Iodine Deficiency Still a Concern, Despite Vast Improvements

Nancy A. Melville

May 21, 2013

Despite significant improvements in global iodine adequacy in recent decades, median urinary iodine concentration levels among US adults have been on the decline, with low rates among women of childbearing age of particular concern, according to an article in the May issue of Thyroid.

Approximately 70% of households worldwide currently have access to adequately iodized salt, up from just 10% in 1990, when the United Nations World Summit for Children established its goal to eliminate iodine deficiency worldwide, the new report says.

But the efforts appear to be losing momentum for a variety of reasons, the authors assert.

"Progress over the last decade has slowed, limited primarily by the technical challenges of reaching small salt producers, poor quality control of salt iodization, waning interest by governments, and difficulties in enforcing iodized salt legislation," Elizabeth N. Pearce, MD, associate professor of medicine at Boston University School of Medicine, Massachusetts, and colleagues write.

As of 2013, 111 countries have sufficient iodine intake, defined as median urinary iodine concentrations (UIC) of 100 to 299 µg/L in school-aged children and ­>150 µg/L in pregnant women. Thirty countries are considered iodine deficient, 21 are mildly deficient, 9 are moderately deficient, and none are considered severely deficient.

Concern as Iodine Concentrations Fall Among Women

In the United States, median UIC levels among individuals aged 6 years and older remained in the normal range, at 144 µg/L, from 2007 to 2010, but median UIC among adults has been on the decline, with a drop of more than 50% between the 1970s and 1990s, say the authors.

And the prevalence of UIC below 50 µg/L among women of childbearing age has increased nearly 4-fold during the same period, from 4% to 15%, while the median UIC among pregnant US women has declined to less than 150 µg/L, suggestive of mild iodine deficiency, according to the National Health and Nutrition Examination Surveys (NHANES), they note.

These reductions are a concern because iodine deficiency in pregnant women is linked to neonatal hypothyroidism, increased pregnancy loss, and infant mortality.

"In women of childbearing age, the concern [of iodine deficiency] is less for overt thyroid dysfunction than for the effect on the fetus and in a breast-fed infant who may not have adequate iodine for growth and development," said Dr. Pearce.

The cause of the decline among pregnant women is unknown but may have to do with reductions in dairy intake, she speculated.

"We think the major source of iodine in the US diet at the moment is dairy, so the decline may relate to a lower intake of milk among pregnant women, but no one knows for sure."

Cow's milk is a key source of iodine, not because it is fortified as a public-health measure but because iodine is put in cattle feed to keep cows healthy. Iodine-containing cleansers used by the dairy industry also get into the milk supply, Dr. Pearce explained.

Recommendations to reduce salt intake for cardiovascular reasons likely play less of a role in iodine concentration declines, because only about 70% of salt is iodized, and about 85% of salt ingested in the United States is from commercially processed foods that do not contain iodized salt, she added.

Despite Recommendations, Message Fails to Get Out

In clinical practice, the diagnosis and treatment of iodine deficiencies are challenging at every level: iodine levels vary in individuals from day to day and even hour to hour; therefore it is not possible to measure the iodine status in an individual. Rather, urinary iodine concentrations have to be gauged more according to population markers. And since iodine typically is not included on food packaging labels, it's also difficult for people to even know how much they are consuming, the researchers say.

That leaves public-health recommendations for groups at risk and the recommendation for women who are pregnant, planning a pregnancy, or breast-feeding is to take an iodine-containing supplement of 150 µg of iodine daily in the form of potassium iodide, Dr. Pearce said.

She noted that strenuous efforts have been made to help prevent iodine deficiencies, particularly in pregnant women, but awareness remains low.

"Myself and others have been working hard on this for the past few years, but I don't think that message has gotten out there," she said.

"[The recommendation] is in the 2 latest thyroid and pregnancy guidelines from the American Thyroid Association and Endocrine Society, and the Teratology Society has released similar guidelines, but I think this maybe hasn't reached the audience that needs to see it."

The authors have reported no relevant financial relationships.

Thyroid . 2013:23:523-528.

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