A dramatic rise in thyroid cancer has resulted partly from overdiagnosis and treatment of tumours too small to ever cause harm, according to a U.S. study that found cases nearly tripled since 1975.

The study is the latest to question whether all cancers need aggressive treatment. Other research has suggested that certain cancers of the prostate, breast and lung as well as thyroid grow so slowly that they will never become deadly, and that overzealous screening leads to overtreatment.

UKRAINE CHERNOBYL

A doctor checks the thyroid gland of a Ukranian man for abnormalities. Radiation exposure, such as from the 1986 Chornobyl nuclear plant explosion, is one of the risk factors for thyroid cancers. (Oded Balilty/Associated Press)

The thyroid is a hormone-releasing gland in the neck that helps regulate the body's metabolism. Thyroid cancer treatment often includes surgery to remove the butterfly-shaped gland, followed by lifelong daily hormone pills.

Thyroid removal is done for 85 per cent of all people diagnosed despite guidelines that say less aggressive surgery is reasonable for lower-risk thyroid tumors, the study authors said.

"Our old strategy of looking as hard as possible to find cancer has some real side-effects," said Dr. Gilbert Welch, co-author of the thyroid study and a professor of medicine at the Dartmouth Institute for Health Policy and Clinical Practice.

Welch said patients "can no longer assume" that labelling a disease as cancer means treatment is necessary. "It's a challenging rethinking," he added.

Welch and Dartmouth colleague Dr. Louise Davies analyzed U.S. government data from 1975 to 2009 and found thyroid cancers jumped from 5 cases per 100,000 people to 14 per 100,000. Most of that increase was in papillary thyroid cancers, the most common and least deadly kind; those cases jumped from about three cases per 100,000 to more than 12 per 100,000.

The results suggest there is "an ongoing epidemic of thyroid cancer" nationwide, they said.

The study was published online Thursday in JAMA Otolaryngology.

Despite the increase, thyroid cancer is relatively uncommon. An estimated 5,700 new cases were diagnosed in Canada last year, according to the Canadian Cancer Society's latest statistics. In the society's annual report, authors said more frequent use of diagnostic testing, including ultrasound, CT scans and MRI may mean that more earlier stage, asymptomatic thyroid cancers are being diagnosed.

Risk factors for thyroid cancer include diets low in iodine — rare in the United States and Canada — and radiation exposure. Women are more commonly diagnosed than men.

The new research echoes previous studies but "certainly raises some provocative questions," said Dr. Brian Burkey, a Cleveland Clinic head and neck cancer specialist.

Experts know that better detection methods including CT scans and ultrasound, have led to more thyroid cancers being diagnosed, but they don't know which ones will become aggressive, Burkey said.

"Thyroid cancer even if treated has a fairly high recurrence rate even if it doesn't kill," he said.

Burkey is among researchers planning a major study seeking to provide answers. Patients diagnosed with thyroid cancers will be randomly assigned treatment or just observation.

In the meantime, the study authors offer some advice: Physicians could "openly share with patients the uncertainty surrounding small thyroid cancers — explaining that many will never grow and cause harm to a patient," but that it's not possible to know for certain which ones are harmless.

That would allow patients to make better informed decisions, and some might opt for close monitoring instead of treatment, the authors said.