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.