The exact causes of thyroid cancer in most people are not known, but research is going on all the time to try to find the cause. There are a number of risk factors that can increase your chance of developing thyroid cancer. These are:

People who have certain non-cancerous (benign) thyroid diseases are more likely to develop thyroid cancer. These include:

• an enlarged thyroid (goitre)
• thyroid nodules (adenomas)
• inflammation of the thyroid (thyroiditis).

The more common thyroid conditions of an over- or under-active thyroid (hyperthyroidism and hypothyroidism) do not increase your risk of developing thyroid cancer. 

A poor diet that contains large amounts of butter, cheese and meat may increase your risk of developing thyroid cancer. Large amounts of fresh fruit and vegetables may help to reduce your risk.

People who eat very little iodine in their diet are more likely to develop thyroid cancer. You are more likely to have a low iodine level if you are also exposed to radiation or if you have a history of benign thyroid disease.

Iodine is found in the soil and if you live in an area where the levels of iodine are low, the levels in your drinking water, and any locally grown vegetables or reared animals will also be low.

In a very small number of people, medullary thyroid cancer may be due to an inherited faulty gene. The affected gene is the RET gene. There are two main types of inherited condition in which this occurs:

• Familial medullary thyroid cancer (FMTC), affecting many family members.

• Multiple endocrine neoplasia (MEN) syndrome types 2A and 2B; with this syndrome, family members are at risk of developing a number of different endocrine tumours including medullary thyroid cancer. We have further information about the MEN 2 syndromes.

Family members of someone with medullary thyroid cancer can be tested to see if they have inherited an abnormal RET gene. If someone is found to have the abnormal gene they may be advised to have their thyroid gland removed to prevent cancer developing. This is known as a prophylactic thyroidectomy.

There is also an increased risk of developing thyroid cancer if you have the inherited bowel condition called familial adenomatous polyposis (FAP).

This may be due to radiotherapy given in childhood, or to unusually high levels of radiation in the environment; for example, in the areas surrounding Chernobyl in the Ukraine, following the nuclear power explosion of 1986. Thyroid cancer can develop many years after exposure. However, only a small number of thyroid cancers are caused by radiation exposure.