NEW YORK -- Mothers who are mildly iodine deficient are more likely to have children who perform poorly in spelling, grammar and literacy, according to a new study from Australia.
Severe iodine deficiency during pregnancy is known to cause serious mental disabilities in children, but researchers examined the test scores of nine year olds whose mothers were only slightly iodine deficient during pregnancy and found the kids performed between 6 percent and 10 percent worse than peers born to mothers with sufficient iodine.
"This is to show in areas where there is even mild deficiency it can have long-term impacts on children," said Kristen Hynes, the study's lead author from the University of Tasmania in Sandy Bay.
Throughout life, everyone needs iodine to make thyroid hormones, but it's also crucial that pregnant women get enough of the element to support their children's brain development.
Past research has found that women who are severely iodine deficient give birth to children with motor, cognitive and auditory defects, Hynes' team writes in The Journal of Clinical Endocrinology & Metabolism.
Little is known, however, about what impact a mother's mild iodine deficiency might have on her child.
The U.S. Institute of Medicine recommends the average adult get 150 micrograms (mcg) of iodine every day. Pregnant women are told to get 220 mcg every day and women who are breastfeeding are told to get 290 mcg.
The main source of iodine in the American diet is milk, but it can also be found in some fish and vegetables as well as in "iodized" table salt.
One cup of reduced-fat milk contains about 56 mcg of iodine and one serving of baked cod has about 99 mcg, according to the U.S. National Institutes of Health.
For the new study, Hynes and her colleagues used data on about 228 pregnant women who were patients at The Royal Hobart Hospital in Tasmania between 1999 and 2001 and the children they delivered at the time.
The researchers compared the standardized test scores of the nine-year-old children born to women whose urinary iodine levels fell below 150 mcg (mildly deficient) during pregnancy to the children of women whose iodine levels exceeded 150 mcg (sufficient).
The children of women who were iodine deficient scored about 371 points on the national student test for spelling and about 377 points for grammar. That compared to about 412 points for spelling and 408 points for grammar among children of women who had sufficient levels of iodine while pregnant.
The average score for all of Tasmania at that time was about 390 for spelling and about 410 for grammar in that age group.
The researchers also found that children born to mothers who were mildly iodine deficient scored about 6 percent lower on English literacy scores, compared to those whose mothers had sufficient iodine levels.
Hynes told Reuters Health her group suspects that iodine deficiency may have some impact on a child's auditory pathways to the brain, which may hurt their ability to quickly process the information they hear.
"It's really only a theory at this stage," she cautioned.
The researchers can't say for certain whether the mothers' iodine deficiency led to their children's poor scores, but she said there is an association.
"I think people working in education will realize this is important, but there are a lot of other things going on as well," Hynes said.
Dr. Elizabeth Pearce, who researches iodine but was not involved in the study, said the findings support recommendations that soon-to-be pregnant, pregnant and breastfeeding women take an iodine supplement, but added that it's important to not get too much of the mineral.
"Very excessive intake in susceptible people can lead to thyroid dysfunction and goiter," said Pearce, an associate professor of medicine at Boston University.
The NIH says the upper safe limit of iodine is 1,100 mcg a day.
"People should not read this and think more iodine is better," she said.