IODINE AND ENDEMIC GOITER

Iodine Nutrition Status in Pregnant Women in Mexico

Pablo Garcıa-Solıs, Juan Carlos Solıs-S, Ana Cristina Garcıa-Gaytan, Vanessa Amaranta Reyes-Mendoza,Ludivina Robles-Osorio, Hebert Luis Hernandez-Montiel, and Guillermo Enrique Leo-Amador

Background: Iodine nutrition during pregnancy has become an important public health concern because of thedeleterious impact of iodine deficiency on brain development during fetal and early postnatal life. Iodinenutrition status can be assessed in a population by the median urinary iodine concentration (UIC). World HealthOrganization, the United Nations Children’s Fund, and the International Council for Iodine Deficiency Disordershave established that a median of UIC between 150 and 249 lg/L in pregnant women indicates an adequateiodine intake. The aim of this study was to assess iodine nutrition status in Mexican pregnant women.Methods: Two hundred ninety-four pregnant women receiving prenatal care in the Public Medical Units of theState Ministry of Health for each pregnancy trimester (first, n = 60; second, n = 103; and third, n = 131) in Quer-etaro, Mexico, were enrolled to assess UIC by the Sandell-Kholtoff method.Results: The median of UIC was 273, 285, and 231 lg/L in the first, second, and third trimesters of gestation,respectively. Globally, the median (range) of UIC was 260 (5–1320) lg/L, and the percentage of samples withUIC below 150 lg/L was 28%. There was no significant difference between the UIC of women using iodine-containing multivitamins compared with those who reported the consumption of noniodized multivitamins( p > 0.05). In addition, we found no difference between the UIC of women using iodized table salt comparedwith those who employed noniodized table salt, with those who did not know whether their table salt wasiodized ( p > 0.05).Conclusions: Based on the median UIC, iodine intake in Queretaro, Mexico, is slightly above requirementsduring the first two trimesters, and adequate in the third trimester. The wide Mexican universal iodized saltprogram seems to supply adequate dietary iodine to pregnant women without health insurance in this region.However, regular monitoring of iodine status is recommended during pregnancy throughout Mexico.

Introduction

In recent years, iodine nutrition during pregnancy andlactation have become important public health concerns

because of the deleterious impact of iodine deficiency on braindevelopment during fetal and early postnatal life (1–3). Iodineis an essential micronutrient constituent of thyroid hormones(TH) that participates in neurogenesis, neuronal migration,myelination, synaptic transmission, and brain plasticity (2,3).Severe iodine deficiency during pregnancy causes hypothy-roxinemia, which may lead to irreversible brain damage,mental retardation, and neurologic defects in the fetus (2,3).On the other hand, it is uncertain as to whether moderate andmild iodine deficiency affect motor and cognitive functions(4). Gestation is accompanied by significant physiologicaladjustments in both TH and iodine metabolism that increaseiodine requirements (4,5). Maternal T4 synthesis is increased

*50% to maintain the euthyroidal state in the mother and totransfer a small amount of TH to the fetus. This increase in THis explained by two major reasons (5). These are an increase inthyroxine binding globulin due to estrogen stimulation,which decreases free TH and stimulates pituitary-thyroidaxis, and thyrotropin receptor stimulation by human chor-ionic gonadotropin. In addition, in later gestation, iodine istransferred to the fetus for TH synthesis, and there seems to bean increase in iodine glomerular filtration rate during gesta-tion; although this observation is still under discussion (4,5).On this basis, the World Health Organization, the UnitedNations Children’s Fund, and the International Council forIodine Deficiency Disorders (WHO/UNICEF/ICCIDD) haveset the recommended daily iodine intake in adult men andwomen at 150 lg and in pregnant women at 250 lg (6). On theother hand, the most useful indicator to evaluate iodine nu-trition in a population is the assessment of the median of

Laboratory of Endocrinology and Nutrition, Department of Biomedical Research, Faculty of Medicine, Universidad Autonoma deQueretaro, Queretaro, Mexico.

THYROIDVolume 21, Number 12, 2011ª Mary Ann Liebert, Inc.DOI: 10.1089/thy.2011.0197

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urinary iodine concentration (UIC), and it was establishedthat in pregnant women a median UIC between 150 and249 lg/L indicates an adequate iodine intake (6).

