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Iodine deficiency is a major health problem worldwide, but a new study points out some downsides of too much iodine.
For the new study, reported online in the American Journal of Clinical Nutrition, Chinese researchers randomly assigned healthy adults to take various doses of iodine supplements for four weeks.
They found that at relatively higher doses – 400 mcg a day and up – study participants began developing subclinical hypothyroidism.
People taking 400-mcg supplements were getting around 800 mcg per day when diet was factored in.
So the findings suggest that people – at least in China – should get no more than 800 mcg a day, according to the researchers, led by Wanqi Zhang of Tianjin Medical University.
Don’t use over 150 mcg
That's different from what's recommended in the US, where National Institutes of Health guidelines say the tolerable upper intake level for adults is 1 100 mcg per day.
Still, the typical American would get much less than 800 mcg per day through diet anyway, according to Dr Elizabeth Pearce of Boston University, who was not involved in the study.
That said, Dr Pearce cautioned against using iodine supplements with more than 150 mcg in a daily dose. And most Americans could skip supplements altogether, she said.
"Overall, we're iodine-sufficient," said Dr Pearce, who studies iodine sufficiency and thyroid function.
But she said there are certain people who may need supplements, including pregnant women.
Low iodine increases risk of miscarriage
In the US, the recommended daily allowances are 150 mcg/day for adults; 220 mcg in pregnancy, and 290 mcg while breastfeeding.
The American Thyroid Association recommends that pregnant and breastfeeding women take a vitamin with iodine because low iodine can increase the risk of miscarriage and thyroid problems in moms, in addition to mental disabilities in babies.
According to Dr Pearce, vegans may also want to take a supplement. In a recent study, she and her colleagues found that the average iodine level in a group of 63 vegans was lower than what's recommended - although their thyroid hormone levels were in the normal range.
The current findings are based on 256 healthy adults who had normal thyroids when they entered the study. The researchers, who did not respond to requests for comment, randomly assigned them to take one of 12 doses of supplemental iodine – anywhere from 0 to 2 000 mcg daily, for four weeks.
Iodine levels vary by region in China
Of the people who took 400 mcg, 5% developed subclinical hypothyroidism. The numbers rose in tandem with the iodine dose: in the highest dose group (taking 2 000 mcg per day), 47% developed subclinical hypothyroidism.
"These are interesting data," Dr Pearce said, "because we don't have a lot of information on iodine excess".
In certain parts of the world, the soil is low in iodine, and people who eat mainly local foods have a high risk of deficiency. In other parts of the world – Japan, for example – people have a high iodine intake starting early in life, and they seem to tolerate that high level, Dr Pearce explained.
In China, natural iodine levels vary by region. The country introduced universal salt iodisation in 1996, so the problem of iodine deficiency has been controlled in most areas.
But Dr Pearce said it's not clear if the adults in this study had adequate iodine intake early in life. If not, that could be a factor in their response to iodine supplements.
(Amy Norton, Reuters Health, January 2012)
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