKGROUND

Given that both insufficient and excess maternal iodine have adverse consequences such as poor cognitive performance, delayed physical development and increased fetal and infant mortality, the determination of maternal iodine status is very important. In this study, we established and verified a method involving inductively coupled plasma-mass spectrometry (ICP-MS) technology for the rapid determination of the amniotic fluid iodine concentration (AFIC), breast milk iodine concentration (BMIC) and cerebrospinal fluid (CSF) iodine concentration (CSFIC).

METHOD

Amniotic fluid, breast milk and CSF were collected from residual samples at Peking Union Medical College Hospital (PUMCH). The linearity, detection limit, precision, recovery, carryover and matrix effect of the testing method using ICP-MS technology were thoroughly evaluated according to the EP-10-A2 evaluation protocol approved by the Clinical and Laboratory Standards Institute (CLSI) guidelines. Furthermore, we evaluated the AFIC, BMIC and CSFIC distributions among clinical patients from PUMCH.

RESULTS

The correlation coefficient (r) was higher than 0.99 (0.995-1.000). The limit of detection (LOD) was 0.233 µg/L, and the limit of quantification (LOQ) was 0.778 µg/L. For amniotic fluid, breast milk and CSF, the assay repeatabilities were 1.5%-1.8%, 1.9%-4.0% and 1.8%-4.0%, respectively, and the within-laboratory coefficient of variations (CV%) over five days were 3.3%-9.2%, 7.2%-8.0% and 3.2%-7.8%, respectively. The recovery rates ranged from 97.7% to 109.8%. Moreover, the median concentrations of iodine in the amniotic fluid, breast milk and CSF of the patients from PUMCH were 176.3 µg/L, 136.0 µg/L, and 81.8 µg/L, respectively.

CONCLUSION

A rapid, stable and accurate method that incorporates ICP-MS technology for the determination of iodine concentration was established for amniotic fluid, breast milk and CSF in this study.

Mild to Moderate Iodine Deficiency and Inadequate Iodine Intake in Lactating Women in the Inland Area of Norway

Breastfed infants are dependent on an adequate supply of iodine in human milk for the production of thyroid hormones, necessary for development of the brain. Despite the importance of iodine for infant health, data on Norwegian lactating women are scarce. We measured iodine intake and evaluated iodine status and iodine knowledge among lactating women. From October to December 2018, 133 mother-infant pairs were recruited in a cross-sectional study through two public health care centers in Lillehammer and Gjøvik. Each of the women provided two human milk specimens, which were pooled, and one urine sample for analysis of iodine concentration. We used 24-hour dietary recall and food frequency questionnaire (FFQ) to estimate short-term and habitual iodine intake from food and supplements. The median (P25, P75) human milk iodine concentration (HMIC) was 71 (45, 127) µg/L-of which, 66% had HMIC <100 µg/L. The median (P25, P75) urinary iodine concentration (UIC) was 80 µg/L (52, 141). The mean (± SD) 24-hour iodine intake and habitual intake was 78 ± 79 µg/day and 75 ± 73 µg/day, respectively. In conclusion, this study confirms inadequate iodine intake and insufficient iodine status among lactating women in the inland area of Norway and medium knowledge awareness about iodine.

The Reliability of Iodine Concentration in Diaper-Retrieved Infant Urine Using Urine Collection Pads, and in Their Mothers' Breastmilk

Mild to moderate iodine deficiency is common among women of childbearing age. Data on iodine status in infants are sparse, partly due to the challenges in collecting urine. Urinary iodine concentration (UIC) is considered a good marker for recent dietary iodine intake and status in populations. The aim of this study was to investigate the reliability of iodine concentration measured in two spot-samples from the same day of diaper-retrieved infant urine and in their mothers' breastmilk. We collected urine and breastmilk from a sample of 27 infants and 25 mothers participating in a cross-sectional study at two public healthcare clinics in Norway. The reliability of iodine concentration was assessed by calculating the intraclass correlation coefficients (ICC) and the coefficient of variation (CV). The ICC for infants' urine was 0.64 (95% confidence interval (CI) 0.36-0.82), while the ICC for breastmilk was 0.83 (95% CI 0.65-0.92) Similarly, the intraindividual CV for UIC was 0.25 and 0.14 for breastmilk iodine concentration (BIC). Compared to standard methods of collecting urine for measuring iodine concentration, the diaper-pad collection method does not substantially affect the reliability of the measurements.

