Less Dairy Leads to Decline in Iodine Concentrations

WASHINGTON, DC — Median levels of iodine among women of reproductive age in the United States have declined significantly in the past two decades, while levels among pregnant women have remained below those recommended to maintain healthy thyroid hormone status during pregnancy and prevent adverse outcomes.

"The study uncovered a worrying decline in urinary iodine concentration levels in reproductive age women in the US over the last two decades," report the authors in research presented at the American Thyroid Association (ATA) Annual Meeting and Centennial Celebration.

The declines underscore "a lack of awareness about the importance of iodine nutrition among both medical providers and the general public," senior author Elizabeth Pearce, MD, a professor of medicine at Boston University Chobanian & Avedisian School of Medicine, told Medscape Medical News.

A drop in dairy consumption is directly linked to the decline in iodine concentrations and inversely associated with education level and socioeconomic status among women of reproductive age.

Among pregnant women, the only factor found to be significantly associated with urinary iodine concentration was dairy consumption.

The data detail the significant trends of reduced dairy consumption, with the percentages of reproductive aged women reporting "rare" frequency of dairy consumption significantly increasing over the study period, while those reporting "often" consuming dairy products dropped from more than 50% in 2001-2004 to less than half that by 2017-2020 — and patterns were similar for pregnant women (P < .01 in both groups).

Changes in dietary trends linked to the decline in dairy consumption include increased consumption of cow's milk substitutes such as almond milk and soy milk, which do not contain iodine.

Those reporting higher consumption of soy milk or plant-based substitutes indeed showed trends of lower median urine iodine concentrations compared with those consuming more dairy products among reproductive aged women (P for trend < .001) as well as pregnant women (P for trend = .019).

"The median urinary iodine levels of women consuming plant-based milk substitutes are lower than those in women consuming cow's milk," first author Cheng Han, MD, also from Boston University, said in presenting the findings.

Misconceptions About Need for Iodine

Commenting on the study, Whitney Goldner, MD, a professor of medicine and director of the Thyroid and Endocrine Tumor Program at the University of Nebraska Medical Center in Omaha, agreed there may be misconceptions about the need for sufficient iodine intake.

"Even though many foods in the US are fortified with iodine, that does not mean all persons are iodine sufficient," she told Medscape Medical News.

"Iodine sufficiency is diet-dependent and women of childbearing age should be aware of the recommended daily intake and try to fortify their diet if necessary."

With iodine playing a key role in fetal brain development and growth, its deficiency in pregnancy is especially concerning, as it can have potentially adverse effects on child cognitive impairment. Thus, the need for adequate levels is critical not just among pregnant women, but for all women of reproductive age.

To investigate trends in iodine levels over the past two decades, the investigators evaluated data on women of reproductive age and who were pregnant and enrolled in the National Health and Nutrition Examination Survey (NHANES) database during five subsequent periods between 2001 and 2020.

Overall, the study included data on 24,145 women of reproductive age and 1702 pregnant women.

The median levels of urinary iodine concentrations among women of reproductive age over the course of the study declined significantly from 142  μg/L to 106  μg/L (P  <  .001). 

And while the median iodine concentrations among pregnant women remained relatively stable over the study period, that's not necessarily favorable because levels remained below the WHO recommended cutoff of 150  μg/L, ranging from 147 μg/L to 133  μg/L (P trend =  .68).

Persistent Trends

While the US has, in general, been iodine sufficient since the 1940s, the lower levels observed in pregnant women have previously been reported.

Nevertheless, "it is concerning that this has persisted," Pearce said.

In addition to the association with declines in dairy consumption, other factors that likely contribute to the iodine declines include that (a) iodized salt has never been mandated in the US; (b) the use of iodized salt may be decreasing due to the increased use of sea salt, which has lower iodine; and (c) the use of other forms of salt.

The use of iodine-containing prenatal multivitamins is currently recommended in the US; however, not all prenatal vitamin formulations even contain iodine.

Furthermore, "there is a lack of awareness about the importance of iodine nutrition among both medical providers and the general public," Pearce noted.

Global Improvements Observed

While declines in iodine have been reported in other countries around the world, many have responded with implementation of mandated use of iodized salt, as has been recommended by the WHO.

In fact, while 113 countries had documented iodine deficiency three decades ago, the number has dropped remarkably to just 21 countries in 2022, corresponding with the implementation of salt iodization programs, Han reported.

While Pearce noted that similar policies in the US are "likely politically infeasible," clinicians can nevertheless make a difference by urging patients to follow ATA recommendations that women who are planning pregnancy, pregnant, or lactating should take a daily supplement that contains 150 mcg iodine.

Goldner added that part of the problem is that "since there has been iodine fortification of foods, there has been less concern about iodine sufficiency in the US, except in populations with very restrictive diets."

"But this study highlights the fact that iodine sufficiency is declining in women of reproductive age, which is concerning," she said. "If women are avoiding or reducing dairy consumption and that was a major source of iodine in their diet, they should consider ways of finding other sources of iodine in the diet or supplements." 

Further commenting, Leonidas Duntas, MD, PhD, an endocrinologist and head physician of the Unit of Endocrinology, Metabolism, and Diabetes at the Evgenidion Hospital, University of Athens, Greece, underscored the need for more proactive measures to reverse the iodine decline.

"Salt iodization alone is often not enough and, therefore, regular intake of iodine with meals, including fish, shellfish, and dairy products, is essential," he explained. "If the nutritional recommendations during pregnancy are difficult to achieve, iodine supplements (150-200 μg), especially in iodine-deficient regions, should be prescribed."