In recent years, several countries have evaluated iodinenutrition in pregnant women by using the UIC (7–13). It hasalso been shown that in some countries, although iodine in-take is sufficient in school-age children, there is iodine defi-ciency in pregnant women (8,9). This finding justifies the needfor a continuous monitoring of iodine nutrition in these twovulnerable populations. One of the most cost-effective actionsto guarantee an adequate iodine intake is universal salt iodi-zation (USI), and special attention should be paid to the cor-rect application of this kind of program (6). In Mexico, thereare no recent data regarding UIC in children and pregnantwomen, but there is a wide USI program (14). It was estimatedthat in 2009, 81% of all marketed salt in Mexico contained anadequate iodine concentration (20–40 ppm), and 94% con-tained ‡ 15 ppm of iodine (14). This is the minimum quantityrecommended by international organizations (6). However,the quality of the salt was not homogenous throughout thecountry. There were places where only 25% of the salt sampleshad ‡ 15 ppm of iodine (15). This could contribute to insuffi-cient iodine intake, and to a heterogeneous iodine nutritionstatus. Considering the WHO/UNICEF/ICCIDD recom-mendation for a periodic surveillance of the iodine nutritionstatus in vulnerable populations, in the current study, weanalyzed the iodine nutrition of pregnant women in the mu-nicipality of Queretaro, Mexico.

Materials and Methods

Setting and subjects

This study was performed in the municipality of Queretaro,which is one of the most populated municipalities in thecountry with important industrial and commercial activity(16). The municipality of Queretaro is located at the center ofthe country (20�30¢–20�56¢ N, 100�17¢–100�36¢ W), at a dis-tance of 221 km northwest from Mexico City. According to the2010 national census, the municipality of Queretaro has801,883 inhabitants (1,174.5 inhabitants per km2) representing43.9% of the total population of the State (16).

In 2008, the municipality of Queretaro registered 22,324births, and 43.5% of these were attended by the Ministry ofHealth of Mexico (Secretaria de Salud, SSA), 41.0% were at-tended by the Mexican Institute of Social Security, and 13.6%were attended by private medical clinics (17). The SSA pro-vides public health care to the population without health in-surance. The SSA in conjunction with the Ministry of Health ofthe State of Queretaro (Secretaria de Salud del Estado deQueretaro, SESEQ) maintains 23 Public Medical Units, pro-viding both primary and antenatal care.

The population included in this cross-sectional study con-sisted of 294 consecutive healthy pregnant women who re-ceived antenatal care in 23 Public Medical Units of the SSA/SESEQ, of which 60 attended during the first trimester ofgestation, 103 in the second trimester, and 131 in the thirdtrimester. All women with a previous history of thyroid dis-ease or medications affecting thyroidal status, including sys-temic illnesses, were excluded from the study. Once enrolled,the women completed a questionnaire. This included demo-graphic data, age, gestational age (based on the date of the lastmenstrual period and corroborated by the Medical Unit re-

cords), smoking habits, consumption of iodized salt, intake ofmultivitamins, exposure to contrast media or other iodine-containing drugs or products such as amiodarone, vaginaldouches, antiseptic skin cleaners, or fruit and vegetable dis-infectants. Special diet consumption such as Japanese foodwith iodine-rich content was also registered. Data were col-lected from April 2009 to July 2010.

This study was approved by the Bioethical Committee ofthe School of Medicine of the Autonomous University ofQueretaro. All participants did so voluntarily by signing awritten informed consent. In the case of pregnant women < 18years of age, written consent was obtained from at least oneparent or guardian.

Urinary iodine concentration

Spot urine samples were obtained from pregnant womeninto a 40 mL plastic urine sample container labeled with theiridentification code. Samples were immediately placed inpolyethylene tubes and stored at - 20�C until analysis. Urinesamples were exclusively employed for UIC determination,and urine test strips were not employed to avoid iodinecontamination (18). The UIC determinations were performedby the Sandell-Kolthoff method after sample digestion withammonium persulfate, according to Pino and colleges (19).Intra- and interassay coefficients of variation were 5.9% and7.8%; respectively. The UIC was expressed as micrograms perliter (lg/L).

The iodine nutrition status in pregnant women was deter-mined according to the recommended WHO/UNICEF/ICCIDD criteria (6). Insufficient iodine intake was defined asUIC < 150 lg/L; adequate intake of iodine as UIC 150–249;iodine intake above the requirements as UIC 250–499; andexcessive intake of iodine as UIC > 500 lg/L.

