Current status of iodisation in pregnant women in Asturias, Spain

Abstract

Introduction

Although former studies indicated that Asturias (Spain) is an iodine-sufficient region, preliminary data suggest this trend may be changing.

Objective

To determine the prevalence of iodine deficiency in pregnant women in Asturias.

Material and methods

We designed an observational, prospective, multicenter study. The urinary iodine concentration (UIC) of 371 pregnant women was analyzed between November 2021 and March 2023. Participants were recruited from the three central areas of the Principality of Asturias (Areas III, IV, and V). The project received approval from the Research Ethics Committee of the Principality of Asturias (RECPA).

Results

A total of 52% of pregnant women had urinary iodine levels indicative of iodine deficiency (< 150 μg/L). The average UIC in our cohort was 146 μg/L. We compared thyroid-stimulating hormone (TSH) levels between pregnant women with iodine deficiency and those with sufficient iodine, but found no statistically significant differences in TSH levels between the two groups. We also couldn't establish statistically significant links between iodine deficiency status and body mass index (BMI), number of previous births, nationality, or age. Furthermore, iodine deficiency did not show a significant link to obstetric complications such such as hypertension during pregnancy or premature birth.

Conclusions

These findings support the hypothesis of an increase in iodine deficiency during the first trimester of pregnancy in our population. This highlights the urgent need to review our supplementation strategies and nutritional education programs.

Resumen

Introducción

Según estudios previos, Asturias es una región con suficiencia de yodo, pero datos preliminares sugieren un cambio de tendencia.

Objetivo

Determinar la prevalencia de yododeficiencia en gestantes de Asturias.

Materiales y métodos

Se diseñó un estudio observacional, prospectivo y multicéntrico. Se analizó la concentración urinaria de yodo (CUI) de 371 gestantes entre noviembre del 2021 y marzo del 2023. La captación se realizó en las tres áreas centrales del Principado de Asturias: III, IV y V. El proyecto fue aprobado por el Comité de Ética de Investigación del Principado de Asturias (CEImPA).

Resultados

El 52% de las gestantes presentó niveles de yoduria indicativos de yododeficiencia (<150 μg/L). La mediana de la CUI en nuestra cohorte fue de 146 μg/L. La hormona estimulante del tiroides (TSH) fue comparada entre los dos grupos de gestantes, aquellas con déficit de yodo y las que tenían suficiente cantidad de yodo, sin encontrar diferencias estadísticamente significativas entre las medias de la TSH en ambos grupos. Tampoco se han podido establecer asociaciones estadísticamente significativas entre el estado de yododeficiencia y el índice de masa corporal (IMC), la paridad, la nacionalidad o la edad. Así mismo, el déficit de yodo no tuvo una asociación significativa con complicaciones obstétricas tales como los estados hipertensivos del embarazo o la prematuridad.

Conclusiones

Estos hallazgos respaldan la hipótesis de un aumento de la yododeficiencia en el primer trimestre del embarazo en nuestra población, lo que resalta la urgencia de revisar las estrategias de suplementación y los programas de educación nutricional.

Introduction

Iodine is an essential micronutrient indispensable for human health due to its role in the synthesis of thyroid hormones. These hormones are crucial for cellular metabolism and play a fundamental role in organ growth and development, particularly of the brain. Since brain development occurs primarily during prenatal life and the first years of life, iodine deficiency during these periods can cause hypothyroxinemia and negatively and irreversibly affect child neurological development.¹

Although iodine is present throughout nature, its highest concentration is found in the oceans, with levels up to 50 μg/L. Through evaporation, iodine enters the atmosphere and returns to the soil with rain. However, many land areas, especially mountainous regions and those far from the sea, have iodine-poor soils.

Daily iodine requirements vary depending on the stage of life. During pregnancy and lactation, the requirement increases to 250 μg/day. Since soils—and consequently food and water—are deficient in iodine, diet alone often does not cover these needs. Only marine products, such as fish, shellfish, and seaweed, contain significant amounts of iodine. This fact makes it essential to maintain active iodine prophylaxis strategies to prevent iodine deficiency states.

