The iodide dye used in heart scans and other medical imaging might damage some people's thyroid glands, which could cause important health problems later on, researchers say.
In a new study, they found 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.
The findings aren't ironclad proof that the dye itself is responsible, but experts agree that's a likely explanation since high doses of iodide are known to throw the thyroid off balance.
And the amounts typically given during a scan may be several hundred times greater than the recommended daily intake of 150 micrograms.
"It's a tremendous, tremendous dose, really exceeding anything that patients are ever exposed to," said Dr. Steven Brunelli of Brigham and Women's Hospital in Boston, who worked on the study.
Some 80 million doses of iodide dye are administered worldwide every year, he added. The chemical is known to take a toll on the kidneys, but until now there has only been anecdotal evidence that it could also hurt the thyroid.
As a consequence, doctors don't usually tell patients about possible side effects on the thyroid.
"As a community, we have not been aware of this until just now, so this wouldn't be part of the informed consent form," Brunelli told Reuters Health.
His study, published in the Archives of Internal Medicine, is based on data on nearly 400 patients treated at Brigham and Women's or Massachusetts General, 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.
And when the 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.
Brunelli said thyroid disease can cause a plethora of health problems, including heart disease, high blood pressure, weight changes and sexual and psychological problems.
From the study, it appeared hypothyroidism corrected itself over time, while hyperthyroidism did not, he added.
Brunelli said the findings should make doctors more cautious about using scans that employ the dyes, but cautioned that patients shouldn't necessarily avoid the procedures to stave off thyroid problems.
"All of these thyroid diseases are eminently treatable," he told Reuters Health.
Dr. Jeffrey A. Kline, who has studied the effects of iodide dyes, said he was happy to see the new work, but pointed out several limitations.
"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," said Kline, who heads emergency medicine research at the Carolinas Medical Center in Charlotte, North Carolina.
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 the doctor.
Kline said kidney damage, which occurs in about one in ten patients who get iodide dye, is a bigger problem. What's more, the scans also expose people to radiation, which may raise their cancer risk slightly.
"What really worries me is pregnant patients getting dye," he told Reuters Health.
Pregnant women regularly get chest scans to look for blood clots in the lungs, and the dyes pass through the placenta into the fetus's blood stream.
"Ultimately the cognitive function of the baby could be damaged," Kline said.
He said doctors can often use a simple blood test, an ultrasound or an MRI scan instead of the CT scans that usually require iodide.
"I would always ask the doctor if there is a blood test that can do the job, and a lot of time there is," Kline said. "If the doctor says no, then do as the doctor says but at least you've made him or her think."