Statistical analysis

We used Microsoft Excel 2007 (Microsoft Corporation,Redmond, WA), SPSS 16.0 (SPSS Inc., Chicago, IL) andGraphPad Prism 5 (GraphPad Software, Inc., La Jolla, CA)to perform statistical analyses. Values shown are means –standard deviation, or in case of UIC data that are not nor-mally distributed, correspond to medians and ranges. Thenonparametric tests, Mann–Withney and Kruskal–Wallis,were used for comparisons of unpaired groups of UIC. Mul-tiple regression analysis was performed with continuousvariables (UIC, age, and gestational age), and logistic regres-sion analysis was performed with UIC as a dependent vari-able (more than and less than 150 lg/L), and iodine sources,age, and gestational age as independent variables. p-values< 0.05 were considered statistically significant.

Results

The age, gestational age, and the median of UIC by tri-mesters are shown in Table 1. The UIC in the first, second, andthird trimesters of gestation was 273, 285, and 231 lg/L, re-spectively. According to the median of UIC, iodine intake inthe first and second trimesters of gestation was above therequirements, and during the third trimester of gestation, itwas adequate. On the other hand, when the median of UICbetween each trimester was compared, there were no signif-icant differences ( p > 0.05). In the 294 pregnant women

1368 GARCIA-SOLIS ET AL.

studied, the median of UIC was 260 lg/L, with a range of5–1320 lg/L, and the percentages of samples with a UICbelow 150 and 50 lg/L were 28% and 6.8%, respectively.

On the one hand, 69.4% of the participants reported con-suming iodized salt, whereas 4.7% declared no consumptionof iodized salt, and 25.8% reported that they did not knownwhether their salt was iodized. Twelve percent of subjectsreported daily intake of vitamin and mineral supplementswith at least 100 lg of iodine. The use of iodine-containingproducts (v. gr. vaginal douches, antiseptic skin cleaners, orfruit and vegetables disinfectants) or the consumption of io-dine-rich foods such as seaweed, as well as smoking, wasmarginal, being < 0.5%. No subjects reported exposure tocontrast media. We found no difference between the UIC ofwomen using iodine-containing multivitamins and those whoreported the consumption of noniodized multivitamins( p > 0.05). In addition, we found no difference between theUIC of women using iodized table salt and those who em-ployed noniodized table salt, or those who did not knowwhether their table salt was iodized ( p > 0.05). Finally, mul-tivariate analyses were performed (multiple and logistic re-gressions) to identify significant predictors of UIC as iodinesources, age, and gestational age; the results were not signif-icant.

Discussion

We found that pregnant women without public healthinsurance in Queretaro, an urban and highly populated lo-cality in Mexico, had an intake of iodine above requirements.This result is in agreement with the fact that only 5% ofpregnant women declared not using iodized salt to cook,although a significant percentage of the subjects (25.8%) didnot know whether their salt was iodized. This suggests apoor knowledge about the importance of iodine in humannutrition. In the current study, the iodine concentration intable salt samples was not determined. This is a limitation ofthe study. Other iodine sources such as multivitamin sup-plements with iodine were consumed only by 12% of preg-nant women. These data suggest that iodized salt plays acentral role in sustaining iodine nutrition in pregnantwomen in Queretaro, and this is in agreement with recentnational reports of the USI program (14). However, a peri-odic surveillance of UIC is not performed in Mexico; theNational Nutrition Survey in 1999 showed that the medianUIC in nonpregnant women of 12–49 years of age was281 lg/L (20). These data indicate an iodine intake aboverequirements. However, the only study to our knowledge

conducted in Mexican pregnant women showed that, inthree geographic regions in the state of Hidalgo (Pachuca,Ixmiquilpan, and Huejutla), the median UIC was 116, 124,and 109 lg/L, respectively (21). According to the currentWHO/UNICEF/ICCIDD criteria, these values correspondto an insufficient iodine intake for pregnant women. The lackof relation between UIC and other variables (multivariateanalyses) could be explained by other factors or sources notconsidered in this study, such as dietary habits or the intakeof processed foods (11).

Our data contrast with several countries that analyzediodine nutrition status in pregnant women. On one hand, ithas been shown that not fully implemented USI programsfail to reach optimal iodine nutrition in gestation (v. gr.Bosnia and Herzegovina, India, Thailand) (7–9). On theother hand, countries such as Iran found that the USI strat-egy seems not to be enough to satisfy iodine requirementsduring the entire gestation period (10). In contrast, devel-oped countries such as Switzerland and the United Statesreached overall optimal iodine nutrition without mandatorysalt iodization; however, they established permanent mon-itoring iodine nutrition programs in their population, thusallowing the timely implementation of corrective actions(1,11,12). This continuous monitoring has recently revealedthat some subgroups of pregnant women in the UnitedStates may be at risk of mild iodine deficiency (22), whichfurther highlights the importance of a permanent surveil-lance program.

