Introduction to Thyroid Cancer

Risk Factors for Thyroid Cancer

Thyroid cancer is the most common of all cancers that involve the endocrine system. According to the American Cancer Society, approximately 26,000 new cases of thyroid cancer are diagnosed in the United States each year. For reasons that are not completely understood, the incidence of thyroid cancer has nearly doubled over the past 20 years. Numerous studies have shown that thyroid cancer develops 2 to 4 times more frequently in women than men suggesting that female hormones (estrogens) and the hormonal changes that occur during pregnancy may predispose women to thyroid cancer. Over 90% of all thyroid cancers are differentiated thyroid carcinomas (papillary or follicular thyroid carcinoma). Medullary thyroid carcinoma and anaplastic thyroid carcinoma occur much less frequently and represent only 5% and 1% of cases, respectively.

A risk factor is anything that increases a person's chances for developing a particular disease, such as cancer. Risk factors for thyroid cancer include:

• Female gender - Thyroid cancer occurs about 2 to 4 times more frequently in women than men.

• Age - Although thyroid cancer can also occur in children and adolescents, most cases of differentiated thyroid cancer occur in adults between the ages of 30 and 50.

• Genetic factors - Although most cases of thyroid cancer are not inherited, familial medullary thyroid carcinoma is an inherited form of thyroid cancer that runs in families. This type of thyroid cancer can be passed-on from parents to their offspring through the genes.

• Low iodine intake - The incidence of differentiated thyroid cancer is higher in some parts of the world where dietary intake of iodine is low. In the U.S. and other developed countries, certain foods (e.g., salt) are supplemented with iodine to ensure an adequate dietary intake of iodine.

• Geographic variation - The incidence of thyroid cancer in Iceland and Hawaii is almost two times higher than in Northern Europe, Canada, or the United States. The reasons for this are not known but may be due to as yet unidentified environmental factors.

• Radiation exposure - In the past, before the risks associated with radiation treatment were fully realized, children with certain benign conditions, such as enlarged tonsils or adenoids, would be treated with radiation. Subsequent studies demonstrated a link between childhood radiation exposure and the development of differentiated thyroid carcinoma. Radiation therapy in adults, however, has not been identified as a major risk factor for thyroid cancer. Radiation exposure among atomic bomb survivors in Japan and survivors of the nuclear reactor accident in Chernobyl has also been linked to a higher incidence of thyroid cancer.