Breast Milk Iodine Concentration Is Associated with Infant Growth, Independent of Maternal Weight

In breastfed infants, human milk provides the primary source of iodine to meet demands during this vulnerable period of growth and development. Iodine is a key micronutrient that plays an essential role in hormone synthesis. Despite the importance of iodine, there is limited understanding of the maternal factors that influence milk iodine content and how milk iodine intake during infancy is related to postnatal growth. We examined breast milk samples from near 2 weeks and 2 months post-partum in a mother-infant dyad cohort of mothers with pre-pregnancy weight status defined by body mass index (BMI). Normal (NW, BMI < 25.0 kg/m2) is compared to overweight/obesity (OW/OB, BMI ≥ 25.0 kg/m2). The milk iodine concentration was determined by inductively coupled plasma mass spectrometry. We evaluated the associations between iodine content at 2 weeks and infant anthropometrics over the first year of life using multivariable linear mixed modeling. Iodine concentrations generally decreased from 2 weeks to 2 months. We observed no significant difference in iodine based on maternal weight. A higher iodine concentration at 2 weeks was associated with a larger increase in infant weight-for-age and weight-for-length Z-score change per month from 2 weeks to 1 year. This pilot study shows that early iodine intake may influence infant growth trajectory independent of maternal pre-pregnancy weight status.

Selenium and iodine in diabetes mellitus with a focus on the interplay and speciation of the elements

Diabetes mellitus is a chronic metabolic disease caused by insulin deficiency (type I) or dysfunction (type II). Diabetes is a threatening public health concern. It is considered as one of the priority non-communicable diseases, due to its high and increasing incidence, the associated healthcare costs, and threatening medical complications. Two trace elements selenium (Se) and iodine (I) were intensively discussed in the context of diabetic pathology and, possibly, etiology. It seems there is a multilayer involvement of these essential nutrients in glucose tolerance, energy metabolism, insulin signaling and resistance, which are mainly related to the antioxidant selenoenzymes and the thyroid hormones. Other factors might be related to (auto)immunity, protection against endoplasmic reticulum stress, and leptin signaling. The aim of the current review is to evaluate the current understanding of the role of selenium and iodine in diabetes with a focus on the biochemical interplay between the elements, their possible role as biomarkers, and their chemical speciation. Possible impacts from novel analytical techniques related to trace element speciation and isotopic analysis are outlined.

Variation of iodine concentration in breast milk and urine in exclusively breastfeeding women and their infants during the first 24 wk after childbirth

OBJECTIVE

The aim of this study was to observe the variation of iodine concentration in breast milk and urine in exclusively breastfeeding women and their infants during the first 24 wk after childbirth.

METHODS

In all, 634 exclusively breastfeeding mother-infant pairs were enrolled at hospital and followed at the 1, 4, 8, 12, 16, and 24 wk postpartum. Spot infant urinary iodine concentration (I-UIC), maternal urinary iodine concentration UIC (M-UIC), and breast milk iodine concentration (BMIC) in bilateral breasts were measured.

RESULTS

During the first 24 wk, the median I-UIC was 216 (139-362) and 122 (68-217) μg/L in lactating mothers, both indicating iodine sufficiency. A strong correlation and no difference were found between BMIC in bilateral breasts. The mean BMIC (M-BMIC) of the two breasts was 165 (112-257) μg/L with a Bland-Altman index of 2.1%. Positive correlations were found between M-BMIC and I-UIC (r = 0.353, P < 0.001), between M-BMIC and M-UIC (r = 0.339, P < 0.001), and between I-UIC and M-UIC (r = 0.222, P < 0.001). M-BMIC was significantly higher than M-UIC (P < 0.001) and lower than I-UIC (P < 0.001). M-BMIC declined from week 1 to week 8 postpartum, both I-UIC and M-UIC dropped from week 1 to week 4 postpartum and stabilized thereafter.

