A reported epidemic of thyroid cancer in rich countries is in fact mainly due to overdiagnosis driven by new technologies, the UN's cancer research agency said Thursday.

Up to 90 percent of the thyroid cancer cases diagnosed in recent decades are, instead, tumours that are "very unlikely" to cause any symptoms at all, let alone death, according to findings published in The New England Journal of Medicine.

Experts from the International Agency for Research on Cancer (IARC) and Italy's Aviano National Cancer Institute combed through data collected by the UN agency from 12 high-income countries -- eight in Europe, along with the United States, Japan, South Korea and Australia.

Starting in the 1980s, the highest rates of overdiagnosis occurred in the US, Italy and France, they found.

The jump in reported cases of thyroid cancer coincided with the arrival of ultrasound as a diagnostic tool, they noted.

"The most recent and striking example is the Republic of Korea," said IARC scientist Salvatore Vaccarella, who led the study.

"A few years after ultrasonography of the thyroid glands started being widely offered..., thyroid cancer has become the most commonly diagnosed cancer."

The researchers estimate that 90 percent of cases in South Korea from 2003 to 2007 were due to overdiagnosis.

During the same period, 70 to 80 percent of cases in Australia, France, Italy and the US can be attributed to overdiagnosing, and 50 percent of cases in Japan, the Nordic countries, England and Scotland.

The more recent introduction of computed tomography (CAT scans) and magnetic resonance imaging (MRI) have also led to the detection of many non-lethal anomalies in the thyroid glands of healthy people of all ages, the study said.

- More monitoring, less surgery -

Thyroid cancer -- which strikes women far more than men -- attacks the butterfly-shaped gland in the neck that produces hormones regulating how the body uses energy.

Patients diagnosed with the disease typically undergo partial or total removal of the gland, which can trigger chronic pain and requires hormone treatment for life.

In total, nearly half-a-million women and 90,000 men in the 12 countries examined may have been told they had the cancer when, in fact, their lives were not threatened, the study concludes.

The authors caution against systematic screening of the thyroid gland, and suggest "careful monitoring" instead of surgery for patients affected by low-risk tumours.

"The drastic increase in overdiagnosis and overtreatment of thyroid cancer is already a serious public health concern in many high-income countries," IARC director Christopher Wild said.

There are "worrying signs" of the same trend in low- and middle-income nations, he added.

Experts not involved in the study backed its conclusions.

"Finding cancers that would never cause someone harm in their lifetime -- 'overdiagnosis' -- is increasingly being recognised as a problem in thyroid cancer around the world," said Jana Witt, Cancer Research UK's health information officer.

"It's likely to be at least partly caused by advances in diagnostic and screening tests making it easier to find cancers that don't need treating and would not be life-threatening."

Witt speculated that the larger rise in women being diagnosed with the disease could be because they are more likely to be offered thyroid examinations as part of regular check ups.