No one knows the exact causes of thyroid cancer. Doctors can seldom explain why one person gets this disease and another does not. However, it is clear that thyroid cancer is not contagious. No one can "catch" cancer from another person.
Research has shown that people with certain risk factors are more likely than others to develop thyroid cancer. A risk factor is anything that increases a person's chance of developing a disease.
The following risk factors are associated with an increased chance of developing thyroid cancer:
- Radiation. People exposed to high levels of radiation are much more likely than others to develop papillary or follicular thyroid cancer.
One important source of radiation exposure is treatment with X-rays. Between the 1920s and the 1950s, doctors used high-dose X-rays to treat children who had enlarged tonsils, acne, and other problems affecting the head and neck. Later, scientists found that some people who had received this kind of treatment developed thyroid cancer. (Routine diagnostic X-rays -- such as dental X-rays or chest X-rays -- use very small doses of radiation. Their benefits nearly always outweigh their risks. However, repeated exposure could be harmful, so it is a good idea for people to talk with their dentist and doctor about the need for each X-ray and to ask about the use of shields to protect other parts of the body.)
Another source of radiation is radioactive fallout. This includes fallout from atomic weapons testing (such as the testing in the United States and elsewhere in the world, mainly in the 1950s and 1960s), nuclear power plant accidents (such as the Chornobyl [also called Chernobyl] accident in 1986), and releases from atomic weapons production plants (such as the Hanford facility in Washington state in the late 1940s). Such radioactive fallout contains radioactive iodine (I-131). People who were exposed to one or more sources of I-131, especially if they were children at the time of their exposure, may have an increased risk for thyroid diseases.
- Family history. Medullary thyroid cancer can be caused by a change, or alteration, in a gene called RET. The altered RET gene can be passed from parent to child. Nearly everyone with the altered RET gene will develop medullary thyroid cancer. A blood test can detect an altered RET gene. If the abnormal gene is found in a person with medullary thyroid cancer, the doctor may suggest that family members be tested. For those found to carry the altered RET gene, the doctor may recommend frequent lab tests or surgery to remove the thyroid before cancer develops. When medullary thyroid cancer runs in a family, the doctor may call this "familial medullary thyroid cancer" or "multiple endocrine neoplasia syndrome." People with the MEN syndrome tend to develop certain other types of cancer.
A small number of people with a family history of goiter or certain precancerous polyps in the colon are at risk for developing papillary thyroid cancer.
- Being female. In the United States, women are two to three times more likely than men to develop thyroid cancer.
- Age. Most patients with thyroid cancer are more than 40 years old. People with anaplastic thyroid cancer are usually more than 65 years old.
- Race. In the United States, white people are more likely than African Americans to be diagnosed with thyroid cancer.
- Not enough iodine in the diet. The thyroid needs iodine to make thyroid hormone. In the United States, iodine is added to salt to protect people from thyroid problems. Thyroid cancer seems to be less common in the United States than in countries where iodine is not part of the diet.
Most people who have known risk factors do not get thyroid cancer. On the other hand, many who do get the disease have none of these risk factors. People who think they may be at risk for thyroid cancer should discuss this concern with their doctor. The doctor may suggest ways to reduce the risk and can plan an appropriate schedule for checkups.
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