CONCLUSION

The iodine nutrition in lactating women and infants were adequate during the first 24 wk after childbirth. M-BMIC declined from week 1 to week 8 postpartum. Both I-UIC and M-UIC dropped from week 1 to week 4 postpartum. Further studies are needed to explore a more definitive BMIC and UIC range for an optimal iodine status in lactating women and breastfed infants.

Iodine Nutrition in Weaning Infants in the United States

BACKGROUND

As iodine is a requisite micronutrient for infant brain development, infants are at risk for iodine deficiency during the weaning period when their diet transitions from milk (breast-milk, infant formula, or follow-on formula) to solid food. Dietary iodine intake during this weaning period is likely minimal, as the iodine content of commercial baby food is not regulated, and the addition of salt to baby food is not recommended. This study reports the current status of iodine nutrition among weaning infants in the United States.

METHODS

Subjects (n = 60; 50% Caucasian, 30% black) were infants <12 months of age who were fed any combination of formula and/or baby food. Samples of all formula and food consumed in the previous 24 hours and a spot urine sample from each infant were obtained for the measurement of iodine. The estimated quantities of ingested formula and baby food were summed from a food diary recorded by the infants' parents.

RESULTS

The mean age of the infants was 6.3 ± 3.5 months. The median urinary iodine concentration (UIC) was 117 μg/L (range 26.9-1302.8 μg/L). Estimated daily iodine intake obtained from the measured iodine content in infant formula/foods was 89 μg (range 0-288 μg). There was a positive correlation between the infants' UIC and the iodine content in the consumed foods (r = 0.4, p < 0.001).

CONCLUSIONS

Although the median UIC of infants fed a combination of infant formula and baby food would meet the criteria for iodine sufficiency in a larger sample, those consuming the lowest quartile of iodine-containing nutritional sources had a median UIC <100 μg/L.

Excess iodine intake: sources, assessment, and effects on thyroid function

Iodine is essential for thyroid hormone synthesis. High iodine intakes are well tolerated by most healthy individuals, but in some people, excess iodine intakes may precipitate hyperthyroidism, hypothyroidism, goiter, and/or thyroid autoimmunity. Individuals with preexisting thyroid disease or those previously exposed to iodine deficiency may be more susceptible to thyroid disorders due to an increase in iodine intake, in some cases at intakes only slightly above physiological needs. Thyroid dysfunction due to excess iodine intake is usually mild and transient, but iodine-induced hyperthyroidism can be life-threatening in some individuals. At the population level, excess iodine intakes may arise from consumption of overiodized salt, drinking water, animal milk rich in iodine, certain seaweeds, iodine-containing dietary supplements, and from a combination of these sources. The median urinary iodine concentration (UIC) of a population reflects the total iodine intake from all sources and can accurately identify populations with excessive iodine intakes. Our review describes the association between excess iodine intake and thyroid function. We outline potential sources of excess iodine intake and the physiological responses and consequences of excess iodine intakes. We provide guidance on choice of biomarkers to assess iodine intake, with an emphasis on the UIC and thyroglobulin.

Lactating Mothers and Infants Residing in an Area with an Effective Salt Iodization Program Have No Need for Iodine Supplements: Results from a Double-Blind, Placebo-Controlled, Randomized Controlled Trial

BACKGROUND

The necessity of iodine supplementation in lactating mothers residing in countries with sustained salt iodization programs for iodine sufficiency of breast-fed infants remains unclear. The aims of this study were to investigate the effect of iodine supplementation on iodine status and growth parameters of lactating mothers and breast-fed infants and to compare these data with that of formula-feeding mothers and their infants during the first year of infancy.

