Writing in the journal The Lancet, researchers reported that 70 percent of UK girls age 14-15 could have a deficiency of iodine, which is essential to maintaining thyroid hormone function and occurs naturally in seawater and soil. Adding iodine to table salt, as it's done in the U.S., could help matters. About 70 percent of table salt in the U.S. has iodine added to it, says the Salt Institute.