Risk factors for thyroid cancer

A risk factor is something that increases the risk of developing cancer. It could be a behaviour, substance or condition. Most cancers are the result of many risk factors. But sometimes thyroid cancer develops in people who don’t have any of the risk factors described below.

Thyroid cancer can develop at any age, but most cases occur in people 20 to 55 years of age. More women than men develop thyroid cancer.

Risk factors are generally listed in order from most to least important. But in most cases, it is impossible to rank them with absolute certainty.

Exposure to ionizing radiation

Non-cancerous conditions of the thyroid

Family history of thyroid cancer

Hereditary conditions

Obesity

Tall height

Acromegaly

There is convincing evidence that the following factors increase your risk for thyroid cancer.

Exposure to ionizing radiation is the strongest risk factor for thyroid cancer. The risk of developing thyroid cancer is related to your age when you were exposed. The younger you are when exposed to radiation, the greater the risk of developing thyroid cancer.

People, especially children, who receive radiation therapy to the head and neck have a higher risk of developing thyroid cancer. When thyroid cancer develops, it usually occurs 20 to 40 years after the radiation exposure. The risk of developing thyroid cancer after radiation therapy depends on the type of radiation used and the dose. The benefit of treating a cancer usually outweighs the risk of developing thyroid cancer later in life.

People who received low-dose radiation as children to treat non-cancerous conditions like fungal infections of the scalp, acne or enlarged tonsils have an increased risk of developing thyroid cancer.

People who are exposed to ionizing radiation from nuclear accidents or weapons have a greater risk of developing thyroid cancer, especially if they were children when they were exposed to the radiation.

Diagnostic imaging tests, such as x-rays and CT scans, use ionizing radiation to make images. There is some evidence that having diagnostic imaging tests may increase the risk for thyroid cancer. The risk of cancer from imaging tests must be weighed against their benefits. Modern imaging equipment delivers the lowest amount of radiation possible.

A history of non-cancerous (benign) thyroid conditions increases the risk of developing thyroid cancer. These include thyroid nodules, goitre (an enlarged thyroid) and inflammation of the thyroid (called thyroiditis).

If you have a first-degree relative who has had thyroid cancer, you have a higher risk of developing thyroid cancer. The higher risk may be due to certain hereditary conditions.

The following rare hereditary conditions are associated with various types of thyroid cancer.

Multiple endocrine neoplasia type 2 (MEN2) is caused by an inherited mutation in the RET gene. Most people with MEN2 will develop medullary thyroid cancer. MEN2 can also cause other types of tumours. There are 3 subtypes of MEN2 that can cause different types of cancer to develop:

• MEN2A is the most common subtype. It can cause medullary thyroid cancer along with tumours of the adrenal gland (called pheochromocytomas) and parathyroid gland.
• MEN2B can cause medullary thyroid cancer along with other tumours, including pheochromocytomas and neuromas.
• Familial medullary thyroid cancer syndrome causes medullary thyroid cancer only.

Familial adenomatous polyposis (FAP) is mainly caused by a mutation of the adenomatous polyposis coli (APC) gene. People with FAP develop large numbers (hundreds to thousands) of polyps called adenomas. Most polyps develop on the lining of the colon and rectum. If the polyps are not found and treated early, most people with FAP will develop colorectal cancer. People with FAP also have an increased risk of developing other types of cancer, including papillary thyroid cancer.

Cowden syndrome is caused by a mutation in the PTEN gene. It can cause many non-cancerous growths (called hamartomas) to form in the thyroid. It is linked with a higher risk of developing some cancers, including thyroid cancer.

Carney complex is a very rare hereditary condition that may cause light brown spots on the skin. It increases the risk of developing several different types of tumours. This includes cancerous tumours in the endocrine glands, including the thyroid.

Werner’s syndrome is a disease that affects connective tissue and makes people age prematurely. It increases the risk for thyroid cancer.

A high body mass index (BMI) increases the risk of developing thyroid cancer. The reason for this higher risk is unclear.

Tall people have a greater risk of developing thyroid cancer, but the reason for this higher risk is not known. It may have to do with hormone levels in childhood, adolescence or adulthood.

Acromegaly is a rare condition that develops when the body makes too much growth hormone. The increased growth hormone causes the bones and organs, including the thyroid gland, to start growing again and become deformed. People with acromegaly have a higher risk of developing thyroid cancer.

The following factors have been linked with thyroid cancer, but there is not enough evidence to show for sure that they are risk factors. More research is needed to clarify the role of these factors for thyroid cancer.

• a diet low in iodine
• not eating enough vegetables or eating very large amounts of cruciferous vegetables (from the cabbage family)
• higher than normal levels of thyroid-stimulating hormone (TSH)
• reproductive and hormonal factors in women
• diabetes

Significant research shows no link between thyroid cancer and alcohol or smoking.

To make the decisions that are right for you, ask your healthcare team questions about risks.