Any substance or condition that increases cancer risk is referred to as a risk factor. The most important risk factor for developing thyroid cancer is radiation exposure. However, most cancers are the result of many risk factors.
*Risk factors are generally listed in order from most significant to least significant. In most cases, it is impossible to rank the relative significance of individual risk factors with absolute certainty.
Thyroid cancer can develop at any age, but most cases occur in people 20–60 years of age. Medullary thyroid cancer can occur in younger people, especially if they carry a mutated (changed) RET gene. For reasons unknown, thyroid cancer occurs about 3 times more often in women than in men.
The following factors are known to increase the risk of developing thyroid cancer.
Radiation exposure
Ionizing radiation is the strongest risk factor for thyroid cancer. People exposed to high levels of radiation are at greater risk of developing papillary thyroid cancer and follicular thyroid cancer. The risk of developing thyroid cancer appears to be related to age at exposure to radiation – the younger the age at exposure, the greater the risk of developing thyroid cancer.
The risk of developing thyroid cancer is greatest for people who, as children, received radiation therapy to the head and neck area. They may have had radiation treatment for benign conditions, such as acne, fungal infections of the scalp (tinea capitis), whooping cough, an enlarged thymus gland or enlarged tonsils and adenoids. This is especially true for children treated between 1920 and 1960. Cancerous tumours tended to start developing in the thyroid 3–5 years after exposure to the radiation therapy, but most cases occurred 20–40 years after exposure. Even after 40 years, people who received this treatment as children are still considered at higher risk for thyroid cancer than those who didn’t.
Radiation therapy may be given for some childhood cancers, such as Wilms’ tumour, neuroblastoma, Hodgkin lymphoma and non-Hodgkin lymphoma. This exposure to radiation may also increase the risk of developing thyroid cancer. However, the benefits of radiation therapy for these types of cancer outweigh the risk of thyroid cancer. A child who has received radiation therapy for one of these cancers will be watched closely for signs of thyroid cancer so it can be found and treated early.
Radiation exposure from radioactive fallout also increases the risk of thyroid cancer. People who were exposed to the Chernobyl nuclear power plant accident in 1986, nuclear weapons testing in Nevada in the 1950s and 1960s, and atomic bombs in Japan in 1945 have a higher risk for thyroid cancer. For example, within 4 years of the Chernobyl accident, cases of thyroid cancer (especially papillary thyroid cancer) were diagnosed in nearby countries that were most affected by the fallout. In the years following the accident, the number of children who developed thyroid cancer in these countries was dramatically higher than children in areas not affected by the accident. Adults in these areas also appear to be at higher risk, but the difference in risk compared to non-exposed adults is not as large. Experts predict that the increased risk of thyroid cancer will continue more than 40 years after the accident.
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Benign thyroid conditions
A history of benign thyroid nodules or goitre (swollen or enlarged thyroid gland) increases the risk of thyroid cancer. The risk appears to be the greatest within 4 years of being diagnosed with thyroid nodules or goitre, but studies have shown the risk remains high even 10 years later. Hashimoto thyroiditis (a chronic inflammation of the thyroid) and Graves’ disease (an autoimmune condition of the thyroid) are other benign conditions that may increase the risk of thyroid cancer. However, the association is either weak or has not been proven yet.
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Family history of thyroid cancer
People with a family history of thyroid cancer have a higher risk of developing the disease themselves. This may be due to inherited conditions, but about 5% of all thyroid cancers occur in families without a known inherited condition.
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Genetic conditions
Some rare inherited genetic conditions are associated with various types of thyroid cancer.
Familial medullary thyroid cancer
About 10%–20% of all medullary thyroid cancer (MTC) cases occur because children inherited an abnormal gene from their parents. The abnormal gene is the RET gene. When medullary thyroid cancer occurs alone, with no evidence of tumours elsewhere in the endocrine system, it is called familial medullary thyroid cancer.
Multiple endocrine neoplasia (MEN) syndrome
When medullary thyroid cancer occurs along with other cancers in the endocrine system, it is called multiple endocrine neoplasia syndrome type 2 (MEN2). There are 2 subtypes of MEN2:
- MEN2A – Medullary thyroid cancer occurs along with tumours in the adrenal glands (pheochromocytomas) and parathyroid gland tumours.
- MEN2B – MTC occurs along with pheochromocytomas and benign neuromas (tumours of nerve tissue). In MEN2B, neuromas commonly occur on the tongue, but they can develop anywhere on the body.
In MEN2, medullary thyroid cancer tends to develop at a young age, often during childhood or young adulthood, and is known to spread early.
Familial adenomatous polyposis (FAP)
Familial adenomatous polyposis (FAP) is caused by a mutation of the adenomatous polyposis coli (APC) gene. FAP is also known as familial polyposis of the colon. It causes hundreds to thousands of polyps to develop on the lining of the colon and rectum in people as early as adolescence. People with FAP are at increased risk for developing other types of cancer, including thyroid cancer.