METHODS

In this multicenter, double-blinded, randomized clinical trial conducted in four healthcare centers in Tehran (Iran), healthy lactating mothers and their term newborns aged 3-5 days were randomly assigned to treatment groups: placebo, 150 μg/day iodine, or 300 μg/day iodine. They were followed up for 12 months. Formula-fed infants aged 30-45 days and their mothers were randomly selected from the same centers. The primary outcomes were maternal and infant urinary iodine concentrations (UICs), breast-milk iodine concentrations (BMICs), and infant growth parameters, measured at 1, 2, 4, 6, 9, and 12 months during routine health visits. The formula-fed group was assessed at 2, 4, 6, 9, and 12 months of age. Analysis was by per protocol principle using mixed-effects models.

RESULTS

Mother-newborn pairs (n = 180) in treatment groups and partially/exclusively formula-feeding mother-infant pairs (n = 60) participated between October 2014 and January 2016. Median baseline UICs in the treatment groups were 84 μg/L (interquartile range [IQR] 41-143 μg/L) in mothers and 208 μg/L (IQR 91-310 μg/L) in their infants. The values in the formula-fed group were 76 μg/L (IQR 40-144 μg/L) in mothers and 121 μg/L (IQR 66-243 μg/L) in infants. The 300 μg/day iodine group showed significantly higher UICs and BMICs than did the other treatment groups; infant UICs in the 150 μg/day iodine, placebo, and formula-fed groups were similar. Infants in all groups showed iodine sufficiency (median UIC ≥100 μg/L) throughout the study period. Anthropometric measurements were similar between the treatment and formula-fed groups over the study period, except at the last follow-up visit at 12 months.

CONCLUSION

Supplementation of breast-feeding mothers with either 300 or 150 μg/day iodine improved their iodine status. However, the iodine status of infants in all groups studied indicated iodine sufficiency during the first year of infancy, demonstrating that in countries with effective salt iodization program, iodine supplementation for lactating mothers is unnecessary.

High Urinary Iodine Concentration Among Breastfed Infants and the Factors Associated with Iodine Content in Breast Milk

Iodine deficiency in infants leads to delayed growth and development. Some studies have reported iodine deficiency among infants and lactating women. We assessed iodine status in infants and lactating women, as well as the iodine content in breast milk. A cross-sectional study enrolled mother-infant pairs (infants aged 4-6 months), who visited Well Child Clinic at Ramathibodi Hospital, Bangkok, Thailand. Infants were classified by feeding type as breastfed (BF), mixed breastfed and formula-fed (MF), and formula-fed (FF). Demographic and perinatal data were collected. The urinary iodine concentration (UIC) of infants and lactating women, and breast milk iodine concentration (BMIC) were analyzed. Seventy-one infants were enrolled. The median UIC of infants was 282 mcg/L. Breastfed infants had higher median UIC than formula-fed infants (553 vs. 192 mcg/L; p = 0.002). Forty-eight percent of infants had a UIC more than 300 mcg/L. The median UIC and BMIC of lactating women were 149 and 255 mcg/L, respectively. Among the BF group, the infant UIC was correlated with maternal UIC (r_s = 0.857, p = 0.014). Multiple linear regression showed the BMIC to be associated with maternal UIC (β = 4.03, 95% CI [1.34, 6.71]) and maternal weight (β = 8.26, 95%CI [2.76, 13.77]). Iodine nutrition among our study population was adequate. The median UIC of infants and lactating mothers were 282 and 149 mcg/L, respectively. Breastfed infants had a significantly higher median UIC than formula-fed infants. The BMIC was associated with maternal UIC and maternal weight.

Effect of Excess Iodine Intake from Iodized Salt and/or Groundwater Iodine on Thyroid Function in Nonpregnant and Pregnant Women, Infants, and Children: A Multicenter Study in East Africa

BACKGROUND

Acute excess iodine intake can damage the thyroid, but the effects of chronic excess iodine intake are uncertain. Few data exist for pregnant and lactating women and infants exposed to excessive iodine intake.

METHODS

This was a multicenter cross-sectional study. At study sites in rural Kenya and urban Tanzania previously reporting iodine excess in children, urinary iodine concentration (UIC), thyrotropin, total thyroxine, and thyroglobulin (Tg) were measured in school-age children (SAC), women of reproductive age, pregnant (PW) and lactating women, and breast-feeding and weaning infants. In a national study in Djibouti, UIC was measured in SAC and PW. At all sites, daily iodine intake was estimated based on UIC, and iodine concentration was measured in household salt and drinking water.

