ON THIS PAGE: You will find out more about what factors increase the chance of this type of cancer. To see other pages in this guide, use the colored boxes on the right side of your screen, or click “Next” at the bottom.
A risk factor is anything that increases a person’s chance of developing cancer. Although risk factors often influence the development of cancer, most do not directly cause cancer. Some people with several risk factors never develop cancer, while others with no known risk factors do. However, knowing your risk factors and talking about them with your doctor may help you make more informed lifestyle and health care choices.
A person with an average risk of thyroid cancer has less than a 1% chance of developing thyroid cancer. Generally, most thyroid cancers are considered sporadic, meaning the genetic changes develop by chance after a person is born, so there is no risk of passing these genetic changes on to one’s children. Inherited thyroid cancers are less common and occur when gene mutations (changes) are passed within a family from one generation to the next (see below). Overall, about 10% of thyroid cancers are inherited. About 5% of papillary and follicular thyroid cancers and 25% of MTCs are thought to be inherited.
The following factors may raise a person’s risk of developing thyroid cancer:
Gender. Women are two to three times more likely to develop thyroid cancer than men.
Age. Thyroid cancer can occur at any age, but about two-thirds of all cases are found in people between the ages of 20 and 55. Anaplastic thyroid cancer is usually diagnosed after age 60. Older infants (10 months and older) and adolescents can develop MTC, especially if they carry the RET proto-oncogene mutation (see below).
Family history/Genetics. The risk of inherited thyroid cancer is low. Thyroid cancer may run in the family if first-degree relatives (parents, brothers, sisters, children) or many close relatives (first-degree relatives, grandparents, aunts, uncles, nieces, nephews, grandchildren, cousins) have been diagnosed with thyroid cancer, especially before age 30. If a person's first-degree relatives developed thyroid cancer, his or her risk of thyroid cancer is higher than the average risk. The risk increases if other close relatives have thyroid cancer. The following factors increase a person’s risk of developing thyroid cancer:
- A family history of goiters increases the risk of developing papillary thyroid cancer.
- A family history of precancerous polyps in the colon (large intestines) increases the risk of developing papillary thyroid cancer.
- A family history of MTC increases a person’s risk. Doctors often recommend that a person who develops MTC be tested for an inherited genetic mutation, because he or she may be the first person diagnosed with the tumor in the family with a mutation. Relatives, siblings, and children should then be tested for the mutation.
Known inherited genetic mutations and rare hereditary cancer syndromes that affect thyroid cancer risk include:
- An abnormal RET oncogene, which can be passed from parent to child, may cause MTC. Not everyone with an altered RET oncogene will develop cancer. Blood tests and genetic tests can detect the gene. Once the altered RET oncogene is identified, a doctor may recommend surgery to remove the thyroid gland before cancer develops. People with MTC are encouraged to have genetic testing  to determine if a mutation of the RET proto-oncogene is present. If so, genetic testing of your siblings and children will be recommended.
- People with MEN2 syndrome  have an increased risk of MTC and other cancers.
- People with familial adenomatous polyposis (FAP)  and Cowden syndrome (CS)  have an increased risk of papillary and follicular thyroid cancer.
It is important to talk with your family members about your family’s history of thyroid cancer. If you think you may have a family history of thyroid cancer, first talk with a genetic counselor  before you have any genetic testing. Only genetic testing  can determine if you have a genetic mutation and genetic counselors are trained to explain the risks and benefits of genetic testing.
Radiation exposure. Exposure to moderate levels of radiation may increase the risk of papillary and follicular thyroid cancers. Such sources of exposure include the following:
- Low-dose to moderate-dose x-ray treatments used before 1950 to treat children with acne, tonsillitis, and other head and neck problems may increase the risk of papillary and follicular thyroid cancers.
- People who have been treated with radiation therapy for Hodgkin lymphoma  or other forms of lymphoma  in the head and neck are at an increased risk for developing papillary or follicular thyroid cancer.
- Exposure to radioactive iodine (also called I-131 or RAI), especially in childhood, may increase the risk of papillary and follicular thyroid cancers. Sources of I-131 include radioactive fallout from atomic weapons testing during the 1950s and 1960s, nuclear power plant fallout (for example, the 1986 Chernobyl nuclear power plant accident and the 2011 earthquake that damaged nuclear power plants in Japan), and environmental releases from atomic weapon production plants.
Diet low in iodine. Iodine is needed for normal thyroid functioning. In the United States, iodine is added to salt to help prevent thyroid problems.
Race. White people and Asian people are more likely to develop thyroid cancer, but this disease can affect a person of any race or ethnicity.
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