Insufficient iodine intake can cause deficiency disorders, the severity of which depends on both the level of deficit and the stage of life at which it occurs. Fetal brain damage is the most severe consequence of this deficiency.² During the first trimester of pregnancy, maternal thyroxine (T4), which crosses the placenta, is the only source of this hormone for the fetus, since the fetal thyroid gland begins to function between gestational weeks 10 and 12. Therefore, cortical brain development of the fetus depends exclusively on maternal T4 and, consequently, on adequate maternal iodine intake. Iodine deficiency during pregnancy causes permanent brain damage that may reduce the child’s IQ by 8–10 points.³

Urinary iodine concentration (UIC) directly reflects dietary iodine intake and is the most widely used indicator worldwide to assess iodine status.⁴ Approximately 90% of ingested iodine is excreted in urine. The high day-to-day variability in individuals’ dietary iodine intake results in very high daily variation in UIC, limiting its usefulness for evaluating an individual’s iodine status. It has been estimated that 10 UIC measurements from spot samples or 24-h collections are required to determine an individual’s iodine status with 20% accuracy.⁵ However, at the population level, UIC from spot samples has proven to be a reliable biomarker of recent iodine intake for the population as a whole. Median UIC is used to evaluate population groups.⁴ Recommended values are 100–199 μg/L in the general population and 150–249 μg/L in pregnant women.

One of the first studies on iodine status in pregnant women in Asturias (Spain) was conducted in 2013.⁶ The results showed a median UIC of 197 μg/L. This study highlighted that women who consumed iodized salt but did not take iodine supplements had a median UIC of 190 μg/L. This confirmed that it is possible to achieve adequate nutritional iodine status without pharmacological supplementation. Based on these findings, in January 2015 the Asturias Regional Health Department published a technical report recommending that prescription of iodine supplements in pregnant or lactating women should be individualized. The report discouraged the use of supplements in women who regularly consumed iodized salt and 3 daily servings of dairy products.⁵

In 2017, a new study was carried out in Asturias, focusing on a cohort of 316 pregnant women from Health Area IV. The results, applying the selective supplementation strategy, showed that the median UIC remained above the recommended threshold of 150 μg/L, at 171 μg/L. These findings, published in 2021 and 2023,^7,8 reinforced the idea that pregnant women in Asturias generally presented with adequate iodine status.⁸

The primary endpoint of this new study was to comply with the universal recommendation to ensure epidemiological surveillance of iodine nutritional status in pregnant women. A cohort of 371 pregnant women representative of the 3 central health areas of the Principality of Asturias was designed for this purpose.

Secondary endpoints included analyzing the current consumption of iodized salt and use of iodine supplements among our pregnant women, as well as investigating potential consequences of iodine deficiency during pregnancy.

Materials and methods

• •

  Design: Observational, prospective, multicenter study.

• •

  Population: UIC was analyzed in 371 pregnant women between November 2021 and March 2023. Sample collection was conducted in 3 health areas of Asturias: III, IV, and V. Participants provided informed consent and received a written study information sheet. Urine samples were collected between gestational weeks 9 and 11. The study was approved by the Research Ethics Committee of the Principality of Asturias (CEImPA).

Inclusion criteria:

• •

  Singleton pregnancies followed in the public health care system of Asturias.

• •

  Ability to understand the contents of the informed consent.

• •

  Provision of informed consent.

Exclusion criteria:

• •

  Preexisting thyroid disease.

• •

  Multiple pregnancies.

At the first midwife visit, conducted at week 8, data were collected on habitual consumption of iodized salt (yes/no) as well as pre- or postconception iodine supplementation. All participants were asked about dairy consumption, but these data were not included in our study due to inconsistencies in collection. Pharmacological supplementation with 200 μg/day of potassium iodide was recommended to all women in our study, following the guidelines of the Spanish Society of Gynecology and Obstetrics and the World Health Organization (WHO).⁹ Most participants chose to take multivitamin complexes, and supplementation generally began after conception, starting from the first midwife visit at week 8.

In women from Health Area III (n = 192), possible associations of iodine deficiency with age, nationality, or parity were analyzed, and whether iodine-deficient women had higher rate of prematurity, growth restriction, hypertensive disorders of pregnancy, or gestational diabetes. The same analysis could not be performed in Areas IV and V, as access to health records was denied; therefore, only UIC and questionnaire data were available for these women.

• •

  Variables: UIC, thyroid-stimulating hormone (TSH), iodized salt intake during pregnancy, preconception iodine supplementation, supplementation during pregnancy, age, body mass index (BMI), nationality, gestational age at delivery, hypertensive disorders of pregnancy, gestational diabetes, miscarriages, smoking status.

• •

  Measurement: To determine maternal UIC, random urine samples were collected, centrifuged at 3,500 × g for 10 min. Supernatant was separated and aliquots frozen at −20 °C until analysis. Iodine concentration was determined by inductively coupled plasma mass spectrometry (ICP-MS) using an ICP-MS 7700× analyzer (Agilent Technologies, Santa Clara, CA, USA). All samples were analyzed at the reference hospital (Area IV) following the established laboratory protocol for routine urinary iodine analysis and meeting required quality standards.

