The iodide dye used in heart scans and other medical imaging, such as CT scans, may in some cases damage patients' thyroid glands, possibly leading to such health problems as thyroid disease, according to a US study. But the diseases occur rarely, are eminently treatable and should not lead patients to put off having scans, experts say.

Patients who had signs of thyroid disease were between two and three times as likely to have had a scan using iodide as a comparison group of people without thyroid problems, researchers write in the Archives of Internal Medicine.

Some 80 million doses of the dye are administered worldwide every year, and while the chemical is known to take a toll on the kidneys, there has up to now only been anecdotal evidence that it could also hurt the thyroid, says Steven Brunelli of Brigham and Women's Hospital in Boston, who worked on the study.

While the findings aren't ironclad proof that the dye itself is responsible, experts agree that the dye was a likely explanation, since high doses of iodide are known to throw the thyroid off balance - and the amounts given during a scan may be several hundred times greater than recommended daily intakes.