But risk factors don't tell us everything. Having a risk factor, or even several risk factors, does not mean that you will get the disease. And many people who get the disease may not have had any known risk factors. Even if a person with thyroid cancer has a risk factor, it is very hard to know how much that risk factor may have contributed to the cancer.

Scientists have found a few risk factors that make a person more likely to develop thyroid cancer.

Gender and Age

For unclear reasons thyroid cancers occur about 3 times more often in women than in men.

Thyroid cancers can occur in people of all ages, but most cases of papillary and follicular thyroid cancer are found in people between the ages of 20 and 60 years.

Diet Low in Iodine

Follicular thyroid cancers are more common in areas of the world where people's diets are low in iodine. In the United States, dietary iodine is plentiful because iodine is added to table salt and other foods. A diet low in iodine may also increase the risk of papillary cancer if the person also is exposed to radioactivity.

Radiation

Exposure to radiation is a proven risk factor for thyroid cancer. Sources of such radiation include certain medical treatments and radiation fallout from power plant accidents or nuclear weapons.

Having a history of head or neck radiation treatments in childhood is a risk factor for thyroid cancer. In the past, children were sometimes treated with radiation for things we wouldn't use radiation for now, like acne, fungus infections of the scalp (ringworm), an enlarged thymus gland, or to shrink tonsils or adenoids. Years later, studies linked these treatments to an increased risk of thyroid cancer.

Radiation therapy in childhood for some cancers such as Hodgkin disease also increases risk. In general, the risk is higher with younger children. Radiation exposure as an adult carries little risk of thyroid cancer.

Several studies have pointed to an increased risk of thyroid cancer in children because of radioactive fallout from nuclear weapons or power plant accidents. For instance, thyroid cancer is several times more common than normal in children living near Chernobyl the site of a 1986 nuclear plant accident that exposed millions of people to radioactivity. Adults involved with the cleanup after the accident and those who lived near the plant have also had a higher rate of thyroid cancer. Children with more iodine in their diet appeared to have a lower risk.

Some radioactive fallout occurred over certain regions of the United States after nuclear weapons testing in western states during the 1950s. This exposure was much, much lower than around Chernobyl. At such low exposures, a higher risk of thyroid cancer has not been proven. If you are concerned about possible exposure to radioactive fallout, discuss this with your doctor.

Hereditary Conditions

Several inherited conditions have been linked to different types of thyroid cancer.

Medullary thyroid cancer (MTC): About 1 out of 5 medullary thyroid carcinomas (MTCs) result from inheriting an abnormal gene. These cases are known as familial medullary thyroid carcinoma (FMTC). FMTC can occur alone, or it can be seen along with other tumors.

The combination of FMTC and tumors of other endocrine glands is called multiple endocrine neoplasia type 2 (MEN 2). There are 2 subtypes, MEN 2a and MEN 2b:

• In MEN 2a, MTC occurs along with pheochromocytomas (tumors in the adrenal glands, which are located on top of the kidneys) and with parathyroid gland tumors. 
• In MEN 2b, MTC is associated with pheochromocytomas and with benign growths of nerve tissue on the tongue and elsewhere called neuromas. This subtype is much less common than MEN 2a.

In these inherited forms of MTC, the cancers often develop during childhood or early adulthood and can spread early. MTC is most aggressive in the MEN 2b syndrome. If MEN 2a, MEN 2b, or isolated FMTC runs in your family, then you may be at very high risk of developing MTC. Ask your doctor for information about having regular blood tests to look for problems and the possibility of genetic testing.

Other thyroid cancers: People with certain inherited medical conditions are at higher risk for more common forms of thyroid cancer. Higher rates of the disease occur among people with uncommon genetic conditions such as Gardner syndrome, Cowden disease, and familial adenomatous polyposis (FAP).

Papillary and follicular thyroid cancers do seem to run in some families without a known inherited syndrome; this may account for about 5% of thyroid cancers. The genetic basis for these cancers is not totally clear.