• •

  Statistical analysis: Descriptive analysis was performed, reporting measures of central tendency and dispersion for numerical variables and absolute and relative frequencies for categorical variables. To evaluate differences in quantitative variables between 2 groups, Student’s t test for independent samples was used. For relationships between 2 categorical variables, Pearson’s chi-square test or Fisher’s exact test was applied depending on expected frequencies. Statistical analyses were performed with R software (R Development Core Team), version 4.3.1.

Results

A total of 371 pregnant women from 3 health areas of the Principality of Asturias were included in the study. The mean age of participants was 34 years, ranging from 19 to 45 years.

Urinary iodine concentration

Participants were divided into 3 groups according to their UIC:

• •

  <150 μg/L: iodine deficiency

• •

  150–500 μg/L: iodine sufficiency

• •

  >500 μg/L: iodine excess

Of the 371 pregnant women, 191 (52%) presented with a UIC < 150 μg/L (Fig. 1). In other words, more than half of the women in our cohort had iodine deficiency during the first trimester of pregnancy.

Figure 1.

Distribution of pregnant women according to urinary iodine concentration. Fifty-two percent show iodine deficiency. UIC: urinary iodine concentration.

The UIC variable showed positive skewness (skewness coefficient = 2.04), and this skewness was statistically significant (p < 0.001). The kurtosis coefficient was 6.39 with p < 0.001, indicating that UIC followed a leptokurtic distribution (Fig. 2). The D’Agostino–Pearson test rejected the null hypothesis of normality, confirming that the data were not normally distributed. Therefore, the median was calculated as the measure of central tendency, and percentiles were used as measures of dispersion (Table 1).

Figure 2.

Histogram of UIC (μg/L) within the first trimester of pregnancy. The black line indicates that the data are not normally distributed.

Table 1.

Percentiles of urinary iodine concentration.

Percentiles	Value	95% Confidence Interval
2.5	43	26–51
5	53	45–59
10	66	59–72
25	91	85–99
75	233	207–270
90	386	345–440
95	476	436–585
97.5	594	504–666

The median UIC for the entire cohort was 146 μg/L, below the 150 μg/L recommended during pregnancy, and lower than the medians reported in previous studies conducted in our region (Fig. 3).

Figure 3.

Temporal trend in median UIC among pregnant women in Asturias.

Of note, women from Health Area IV had an adequate median UIC of 157 μg/L, which is consistent with previous studies in Asturias, as those investigations included only women from this area.⁷

Nonetheless, a comparative study of the medians across the 3 health areas was conducted using the Kruskal–Wallis test: no significant inter-group differences were observed (p = 0.168) (157, 146, and 135 μg/L, respectively) (Table 2).

Table 2.

Comparison of median urinary iodine concentrations among different health areas.

Origin	n	25th Percentile	Median	75th Percentile
Area III	186	99	146	236
Area IV	86	91	157	263
Area V	99	82	135	207

Iodine intake

At the first midwife visit, a questionnaire was administered to assess iodine intake. In total, 228 questionnaires were completed across the 3 health areas. The following data were collected:

• •

  Usual consumption of iodized salt (yes/no)

• •

  Preconception iodine supplementation (yes/no)

• •

  Postconception iodine supplementation (yes/no)

For iodized salt consumption, the frequency distribution was: yes, 64.04%; no, 35.96%.

For preconception iodine supplementation, the frequency distribution was: no, 81.14%; yes, 18.86%.

For postconception iodine supplementation (starting at week 8 after the first midwife visit) the frequency distribution was: yes, 89.91%; no, 10.09%.

Association between iodine deficiency and other variables

From the total cohort, women from Health Area III (n = 186) were selected for a comparative study between those with iodine deficiency (UIC < 150 μg/L) and those without deficiency (UIC 150–500 μg/L). The following variables were analyzed:

• •

  First-trimester TSH

• •

  BMI

• •

  Age

• •

  Nationality

• •

  Parity

• •

  Smoking status

• •

  Use of iodine supplements during pregnancy

No statistically significant differences were found in any of these variables (Table 3). However, the median TSH in the iodine-deficient group was substantially higher than in the iodine-sufficient group, although the small sample size did not allow this difference to reach statistical significance.

Table 3.

Comparison according to iodine status.

