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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 15–49 years of age. It is the most commonly diagnosed cancer in people in the 15–29 age group.
More women than men develop thyroid cancer. The incidence rates of thyroid cancer in women have been steadily rising since 1986.
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.
Significant evidence shows no link between alcohol or smoking and increased risk of thyroid cancer.
Known risk factors
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.
The following are sources of ionizing radiation.
People, especially children, who receive radiation therapy to the head and neck have a higher risk of developing thyroid cancer. If thyroid cancer develops, it tends to develop 20–40 years after the radiation exposure.
Radiation therapy may be used to treat some childhood cancers, such as leukemia, neuroblastoma, Hodgkin lymphoma and non-Hodgkin lymphoma. 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. The healthcare team will closely follow a child who has received radiation therapy for one of these cancers for signs of thyroid cancer so it can be found and treated early.
In the past, low doses of radiation therapy were used to treat non-cancerous conditions like acne, fungal infections of the scalp or an enlarged thymus gland or tonsils. People who received these treatments as children have a risk of developing thyroid cancer.
Nuclear accidents and weapons
Studies show that 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. For example, after the Chernobyl nuclear power plant accident, people were diagnosed with thyroid cancer in countries that were most affected by the nuclear fallout. In the years following the accident, there was a large increase in the incidence of childhood thyroid cancer in these areas.
Diagnostic imaging tests
Some 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 of thyroid cancer. The risk of cancer from imaging tests must be weighed against their benefits. Often the benefits outweigh the risks.
In the past, routine x-rays used higher doses of radiation. It is important to note that modern imaging equipment delivers the lowest amount of radiation possible.
A history of non-cancerous, or 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 relativefirst-degree relativeA person’s parents, brothers, sisters or children. who has had thyroid cancer, you have a higher risk of developing thyroid cancer. The higher risk may be due to certain hereditary conditions, but some families don’t have a known hereditary condition.
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 only causes medullary thyroid cancer.
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 polypspolypsA small cauliflower-like growth on a mucous membrane, such as the lining of the colon, bladder, uterus (womb), vocal cords or nasal passage. called adenomas. Most polyps develop on the lining of the colon and rectum. They increase the risk of developing colorectal cancer. In addition to colorectal cancer, people with FAP have a higher 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 associated 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 glandsendocrine glandsA type of gland without a duct that releases hormones directly into the blood., including the thyroid.
Werner’s syndrome is a disease that affects connective tissue and makes people age prematurely. It increases the risk of thyroid cancer.
Research has shown a link between obesity and thyroid cancer. A high body mass index (BMI)body mass index (BMI)A measure that relates body weight to height (calculated by dividing weight in kilograms by height in metres squared). increases the risk of developing thyroid cancer, although the reason for this increased risk is unclear.
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 also have a higher risk of developing thyroid cancer.
Possible risk factors
The following factors have been linked with thyroid cancer, but there is not enough evidence to show they are known risk factors. Further study is needed to clarify the role of these factors for thyroid cancer.
The thyroid needs iodine to make thyroid hormones. Our bodies don’t make iodine so we need to get it from our diets. Having a diet low in iodine may increase the risk of developing thyroid cancer. This higher risk is usually seen in areas that do not have foods fortified with iodine or in areas where there are low levels of iodine in the soil. In developed countries like Canada, iodine is added to table salt to make sure it is part of the diet and to prevent low thyroid hormone levels.
It is unclear what effect a diet high in iodine has on the risk of thyroid cancer. There is some evidence that a diet high in iodine may increase the risk of developing thyroid cancer. Research has shown that people who eat large amounts of fish, an important source of iodine, have a slightly higher risk for thyroid cancer.
Some studies suggest that not eating enough vegetables increases the risk of developing thyroid cancer. Vegetables contain many protective substances, like antioxidantsantioxidantsAny substance that help protect cells from damage caused by free radicals (unstable molecules made during metabolism, which is the chemical processes needed for cell function, growth and reproduction). and phytochemicals. A diet high in vegetables may actually protect against thyroid cancer.
Other studies show that eating large amounts of cruciferous vegetables may increase the risk of thyroid cancer. Cruciferous vegetables belong to the cabbage family and include broccoli, cauliflower and Brussels sprouts.
Some studies have shown that people with higher than normal levels of TSH in the blood may have a higher risk of developing thyroid cancer. One explanation for this is that TSH may promote the growth of cancer once it has already started. Another possible reason is that high levels of TSH cause thyroid cells to reproduce and grow faster than normal. This increases the number of cells that could have a mutation, which in turn increases the risk of developing cancer.
Researchers suspect that factors related to women’s reproduction and hormones may increase the risk of thyroid cancer. This is because thyroid cancer is a common disease in young women.
Generally, most of the studies show a weak or inconsistent link between thyroid cancer and the following reproductive aspects:
- the number of pregnancies a woman has
- a woman’s age when she has her first child
- a woman’s age when she has her first period
- how regular is a woman’s menstrual cycle
- a woman’s age at menopause
- the use of oral contraceptives
- the use of hormone replacement therapy (HRT)
There is some evidence that having diabetes may increase the risk of developing thyroid cancer. Diabetes (also called diabetes mellitus) is a chronic disease where the body can't make insulin or it can’t properly use the insulin it makes. Since the body needs insulin to use and store glucose (sugar) for energy, diabetes leads to high blood sugar levels.
Unknown risk factors
It isn’t known whether or not the following factors are linked with thyroid cancer. It may be that researchers can’t show a definite link or that studies have had different results. Further study is needed to see if the following are risk factors for thyroid cancer:
- having a non-cancerous breast condition
- being exposed to certain chemicals, including hydrocarbons, silica and agricultural chemicals
- having had cancer before, including breast cancer
- having hyperthyroidism
- being exposed to ionizing radiation at work
Questions to ask your healthcare team
To make the decisions that are right for you, ask your healthcare team questions about risks.