RESULTS

The total sample size was 4636: 1390, 2048, and 1198 subjects from Kenya, Tanzania, and Djibouti, respectively. In Kenya and Tanzania: (i) median UIC was well above thresholds for adequate iodine nutrition in all groups and exceeded the threshold for excess iodine intake in SAC; (ii) iodine concentrations >40 mg of iodine/kg were found in approximately 55% of household salt samples; (iii) iodine concentrations ≥10 μg/L were detected in 9% of drinking water samples; (iv) Tg was elevated in all population groups, but the prevalence of thyroid disorders was negligible, except that 5-12% of women of reproductive age had subclinical hyperthyroidism and 10-15% of PW were hypothyroxinemic. In Djibouti: (i) the median UIC was 335 μg/L (interquartile range [IQR] = 216-493 μg/L) in SAC and 265 μg/L (IQR = 168-449 μg/L) in PW; (ii) only 1.6% of Djibouti salt samples (n = 1200) were adequately iodized (>15 mg/kg); (iii) the median iodine concentration in drinking water was 92 μg/L (IQR = 37-158 μg/L; n = 77). In all countries, UIC was not significantly correlated with salt or water iodine concentrations.

CONCLUSIONS

Although iodine intake was excessive and Tg concentrations were elevated, there was little impact on thyroid function. Chronic excess iodine intake thus appears to be well tolerated by women, infants, and children. However, such high iodine intake is unnecessary and should be avoided. Careful evaluation of contributions from both iodized salt and groundwater iodine is recommended before any review of iodization policy is considered.

Breast Milk Iodine Concentration Rather than Maternal Urinary Iodine Is a Reliable Indicator for Monitoring Iodine Status of Breastfed Neonates

There is no scientific consensus on whether breast milk iodine concentration (BMIC) accurately reflects iodine status in lactating mothers and breastfed infants. This study aimed to compare BMIC and maternal urinary iodine concentration (UIC) as indicators of iodine status in breastfed neonates. In this cross-sectional study, 147 lactating mothers and their neonates (3-5 days postpartum) were randomly selected from health care centers. Breast milk and urine samples were collected from each mother and neonate, and a heel-prick blood sample was taken from all neonates as part of a congenital hypothyroidism screening program. According to the World Health Organization criteria, median urinary iodine concentration (UIC) ≥ 100 μg/L in lactating mothers and neonates indicates iodine sufficiency. In areas of iodine sufficiency, median BMIC ≥ 100 μg/L is considered an adequate level. Overall, 129 (89.0%) and 16 (11.0%) mothers had BMICs ≥ 100 and ˂ 100 μg/L, respectively. Median (interquartile range [IQR]) maternal UIC was 70 μg/L (42-144 μg/L) and 37 μg/L (25-100 μg/L) in mothers with breast milk iodine levels ≥ 100 and ˂ 100 μg/L, respectively (P = 0.047); values for UIC of neonates born to mothers with BMICs ≥ 100 and ˂ 100 μg/L were 230 μg/L (114-310 μg/L) and 76 μg/L (41-140 μg/L), respectively (P < 0.001). In the linear regression model, neonate UIC was positively associated with BMIC in both unadjusted (β = 0.558, P < 0.001) and adjusted analysis (β = 0.541, P < 0.001). A similar result was found in logistic regression analysis, indicating that neonates born to mothers with BMIC ≥ 100 μg/L were more likely to have UIC ≥ 100 μg/L compared to those whose mothers had BMIC < 100 μg/L in both unadjusted (OR = 7.93, P < 0.001) and adjusted analysis (OR = 7.29, P = 0.001). The present findings indicate that BMIC is a more sensitive indicator than maternal UIC for assessment of iodine status in breastfed neonates. To address low levels of maternal UIC, further studies on the prescription of supplements containing 150 μg/day iodine during lactation period are warranted.