Thyroid Cancer and the Environment
Thyroid cancer is cancer that develops in the small, butterfly-shaped gland below the Adam's apple that regulates the body's metabolism, controlling everything from heart rate to how fast the body burns calories.
Thyroid cancer has a lower fatality rate than most cancers. Since 1973, a 2.4—fold increase in the incidence of this disease has occurred. Much of this is attributable to improved detection of the disease by thyroid ultrasound and ultrasound-guided fine needle aspiration rather than an increase in occurrence. Over 38,000 U.S. residents were diagnosed with thyroid cancer in 2008. Thyroid cancer is much more likely to occur in women and people between the ages of 20 and 55. The American Cancer Society states, "nearly 2 out of 3 cases are found in people between the ages of 20 and 55."
The excessive risk for thyroid cancer associated with exposure to external ionizing radiation has been well-established. No other environmental chemicals or physical agents have been associated with this cancer. Other risk factors for this cancer include:
- dietary factors, especially iodine intake,
- hormonal and reproductive factors,
- benign thyroid nodules and goiter,
- hereditary conditions, and
- gender and age.
Exposure and Risk
Although a person with thyroid cancer may have a risk factor, the degree to which that risk factor contributes to the cancer may be difficult to determine. A few risk factors that increase the likelihood to develop thyroid cancer have been established. These include:
Gender and Age
- For undetermined reasons, thyroid cancers occur about 3 times more often in women.
- Thyroid cancers can occur at any age but most cases of them occur between the ages of 20 and 55 years.
Diet Low in Iodine
- Some types of thyroid cancers are more common in areas of the world where diets are low in iodine.
- In the United States, dietary iodine intake is sufficient because iodine is added to table salt and foods.
Exposure to high levels of radiation during childhood has been proven to increase the risk for thyroid cancer. Sources of exposure include certain medical treatments and radiation fallout from power plant accidents or nuclear weapons.
- X-ray treatments prescribed for conditions like childhood acne or ringworm of the scalp from the 1920s to the 1950s increased the risk for thyroid cancer.
- A history of head or neck radiation treatments in childhood increases the risk for thyroid cancer.
- Radiation therapy in childhood for some cancers such as Hodgkin's disease also increases risk.
- Several studies suggested an increased risk of thyroid cancer in children because of exposure to radioactive iodine (I-131) in fallout from nuclear weapons or power plant accidents such as the Chernobyl accident. Some radioactive fallout occurred over certain regions of the United States after nuclear weapons testing in western states during the 1950s. This exposure was significantly lower than that measured around Chernobyl. At such low exposures, a higher risk of thyroid cancer has not been proven.
- Medullary thyroid cancer (MTC): About 1 of 5 MTCs is caused by an inherited abnormal gene.
- Other thyroid cancers: Higher rates of the disease occur among persons with uncommon genetic conditions such as Gardner syndrome, Cowden disease, and familial adenomatous polyposis (FAP).
- Papillary and follicular thyroid cancers: These cancers seem to run in some families without a known inherited syndrome. This trend may account for about 5% of all thyroid cancers. The genetic basis for these cancers is undecided.
Most people with thyroid cancer have no known risk factors; therefore it is not possible to prevent the disease in those instances. Because of the availability of genetic blood tests, most of the hereditary cases of Medullary thyroid cancer (MTC) can be either prevented or treated early. If the disease is diagnosed, the patient's family members can be tested also. If a family history of MTC exists, a doctor who is familiar with the latest advances in genetic counseling and genetic testing for this disease should be consulted. Removing the thyroid gland in children who carry the abnormal gene will prevent a cancer that might be fatal.
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