So the findings suggest that people -- at least in China -- should get no more than 800 micrograms a day, according to the researchers, led by Wanqi Zhang of Tianjin Medical University.
That's different from what's recommended in the U.S., where National Institutes of Health guidelines say the safe upper limit for adults is 1,100 micrograms of iodine per day.
The current findings are based on 256 healthy adults who had normal thyroid when they entered the study. Zhang's team, which 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 micrograms per day, for four weeks.
Of the people who took 400 micrograms, 5 percent developed subclinical hypothyroidism. And the numbers rose in tandem with the iodine dose: Of people on the highest dose (2,000 micrograms per day), 47 percent developed subclinical hypothyroidism.
"These are interesting data," Pearce said, "because we don't have a lot of information on iodine excess."
Subclinical hypothyroidism has no obvious symptoms, but studies have linked it to an increased risk of heart disease over the long term, Pearce noted.