Experts do not know the exact cause of thyroid cancer. But they do know that people who have been exposed to a lot of radiation-either from the environment or from medical treatment-have a greater chance of developing thyroid cancer. An occasional dental X-ray will not increase your risk of developing thyroid cancer. But past radiation treatment of your head, neck, or chest (especially during childhood) can increase your chances of developing thyroid cancer. Radiation was used from the 1920s to the 1950s to treat problems such as acne or enlarged tonsils, but it is no longer used to treat noncancerous conditions.

One rare type of thyroid cancer, which is called medullary thyroid cancer (MTC), runs in families. You can inherit a specific gene that increases your risk of developing MTC.