	UIC < 150 μg/L	UIC > 150 μg/L
TSH (median)	1.48	1.79
BMI (mean)	26.07	25.43
Age (mean)	34.96	34.73
Asturian women (%)	77.17	74.16
Primiparous women (%)	52.75	54.55
Smokers (%)	14.94	13.79
Iodine supplementation (%)	89.01	95.45

For the comparison of these variables, the Student t test, Fisher’s exact test, and Pearson’s chi-square test were used. BMI: body mass index; TSH: thyroid-stimulating hormone.

Obstetric complications

Again, among the 186 women from Health Area III, a comparative study was conducted between those with iodine deficiency (UIC < 150 μg/L) and those without (UIC 150–500 μg/L). The following variables were analyzed:

• •

  History of miscarriage

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  Gestational age at delivery

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  Neonatal birth weight

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  Hypertensive disorders of pregnancy

• •

  Gestational diabetes

No differences were observed in any of these complications except for gestational diabetes, which was more prevalent in the iodine-deficient group.

Discussion

The median urinary iodine concentration (UIC) obtained in our study confirms iodine deficiency among pregnant women in Asturias. This finding modifies the previously available evidence, which had shown iodine sufficiency in pregnant women from Health Area IV.⁷ Nevertheless, it is noteworthy that, in our study as well, women from that health area had a median UIC > 150 μg/L.

There are no nationwide studies in Spain on iodine nutritional status in pregnant women. However, over the last decade, several local or regional studies have evaluated iodine status in pregnant women in Spain and report heterogeneous results,^6,10–15 with median UIC ranging from 57 to 242 μg/L.

In Asturias, the first study in pregnant women (2002) showed severe iodine deficiency in more than 50% of women. In 2013, a new assessment found a first-trimester median UIC of 197 μg/L.¹⁶ In 2017, another study in pregnant women from Health Area IV reaffirmed iodine sufficiency with a median UIC of 171.5 μg/L.⁷ By contrast, our study shows a clear downward shift in median UIC.

Moreover, this trend toward iodine deficiency does not seem to be unique to our region, but quite possibly reflects patterns in Spain and across Europe.

In 2024, the WHO and the Iodine Global Network (IGN) published a report—Prevention and control of iodine deficiency in the WHO European Region: adapting to changes in diet and lifestyle—warning of increasing iodine deficiency in Europe, attributed largely to recent dietary shifts such as replacing dairy with plant-based alternatives that, in most cases, are not fortified with iodine.¹⁷

Pregnant women are particularly vulnerable due to increased requirements. The report highlights that declining intake of dairy products—a traditional iodine source in Western and Central Europe—raises deficiency risk. It also notes that only a minority of processed foods contain iodized salt, compounding the problem. To address this, WHO recommends:

• •

  Fortifying foods: add iodine to plant-based products that replace dairy, ensuring adequate intake for those choosing these alternatives.

• •

  Using iodized salt: promote iodized salt use in the food industry and at home to guarantee sufficient iodine intake.

• •

  Providing nutrition education: inform the public about the importance of iodine and dietary sources that contain it.

These measures aim to adapt to evolving European dietary patterns to ensure adequate iodine intake and prevent deficiency. Progress in optimizing iodine intake seems to have stalled—or even regressed—in some countries; if this trend continues, iodine deficiency disorders could reemerge.

Regarding iodized salt use among pregnant women, no study has covered the entire Spanish territory, according to the Scientific Committee Report of the Spanish Agency for Food Safety and Nutrition.¹⁸ Nonetheless, multiple provincial or regional studies over the last decade report iodized-salt use ranging from 32% to 55%—well below WHO targets. In a study from Catalonia, midwife-led education was associated with a marked increase in iodized-salt use from the first to the third trimester (35.71%–87.61%).¹⁹

In Asturias, a 2013 study in Oviedo found that 47% of pregnant women took iodine supplements; their median UIC was higher than among those not taking supplements, as was TSH (though not significantly). Dairy consumption (present in 70.29% of women) did not correlate with UIC; however, median UIC was lower among those consuming ≤2 daily servings (191 μg/L) vs >2 servings (230 μg/L). Among women not taking supplements, habitual use of iodized salt in cooking (47%) was associated with a median UIC of 190 μg/L, indicating iodine sufficiency. The authors concluded that iodine supplements were unnecessary in pregnant women who habitually consumed iodized salt.¹⁶

Subsequently (2021), 51.10% of pregnant women in Health Area IV reported using iodized salt, 48.90% consumed ≥2 daily dairy servings, and 87.08% were on iodine supplements. In multivariable logistic regression, iodized salt had a protective effect against UIC < 150 μg/L, whereas iodine supplements did not.⁷

In our study, despite apparent improvement in iodoprophylaxis—64.04% reported iodized-salt use and 89.91% initiated iodine supplementation after the first midwife visit—first-trimester UIC was lower than previously reported.

