Increase in thyroid cancer rates studied

01 February 2012

February 2012 – Thyroid cancer incidence rates have been increasing substantially worldwide, but the reasons behind this increase are unclear.

In an effort to understand this trend, researchers at the Canadian Cancer Society examined the role of suspected lifestyle and reproductive factors related to thyroid cancer risk. The Society researchers reviewed 37 studies that investigated factors including body fatness (BMI), iodine-rich diets and female reproductive factors (such as age at first pregnancy, number of births and oral contraceptive use). The researchers studied these and other factors to gain a better understanding of the major known and suspected factors to determine which might pose the greatest risk for thyroid cancer.

In reviewing the existing evidence, the researchers found a positive association between BMI and an increased risk of thyroid cancer. The risk was higher for women than men, and among women the risk was up to 7 times greater in those with a high BMI compared to a lower BMI. A high BMI is an indication of overweight or obesity. Even though the association between BMI and thyroid cancer was stronger than that of the other risk factors that were examined, the overall association was still relatively weak and inconsistent across studies and therefore cannot fully account for the significant increases in thyroid cancer rates.

“Our finding suggests that obesity plays an important role in thyroid cancer and reducing it can be one way of lowering the risk of thyroid cancer. But there are likely other factors, related to lifestyle or the environment, that together heighten the risk of thyroid cancer,” says study leader Dr Prithwish De, Epidemiologist, Canadian Cancer Society. “The big question we still need to answer is whether there is a true increase in risk or if we are simply getting better at detecting this type of cancer.”

Among the other factors examined – diet and female reproductive factors – there was no consistent association with risk.

The authors cite a previous analysis of data from 5 continents that showed the average increase in thyroid cancer rates was 67% in females and 48% in males between 1973 and 2002. The increase in thyroid cancer incidence has been almost universal across countries.

From the current review, the researchers conclude that more research on risk factors that affect the general population may help identify and support prevention efforts to reduce the risk of thyroid cancer.

The review was published in the journal PlosONE on January 19. It is the broadest and most comprehensive review of thyroid cancer risks that has been published to date.

Thyroid cancer is the 7th most common cancer in Canada. There were approximately 5,700 new cases expected in 2011. The good news is that the survival rate for thyroid cancer patients is high (98% five-year relative survival) because modern treatments are highly effective.