Even more, it is well established that the use of a medianbetween 100 and 199 lg/L UIC in children of 6–12 years of ageas an indicator of adequate iodine nutrition in a population isnot useful to assume adequate iodine nutrition during ges-tation (6–12). WHO/UNICEF/ICCIDD has highlighted theimportance of periodically monitoring iodine nutritionduring gestation, and not only in school-age children (6).All these data underline the need of a new national surveyof iodine nutrition in Mexico.

We noticed a slight nonsignificant trend toward a reductionof median UIC from the second to third trimester of gestation,together with an increase in the percentage of samples below150 lg/L of UIC (Table 1). Other studies in countries such asBosnia and Herzegovina, Iran and Thailand have shown asimilar behavior in the median of UIC in the last two tri-mesters of gestation (7–9). This reduction of UIC duringpregnancy is more clearly evident in places where iodine in-take is borderline sufficient or frankly deficient (13). Stilwellet al. showed in Australian pregnant women a clear reductionin UIC through gestation, with 22.2% of samples below 50 lg/

Table 1. Summary of Characteristics of Pregnant Women

First trimester Second trimester Third trimester All(n = 60) (n = 103) (n = 131) (n = 294)

Age, yearsa 23.7 – 5.4 23.4 – 5.6 23.7 – 5.8 23.6 – 5.6Gestational age, weeksa 9.7 – 2.4 19.9 – 3.6 30.3 – 3.6 22.4 – 8.6Median UIC, lg/L 273 285 231 260UIC below 150 lg/L, % 31.7 22.3 30.5 28.0UIC below 50 lg/L, % 3.3 6.8 8.4 6.8

aMean – standard deviation.UIC, urinary iodine concentration.

IODINE NUTRITION IN MEXICAN PREGNANT WOMEN 1369

L at the beginning of gestation, and 40% at the end (13).All these data suggest a depletion of maternal iodine storesdue a maternal-fetal use, renal elimination, and/or inade-quate dietary compensation (13).

Finally, due to the current global trend to reduce salt intakefrom 10 g/day per capita to < 5 g/day to prevent heart diseaseand stroke (23), it is important to evaluate iodine nutritionstatus during pregnancy. Iodine intake in pregnancy shouldnot be jeopardized due to a reduction in salt consumption.As an alternative, the iodine concentration of table salt shouldbe increased or a regular iodine supplementation should beestablished for pregnant women. It was shown in one areathat the restriction of iodine fortified salt during preg-nancy was associated with insufficient UIC (7). More com-prehensive studies in children and pregnant women areneeded throughout to address this issue and to sustain ade-quate iodine nutrition. These should identify salt consump-tion patterns, iodine in salt and other food supplies, andcorrelate these parameters with UIC data.

Acknowledgments

The authors are grateful to Dr. Ana Colarossi, from theCayetano Heredia Peruvian University, Laboratory of Mi-cronutrients (a member of the IRLI Network), for her technicaladvice and training in urinary iodine determination. Theywould like to especially recognize Dr. Eduardo Pretell for hisinvaluable suggestions in the preparation of this article. Theyalso acknowledge Dr. Carlos Valverde for his critical reviewof this article. The authors would also like to thank the au-thorities of the SESEQ, especially Dr. Julio Hernandez, Dr.Gildardo Rivera, and Dr. Ramon Mancilla for their assistancein performing this study. The authors are also grateful to theundergraduate iodine study group (Diana Cortes, UlisesHernandez, Jose Ibarra, Juan Macias, Jose Castillo, Ana Iba-nez, Selene Munoz, Adydolbeth Moreno, Erika Reyes, andPaola Garay) for sample collection. This study was supportedby grant PROMEP-SEP UAQ-PTC-094.

Disclosure Statement

The authors declare that no competing financial interestsexist.

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Address correspondence to:Pablo Garcıa-Solıs, Ph.D.

Laboratorio de Endocrinologıa y NutricionDepartamento de Investigacion Biomedica

Facultad de MedicinaUniversidad Autonoma de Queretaro

Clavel 200Fracc. Prados de la Capilla

Queretaro C.P. 76176Mexico

E-mail: pablo.garcia@uaq.mx

IODINE NUTRITION IN MEXICAN PREGNANT WOMEN 1371

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