Thyroid cancer - Causes of thyroid cancer

In most cases, the exact cause of thyroid cancer is unknown.

However, there are a certain things that can increase your chances of developing the condition, including having another thyroid condition and being exposed to radiation.

What is cancer?

Cancer begins with a change (mutation) in the structure of the DNA in cells, which can affect how they grow. This means cells grow and reproduce uncontrollably, producing a lump of tissue called a tumour.

Left untreated, cancer can spread to other parts of the body, usually through the lymphatic system. The lymphatic system is made up of a network of vessels and glands (lymph nodes) located throughout the body.

Lymph nodes produce many of the cells needed by your immune system (the body's natural defence system against infection and illness).

Once the cancer reaches your lymphatic system, it's capable of spreading to other parts of your body, including your blood, bones and organs.

The most common types of thyroid cancer are papillary carcinomas and follicular carcinomas, which are known as differentiated thyroid cancers (DTCs).

They spread much more slowly than other types of cancer. When DTCs are diagnosed, they're usually limited to the thyroid gland itself or nearby lymph nodes.

The rarer types of thyroid cancer are more aggressive and spread faster. By the time medullary thyroid carcinoma is diagnosed, it may have spread to the lymph nodes. In advanced cases, it may have also spread to the bones and lungs.

Anaplastic thyroid cancer often spreads to the windpipe and, in some cases, the lungs.

Risk factors for thyroid cancer

The main risk factors for developing thyroid cancer are:

• having a thyroid condition
• having a family history of thyroid cancer (in the case of medullary thyroid cancer)
• having a bowel condition known as familial adenomatous polyposis (FAP)
• acromegaly (a rare condition where the body produces too much growth hormone)
• having previous benign (non-cancerous) breast conditions
• weight and height
• radiation exposure

These are discussed below.

Thyroid conditions

Your risk of developing thyroid cancer is slightly increased if you have certain non-cancerous (benign) thyroid conditions, such as an inflamed thyroid gland (thyroiditis) or an enlarged thyroid gland (goitre).

Having an overactive thyroid gland (hyperthyroidism) or an underactive thyroid gland (hypothyroidism) doesn't increase your chances of developing thyroid cancer.

Around one in five cases of thyroid cancer occur in people who've had a previous benign thyroid condition.

Family history

Inherited genetic mutations are responsible for a small number of medullary thyroid carcinomas. If the instructions carried in genes are altered, some of the body's processes won't work normally.

This inherited mutation occurs in:

• familial medullary thyroid cancer
• multiple endocrine neoplasia (MEN) syndrome, types 2A and 2B

In cases of MEN2A or MEN2B thyroid cancer, the mutations usually develop during childhood or the teenage years. In familial medullary thyroid cancer, the mutations usually develop in adulthood.

If one of your parents has a history of medullary thyroid carcinoma or MEN syndrome, you should consider having a blood test to find out whether you have the mutated genes. If your test results are positive, it's recommended you have surgery to remove your thyroid gland as a precaution.

Familial adenomatous polyposis

If you have a bowel condition called familial adenomatous polyposis (FAP), your risk of developing thyroid cancer is increased. FAP runs in families and is caused by inheriting a faulty gene.

Find out more about FAP on the Cancer Research UK website.

Acromegaly

Thyroid cancer risk is increased in people who have acromegaly. This is a rare condition where the body produces too much growth hormone.

Previous breast conditions

If you've had a benign (non-cancerous) breast condition in the past, such as a breast cyst or fibroadenoma (non-cancerous tumour), your risk of developing thyroid cancer increases by around half (50%) compared with women who haven't had this type of condition.

Weight and height

If you're overweight, you're more at risk of developing thyroid cancer than someone who isn't overweight.

Research has also shown that taller adults have an increased risk. However, the reasons for this are unclear.

Radiation exposure

Exposure to radiation during childhood is another risk factor for thyroid cancer.

Radiation from nuclear fallout and radiation used for medical treatments have both been associated with thyroid cancer.  

Many recently reported cases of thyroid cancer are thought to have been caused by radiation exposure during medical procedures carried out between 1910 and 1960.

During this time, not much was known about the risks of radiation treatment. Today there are much stricter regulations regarding the use of radiation for medical procedures.

Other risk factors

Diet

If your diet contains low levels of the trace element iodine, you're at an increased risk of developing thyroid cancer.

People exposed to radiation, or those with a history of benign thyroid conditions, are more likely to have low levels of iodine.

Eating a lot of butter, cheese and meat may also increase your risk of developing thyroid cancer. To help reduce your risk, you should include plenty of fresh fruit and vegetables in your diet.

People with a high body mass index (BMI) also have an increased risk of developing thyroid cancer. Use the BMI healthy weight calculator to find out what your body mass index is.

Gender

Women are about two to three times more likely to develop thyroid cancer than men.

It's thought this may be because of the hormones released during a woman's monthly period or during pregnancy. However, there's little scientific evidence to support this theory.

The Cancer Research UK website has more information about women's risk of thyroid cancer.