Researchers from Harvard Medical School and Brigham and Women’s Hospital found that exposure to high levels of iodide—commonly given intravenously to patients for medical imaging procedures—is associated with an increased risk of thyroid disease.

The study, published in the Archives of Internal Medicine, is the first large-scale investigation of how iodide contrast agents used in medical imaging procedures can disrupt thyroid function. The study was based on data gathered over a 20-year period from patients treated at either Brigham and Women’s or Massachusetts General Hospital without pre-existing hyperthyroidism or hypothyroidism.

Approximately 80 million doses of iodinated contrast agents are administered each year to aid doctors in seeing blood vessels and tissues, largely during CT scans and cardiac catheterizations, according to Connie M. Rhee, lead author of the study.

According to Rhee, a single dose of contrast agent could contain 90 to several hundred times the recommended daily intake of iodide.

“Iodine is present in things even beyond contrasts. It is in a lot of medications, in the foods we eat, and it is even in some cleaning agents that we use in the hospital in small amounts,” Rhee said.

In an emailed statement, Elizabeth N. Pearce ’90, a professor at the Boston University School of Medicine, said that “the ‘as low as reasonable achievable’ principle should always be exercised in determining the need for radiation exposure, as potential risks of iodinated contrast studies extend beyond the thyroidal risk from iodine exposure.”

Hypothyroid and hyperthyroid disease are associated with higher risk of cardiovascular disease, heart failure, and high blood pressure and with damage to other organs beyond the heart.

According to Pearce, physicians should take particular care in administering iodinated contrast imaging procedures with patients at high risk of developing thyroid dysfunction. However, Rhee cautioned that patients should not necessarily avoid medical imaging procedures to prevent thyroid problems, since medical imaging with iodinated contrast dyes is often vital for urgent diagnoses.

Both Rhee and Pearce noted that further research is still necessary.

“Future prospective studies are needed to better understand risk factors for the development of iodine-induced thyroid dysfunction following exposure to iodinated contrast,” Pearce said.

—Staff writer Cynthia W. Shih can be reached at cshih@college.harvard.edu.