Gardner syndrome
Gardner syndrome is a rare condition caused by a mutation in the APC gene. It causes large numbers of polyps in the colon, along with tumours in the bone and soft tissues. People with Gardner syndrome have a higher risk of developing other types of cancer, including thyroid cancer.
Cowden syndrome
People with Cowden syndrome have a mutation in the tumour suppressor gene PTEN. Cowden syndrome is a rare disease that makes people more likely to develop certain cancers, including thyroid cancer.
Carney complex
Carney complex is a very rare genetic disorder that increases a person’s risk of developing certain cancers, including thyroid cancer.
Werner syndrome
Werner syndrome is a very rare condition caused by a mutation in the WRN gene. It causes people to age prematurely and very rapidly after puberty. Werner syndrome increases the risk of thyroid cancer.
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Acromegaly
People with acromegaly have an increased risk of developing thyroid cancer. In this condition, the body produces too much growth hormone and insulin-like growth factor. This over-production stimulates cells in the thyroid gland to grow, which could explain the increased risk of thyroid cancer.
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Increased thyroid-stimulating hormone (TSH) secretion
People with higher than normal levels of serum TSH have a higher risk of developing thyroid cancer. However, it is not clearly understood how TSH is related to thyroid cancer. One theory is that TSH does not cause cancer to develop, but rather promotes its growth once it has already started. Another theory is that high levels of TSH cause a rapid increase in the number and growth rate of thyroid cells. This increases the number of cells in which a mutation could occur, which in turn increases the risk of developing cancer.
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Possible risk factors
The following factors have shown some association with thyroid cancer, but there is not enough evidence to say they are known risk factors. Further study is needed to clarify the role of these factors for thyroid cancer.
- body mass index (BMI) – There appears to be a weak link between thyroid cancer and increased body mass index (BMI). Studies have shown a higher BMI (and being overweight or obese) may slightly increase the risk of thyroid cancer, although the reason for this increased risk is unclear.
- diet and iodine intake – The thyroid gland needs iodine to make thyroid hormones. The amount of iodine in a person’s diet may affect the risk of developing thyroid cancer. The relationship between iodine intake and thyroid cancer is unclear.
- Low levels of iodine in a person’s diet may increase the risk of follicular thyroid cancer. This is more common 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, iodine is added to table salt to make sure it is part of the diet and to prevent low thyroid hormone levels.
- Diets that include very large amounts of cruciferous vegetables (vegetables belonging to the cabbage family, such as broccoli, cauliflower and Brussels sprouts) may prevent the body from absorbing iodine and may increase the risk of thyroid cancer. Diets that are high in raw vegetables other than cruciferous vegetables show a slightly reduced risk of thyroid cancer.
- Low iodine levels may also increase the risk of papillary thyroid cancer if the person is also exposed to radiation.
- In contrast, high levels of iodine may increase the risk of papillary thyroid cancer. Papillary thyroid cancer is more common in areas where people eat a lot of seafood and shellfish or use a lot of iodized salt in their diet. Fish and seafood are among the main dietary sources of iodine.
- women’s reproductive and hormonal factors – Researchers suspect that factors related to women’s hormones and reproduction may increase the risk of thyroid cancer. There is a peak incidence of thyroid cancer among women during their reproductive years. The risk of thyroid cancer increases at the time of puberty and decreases after menopause. It is thought that estrogen increases the levels of thyroid-stimulating hormone (TSH) in the body and, in turn, increases thyroid growth.
- Many studies have been done on a variety of reproductive aspects that may affect a woman’s risk of thyroid cancer. Generally, most of the studies show a weak or inconsistent link between thyroid cancer and the following reproductive aspects:
- the number of babies a woman has
- age of a woman when she has her first child
- having had a miscarriage
- use of oral contraceptives
- menopause
- age at menarche
- menstrual cycle irregularity
- use of medications to stop breast milk production (suppress lactation)
- Some studies have shown no link between fertility drugs or hormonal replacement therapy and the risk of thyroid cancer.
- height – There appears to be some association between thyroid cancer and height. People who are tall have a slightly higher risk of thyroid cancer, although the reason why this happens is unclear.
- routine x-rays – Studies have suggested that exposure to low levels of radiation, such as routine x-rays, may be associated with an increased risk of developing thyroid cancer. Before the 1960s, higher doses of radiation were used with routine x-rays. It is important to note that modern x-ray equipment delivers the lowest amount of radiation possible.
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Factors not associated with thyroid cancer
The following are not considered to be risk factors for thyroid cancer because there is significant evidence showing that there is no association:
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Unknown risk factors
The following are factors for which there is not enough evidence or the evidence is inconclusive. In other words, it can’t be determined for sure whether these risk factors are or are not associated with thyroid cancer.
- occupational exposure to chemicals
- hyperthyroidism
- hypothyroidism
- diagnostic use of I-131
- history of other cancers
- occupational exposure to radiation
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See a list of questions to ask your doctor about risks. Find out how to reduce the risk of thyroid cancer.