Chinese researchers have found that excessive iodine consumption may cause thyroid disease. But the problem does not appear to be nearly as bad as disorders caused by too little iodine, a condition public health authorities are seeking to correct worldwide.
Iodine deficiency is a major target of the World Health Organization and other global health programs. It causes mental and physical retardation in children and goiter in both children and adults. This is a swollen neck from an enlarged thyroid gland, which regulates many body functions.
World Health Organization figures show that 750 million people are affected by iodine deficiency in 134 countries and more than two billion others face the risk. Since 1992, the agency has campaigned to boost the iodine content of salt, the main intervention. It says treated salt now reaches nearly 70 percent of households in affected countries.
But the program may have some unintended health consequences.
In China, where salt has been iodized for 10 years, doctors at China Medical University in Shengyang say some people are getting too much of the nutrient. An overabundance can cause the thyroid gland to produce too few hormones, eventually causing fatigue, slow heart rate, confusion, hearing loss, and other side effects.
The doctors tracked people for five years in three regions of China with varying degrees of iodine overconsumption. They report in the New England Journal of Medicine that more than adequate and excessive iodine intake were both associated with slightly increased frequency of mild thyroid disorder in some of the people studied. The percentage increased with the degree of overconsumption, but it was not more than three percent.
A Boston physician who specializes in iodine-related illness says overconsumption is not a serious global problem.
"There is a price to be paid at either end of the spectrum, but I would prefer to avoid iodine deficiency," said Dr. Robert Utiger.
Dr. Utiger of Brigham and Woman's Hospital says thyroid problems associated with too much iodine are outweighed by the substantial health hazards of too little.
"Looked at globally, we are better to err on the side of a little higher intake and accept a very little increase in risk to be sure that there are not a lot of people whose iodine intake is less than, let's say, 100 micrograms per day," he said.
In fact, Utiger says iodine consumption has been declining in countries as diverse as the United States, Australia, Guatemala and Morocco. There are different reasons in different countries, including health warnings against excess salt intake related to heart disease, reduced use of iodine in commercial baking and animal feed, use of uniodized salt in processed foods, and lack of government enforcement of iodine levels in salt.
"In every country, iodized salt should be available, if not mandated by law," noted Utiger.