WEDNESDAY, Jan. 21 (HealthDay News) -- People taking dietary supplements need to be careful that those don't interfere with any medical treatments they might be getting, a new report emphasizes.

The case in point was a 55-year-old man being treated for thyroid cancer who was supposed to be on a low-iodine diet as part of his treatment, but his levels of iodine continued to increase. The researchers found that a selenium supplement he was taking contained kelp, which is a rich source of iodine and significantly increased his iodine levels.

"This was a patient with thyroid cancer who had surgery and was treated with radioactive iodine," said lead author Dr. Lewis E. Braverman, a professor of medicine at Boston University. "It is very important that he consume a low-iodine diet, which would result in an uptake of the radioactive iodine."

The report was published in the Jan. 22 issue of the New England Journal of Medicine.

When doctors discovered the source of the iodine, they stopped all the 20 over-the-counter supplements the patient was taking. After eight weeks, iodine levels dropped to normal. The patient was then put on a low-iodine diet, and after four weeks, iodine levels dropped even further.

"Who would have thought that kelp would be in a selenium tablet?" Braverman said.

"If you want to treat patients with radioactive iodine -- if you want them to be on a low-iodine diet -- you must be extremely inquisitive and cautious, and find out all the over-the-counter remedies they are taking," Braverman noted.

Don't only look at the label that's on the bottle, get a complete description from the manufacturer, Braverman stressed.

There is no reason for people in the United States to be taking selenium, Braverman said, since most people get the selenium they need through diet.