Thyroid cancer is a cancer that starts in the thyroid gland, which is situated under the Adam's apple in the front part of the neck. In most people, it cannot be seen or felt. It is butterfly shaped, with two lobes--the right lobe and the left lobe--joined by a narrow isthmus. 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. The American Cancer Society (ACS) estimates that about 37,340 new cases of thyroid cancer will be diagnosed in the United States in 2008. Thyroid cancer is much more likely to occur in women. ACS states that "nearly 2 out of 3 cases are found in people between the ages of 20 and 55."
Thyroid Cancer and the Environment
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
- 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 usually 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 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.
- Hereditary Conditions
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 uncertain.
Reduce Your Risk
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 familial cases of 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.