In the United States and Europe, where there are no formal, widespread screening programs for thyroid cancer, scans for other conditions, like ultrasound exams of the carotid artery in the neck or CT scans of the chest, are finding tiny thyroid tumors.

Although more and more small thyroid cancers are being found, however, the death rate has remained rock steady, and low. If early detection were saving lives, death rates should have come down.

That pattern — more cancers detected and treated but no change in the death rate — tells researchers that many of the cancers they are finding and treating were not dangerous. It is a phenomenon that researchers call overdiagnosis, finding cancers that did not need treatment because they were growing very slowly or not at all. Left alone, they would probably never cause problems. Overdiagnosis is difficult to combat. Pathologists cannot tell which small tumors are dangerous, and most people hear the word “cancer” and do not want to take a chance. They want the cancer gone.

But cancer experts said the situation in South Korea should be a message to the rest of the world about the serious consequences that large-scale screening of healthy people can have.

“It’s a warning to us in the U.S. that we need to be very careful in our advocacy of screening,” said Dr. Otis W. Brawley, chief medical officer at the American Cancer Society. “We need to be very specific about where we have good data that it saves lives.”