Although the cause of thyroid cancer is not well understood, there are some exposures which are known to increase the risk of thyroid cancer:
Follicular thyroid cancers are more common in areas of the world where people’s diets are low in iodine. A diet low in iodine may also increase the risk of papillary cancer if the person also is exposed to radioactivity.
Thyroid cancer can occur at any age, but risk peaks for women when they are between the ages of 45 and 49 years – for men, risk peaks between the ages of 65 and 69 years. Exposure to radiation significantly increases the risk of thyroid cancer and for unclear reasons females are three times more likely than males to develop thyroid cancer. Thyroid cancer may also be inherited due to genetic mutations.
Thyroid Cancer Causes
A risk factor is anything that affects a person’s chance of getting a disease. Different diseases have different risk factors. For example, smoking is a risk factor for cancers of the lung, mouth, larynx (voice box), bladder, kidney, and several other organs. Risk factors don’t let us know everything. Just because you have a risk factor, or even several risk factors, does not mean that you will get the disease. So 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.
While thyroid cancer causes are not well understood, there are two main factors which give a higher risk of developing the disease:
- Family history of thyroid cancer
- Exposure to radiation
As with many types of cancer, a family history of thyroid cancer is a known risk factor. A family history of thyroid cancer carries the biggest risk for patients with first degree relatives. About 5% of patients who develop papillary thyroid cancer and 20-25% of patients who develop medullary thyroid cancer (MTC) will have a relative who also had thyroid cancer.
Medullary thyroid cancer
MTC can be related to a disorder called Multiple Endocrine Neoplasia type 2 (MEN 2) Syndrome, and is usually the result of a specific genetic mutation in a part of the genome called the RET proto-oncogene. The combination of medullary thyroid cancer and tumours of other endocrine glands is called multiple endocrine neoplasia type 2 (MEN 2).
There are 2 subtypes, MEN 2a and MEN 2b, both of which are caused by mutations (defects) in a gene called RET. In MEN 2a, MTC occurs along with pheochromocytomas (tumours that make adrenaline) and with parathyroid gland tumours. 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 familial MTC runs in your family, then you may be at very high risk of developing MTC. Ask your doctor about having regular blood tests or ultrasound exams to look for problems and the possibility of genetic testing.
Other thyroid cancers
Rarely, papillary thyroid cancer can also be genetically linked and may also be associated with familial forms of colon cancer, breast cancer, and goitres. Papillary and follicular thyroid cancers do seem to run in some families. Having a first-degree relative with thyroid cancer, even without a known inherited syndrome in the family, increases your risk of thyroid cancer. The genetic basis for these cancers is not clear. 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:
Familial adenomatous polyposis (FAP)
People with FAP develop many colon polyps and have a very high risk of colon cancer. They also have an increased risk of some other cancers, including papillary thyroid cancer. Gardner syndrome is a subtype of FAP in which patients also get certain benign tumours. Both Gardner syndrome and FAP are caused by defects in the gene APC.
People with this syndrome have an increased risk of thyroid, endometrial (uterine), and breast cancers. The thyroid cancers tend to be either of the papillary or follicular type. This syndrome is caused by defects in the gene PTEN.
Carney complex, type I
People with this syndrome may develop a number of benign tumours and hormone problems. They also have an increased risk of papillary and follicular thyroid cancers. It is caused by defects in the gene PRKAR1A. If you suspect you may have a familial condition, discuss it with your doctor, who might arrange genetic counselling.
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. Radiation treatments for head and neck diseases in childhood are a risk factor for thyroid cancer. The risk depends on how much radiation is given and the age of the child. In general, the risk increases with larger doses and with younger age at treatment.
Before the 1960s, children were sometimes treated with low doses of radiation for things we wouldn’t use radiation for now, like acne, fungus infections of the scalp (ringworm), an enlarged thymus gland, or enlarged tonsils or adenoids. Years later, the people who had these treatments were found to have a higher risk of thyroid cancer.
Radiation therapy in childhood for some cancers such as lymphoma, Wilms’ tumour, and neuroblastoma also increases risk. While exposure to radiation as an adult carries a lower 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.
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 who had 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 were tested in western states during the 1950s. This exposure was much, much lower than that around Chernobyl. A higher risk of thyroid cancer has not been proven at these low exposure levels.
If you have any questions about thyroid symptoms, you should speak to your local doctor, who will arrange to contact your thyroid surgeon.