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 health problems such as thyroid disease later on, according to a U.S. study.
But the diseases occur rarely, are eminently treatable and should not lead patients to put off having scans, experts said.
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 wrote 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, said 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 agreed 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.
"It's a tremendous, tremendous dose, really exceeding anything that patients are ever exposed to," said Brunelli.
"As a community, we have not been aware of this until just now, so this wouldn't be part of the informed consent form," he told Reuters Health.
The study is based on data on nearly 400 patients treated at Brigham and Women's or Massachusetts General Hospital, who developed either over or underactive thyroids.
Those people were compared to a group of more than 1,400 similar patients with normal thyroid function.
Based on blood samples, the researchers found that people with overactive thyroids so-called hyperthyroidism were twice as likely as the comparison group to have been exposed to iodide dye during CT scans or cardiac catheterization procedures.
When researchers focused on patients who had signs of serious thyroid disease, the link became even stronger and included underactive thyroids, or hypothyroidism.
If that's cause and effect, it means there would be one extra case of hyperthyroidism per 33 patients who get the dye, and one extra case of hypothyroidism per 36 patients.
From the study, it appeared the hypothyroidism corrected itself over time, while hyperthyroidism did not, Brunelli said.
"All of these thyroid diseases are eminently treatable," he added, although he said the findings should make doctors more cautious about using scans that employ the dyes.
Jeffrey Kline, head of emergency medical research at the Carolinas Medical Center in Charlotte, North Carolina, who has studied the effects of iodide dyes, said there were several limitations to the work.
"It just tells us that some of the patients' blood work gets abnormal after (the scans), but we really don't know what it means for the patients," he said.
Based on his own experience, he estimates that only about one in 1,000 patients is bothered enough by thyroid problems from the dye to see a doctor, while kidney damage occurs in about one in ten patients.
He added that doctors can often use a simple blood test, ultrasound or MRI scan instead of the CT scans that usually require iodide.