Several considerations may explain this:

• •

  During pregnancy, iodine requirements increase significantly; for this reason, the WHO recommends a daily intake of 250 μg of iodine during pregnancy and lactation. To achieve this intake, it is necessary to consume 3 daily servings of milk or dairy products along with 2 g of iodized salt. It is essential that this intake be consistent over time, as beginning to use iodized salt only at the onset of pregnancy is insufficient. Adequate iodine stores must be achieved before conception, which requires regular use of iodized salt for at least the 2 years prior to pregnancy.²⁰

• •

  In industrialized countries, milk and dairy products are the main dietary sources of iodine.²¹ However, both the consumption of these products and their iodine concentrations may vary considerably, making them an unpredictable source of this micronutrient. Milk iodine content ranges from 33 to 534 μg/L, depending on factors such as the cows’ diet, the presence of goitrogens, milk yield, season of the year, use of iodine-containing teat dips, type of husbandry, and processing practices.²² It is estimated that milk and dairy products provide between 13% and 64% of the recommended daily iodine intake, depending on specific dietary patterns in each country.

• •

  Regarding multivitamin complexes containing iodine, although the technical data sheet specifies iodine content ranging from 150 to 225 μg, some studies have shown that the actual iodine content after manufacture is highly variable and not always consistent with the label.

Potassium iodide supplements (either marketed alone or combined with folic acid and vitamin B₁₂) should be started preconceptionally, at least 2 months before pregnancy, to ensure adequate iodine stores at conception. The daily recommended dose, present in most pharmacologic supplements marketed in Spain, is 100, 200, or 300 μg per tablet. A daily tablet is recommended from the preconception or early gestational stage and should be continued uninterruptedly until the end of exclusive breastfeeding.²³

In conclusion, according to our results, iodized-salt consumption among pregnant women is well below the WHO target of > 90%, and exclusive postconceptional supplementation is not an effective measure to maintain adequate iodine levels during the first weeks of pregnancy. The ineffectiveness of postconceptional supplementation is likely explained by the short exposure time, which is insufficient to normalize iodine levels in the first trimester. This reinforces the need for preconceptional supplementation strategies, whether through regular iodized-salt use, an adequate diet, or pharmacologic supplementation beginning in the pregestational period.

An interesting finding from our results is that, among the 3 health areas studied, iodine deficiency was greater in the coastal zones (Health Areas III and V) than in the inland zone (Area IV). This may be explained by other factors influencing iodine nutritional status.

Smoking has also been shown to increase iodine deficiency risk, particularly in infants of smoking mothers.²⁴ Although smoking has not been shown to increase the risk of iodine deficiency during pregnancy,¹⁹ pregnant women are advised not to smoke or remain in smoke-filled environments. Other determinants, such as education level and socioeconomic status, also affect iodine intake. In one study, people with no formal education used iodized salt less often than those with primary, secondary, or university education.²⁵ Similarly, in the study by Melero et al. in Madrid,¹⁴ pregnant women with lower educational attainment consumed less iodine. Dairy and fish consumption was also lower in populations with lower socioeconomic status.

In Spain, as in some other European countries, salt iodization is voluntary. The iodine content of iodized salt in Spain (60 ppm) is among the highest in Europe, meaning that even modest iodized-salt consumption could provide the recommended daily intake.

Women especially vulnerable to iodine deficiency include those who do not consume iodized salt, follow a vegan diet (avoiding dairy, fish, and other animal-derived iodine sources),^26,27 and do not obtain iodine from other sources.

Our proposal to counteract the trend toward iodine deficiency in pregnant women would include:

• •

  Universal use of iodized salt in households and the food industry.

• •

  Preconception and pregnancy iodine supplementation for women without consistent iodized-salt use (for at least the 2 years before pregnancy).

• •

  Inclusion of authorized nutrition and health claims on food labels regarding iodine content, which could serve as a useful tool to promote appropriate, informed choices of iodine-containing foods that meet legally established criteria for such claims.

Conclusions

• •

  More than half of the pregnant women in our study had iodine deficiency during the early weeks of gestation.

• •

  Iodine supplementation only during pregnancy does not appear sufficient to ensure required iodine amounts in the first trimester.

• •

  Iodine deficiency among pregnant women in Asturias underscores the need to review supplementation strategies and nutrition education for this population.

• •

  Periodic monitoring of UIC in pregnant women is essential, given its dynamic behavior and the generalized downward trend in our setting.

Funding

None declared.