A century ago, iodine deficiency affected kids across large swaths of the United States. It essentially disappeared after some food makers started adding it to table salt, bread and some other foods, in one of the great public health success stories of the 20th century.

But today, people are getting less iodine because of changes in diet and food manufacturing.

Although most people are still getting enough, researchers have increasingly been reporting low levels of iodine in pregnant women and other people, raising concerns about an impact on their newborns. And there is also a very small, but growing, number of reports of iodine deficiency in kids.

One sign of insufficient iodine is a swelling of the neck, known as a goiter. The thyroid gland in the neck uses iodine to produce hormones that regulate the heart rate and other body functions. When there's not enough iodine, the thyroid gland enlarges as it goes into overdrive to make up for the lack of iodine.

Public health experts realized they couldn't solve the problem by feeding everyone seaweed and seafood, but they learned that iodine can essentially be sprayed on table salt. Iodized salt first became available in 1924. By the 1950s, more than 70% of U.S. households used iodized table salt. Bread and some other foods also were fortified with iodine, and iodine deficiency became rare.

But diets changed. Processed foods now make up a large part of the American diet, and though they contain a lot of salt, it's not iodized. Leading bread brands no longer add iodine.

And for people who do salt their food, the fashion now is to use kosher salt, Himalayan rock salt or other non-iodized products.

“People have forgotten why there's iodine in salt,” said Dr. Elizabeth Pearce of Boston Medical Center.

She noted a reported 50% drop in U.S. iodine levels in surveyed Americans between the 1970s and the 1990s.

Some studies have linked even mild iodine deficiency to lower IQs and language delay in children, although there is debate about at exactly what levels problems start, Pearce said.

This article was provided by The Associated Press.