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A risk factor is anything that increases your chances of getting a disease such as cancer. Different cancers have different risk factors. Some risk factors, like smoking, can be changed. Others, like your age or family history, can’t be changed.

But having a risk factor, or even several, doesn’t mean you will get the disease. And many people who get the disease may have few or no known risk factors. Even if a person with thyroid cancer has a risk factor, it’s hard to know how much that risk factor may have contributed to their cancer.

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

Sex

For unclear reasons, thyroid cancer occurs almost 3 times more often in women than in men.

Age

Thyroid cancer can occur at any age, but it’s most common in people in their 30s through 60s. The risk of thyroid cancer peaks at an earlier age for women than for men. The reasons for this aren’t clear.

Hereditary conditions

Some hereditary (inherited) conditions are linked to an increased risk of certain types of thyroid cancer.

These conditions aren’t common, and they likely account for only a small portion of thyroid cancers overall. Some examples are listed here, although there are others as well.

Multiple endocrine neoplasia type 2 (MEN2)

People with this condition have a high risk of (MTC), as well as tumors of some other endocrine glands.

There are 2 subtypes: MEN2A and MEN2B. These subtypes are caused by different mutations (defects) in the RET gene.

MEN2A

MEN2A is much more common, and it has several variants. In the most common variant, medullary thyroid cancer (MTC) occurs along with pheochromocytomas (tumors that make adrenaline) and with parathyroid gland tumors. In another variant of MEN2A, known as familial MTC (FMTC), MTC is the only tumor.

MEN2B

In people with MEN2B, MTC is often seen along with pheochromocytomas but not with parathyroid tumors. MTC also tends to be more aggressive in people who have MEN2B.

Some people with this syndrome also have benign growths of nerve tissue on the tongue and elsewhere called neuromas.

In people with these conditions, medullary thyroid cancer or other tumors often develop during childhood or early adulthood. MTC also tends to be more aggressive in people who have MEN2B.

If MEN2 runs in your family, you may be at very high risk of developing medullary thyroid cancer. Ask your health care team about the possibility of genetic testing, and about having regular blood tests or ultrasound exams to look for problems.

Familial adenomatous polyposis (FAP)

People with this syndrome develop many colon polyps at an early age and have a very high risk of . They also have a higher risk of some other cancers, including papillary thyroid cancer.

Gardner syndrome is a subtype of FAP in which people also have an increased risk of certain other tumors, including . Both Gardner syndrome and FAP are caused by defects in the APC gene.

Cowden syndrome (multiple hamartoma syndrome)

People with this syndrome have an increased risk of certain benign (non-cancerous) growths, including some called hamartomas.

They also have an increased risk of developing cancers of the thyroid, , and some other organs. These cancers tend to develop at an earlier age. The thyroid cancers tend to be either the papillary or follicular type.

This syndrome is most often caused by defects in the PTEN gene.

Carney complex

People with this rare syndrome typically have pigmented (dark) areas on their skin, as well as an increased risk of certain types of benign (non-cancerous) tumors. They also have an increased risk of papillary and follicular thyroid cancers which tend to occur at a young age.

This syndrome is most often caused by defects in the PRKAR1A gene.

If you suspect you have an inherited condition that increases your risk of thyroid cancer, talk with your health care provider. They might recommend , depending on your medical history.

Family history

If you have a first-degree relative (parent, brother, sister, or child) with thyroid cancer, you are at an increased risk of developing it as well. This is true even without a known inherited syndrome in the family. Still, most people with thyroid cancer do not have a family history of the disease.

Radiation

Radiation exposure is a risk factor for thyroid cancer. This includes radiation exposure from certain medical treatments and tests, as well as radiation fallout from power plant accidents or nuclear weapons.

Radiation from medical treatments

Having head or neck radiation treatments in childhood increase the risk of thyroid cancer. This includes radiation therapy to treat cancers such as Hodgkin lymphoma, as well as radiation treatment given before a stem cell transplant (bone marrow transplant).

The amount of risk depends on how much and at what age the radiation is given. In general, the risk increases with larger doses and with younger age at treatment.

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Before the 1960s, children were sometimes treated with low doses of radiation for things we wouldn’t use radiation for now, like acne, fungal infections of the scalp (ringworm), and enlarged tonsils or adenoids. Years later, the people who had these treatments were found to have a higher risk of thyroid cancer.

Radiation from imaging tests

Imaging tests such as x-rays and CT scans also expose children to radiation, but at much lower doses, so it’s not clear how much these tests might raise the risk of thyroid cancer (or other cancers).

If there is an increased risk, it’s most likely small. But to be safe, experts advise that children should not have these tests unless absolutely necessary. When this type of imaging test is needed, it should be done using the lowest dose of radiation that still provides a clear picture.

Radiation fallout

Thyroid cancer risk is also higher in children exposed to radioactive fallout from nuclear weapons or power plant accidents.

For instance, thyroid cancer risk was many times higher in children who lived near Chernobyl, the site of the 1986 nuclear plant accident. Adults involved with the cleanup after the accident, or who lived near the plant, have also had higher rates of thyroid cancer.

Some radioactive fallout occurred over certain regions of the United States (and other parts of the world) during nuclear weapons testing after World War II. This exposure was generally much lower than the exposure around Chernobyl.

Being exposed to radiation when you are an adult carries much less risk of thyroid cancer compared to exposure that happens when you are a child.

Excess body weight

People with excess body weight have a higher risk of thyroid cancer than those who do not have excess weight. The risk appears to go up as increases.

Iodine in the diet

Follicular thyroid cancers are more common in areas of the world where people’s diets are low in iodine. On the other hand, a diet high in iodine may increase the risk of papillary thyroid cancer. In the United States, most people get enough iodine in their diet because it’s added to table salt and other foods.