Thyroid cancer risks and causes

Thyroid cancer is quite a rare cancer. About 2,340 cases are diagnosed in the UK each year. Less than 1 in every 100 cancers diagnosed in the UK is a thyroid cancer. More than twice as many thyroid cancers are diagnosed in women than in men.

There are different types of thyroid cancer. Papillary thyroid cancer is the most common, followed by follicular thyroid cancer. Medullary tumours and anaplastic tumours are much rarer.

The risk of thyroid cancer seems to be higher in South Asian women. But that is possibly explained by the risk factors related to reproduction below.

Remember that having a lump in your thyroid does not necessarily mean cancer. Only about 1 in 20 thyroid lumps are cancer.

The main risk factors for thyroid cancer are

• Benign thyroid disease
• Radiation and low iodine levels
• Family history of thyroid cancer
• A bowel condition called FAP
• Height and weight
• Acromegaly
• Previous breast conditions

Benign thyroid disease

The two most common thyroid conditions (underactive and overactive thyroid) do not increase the risk of thyroid cancer. But other non cancerous conditions affecting the thyroid do increase your risk, including

• Nodules (adenomas)
• An enlarged thyroid (goitre)
• Inflammation of the thyroid (thyroiditis)

Together, the conditions above make up the most important risk factors for thyroid cancer. 1 in 5 cases of thyroid cancer (20%) occur in someone who has had one of these non cancerous thyroid problems in the past. Adenomas have the largest increase in risk. The risk is particularly strong if you had thyroid nodules from a young age, under 55.

Benign thyroid disease runs in families. If you have this type of condition in your family, then statistically, you have an increased risk of thyroid cancer. The risk is higher if more than one family member is affected.

Radiation and low iodine levels

Thyroid cancer is more common in people who had radiotherapy treatment to the neck area at a young age. The cancer may develop years later – as many as 10 to 30 years after treatment. This risk is highest for people treated with radiotherapy when they were children. But there is a slight increased risk for anyone who has had external beam radiotherapy.

Thyroid cancer is also more common in survivors of atomic explosions or accidents. After the Chernobyl nuclear reactor accident, cases of thyroid cancer in the Ukraine rose in people exposed to radiation, particularly as children or adolescents. And thyroid cancer cases also increased in the USA after nuclear testing in Utah.

There are reports of higher rates of thyroid cancer in people exposed to radiation through their work. But these studies include information about people working before 1985 and we don’t know if people working with radiation now are increasing their risk.

People who have low levels of iodine in their body may have a higher risk of thyroid cancer after exposure to radiation than people with normal iodine levels.

Family history of thyroid cancer

Some people who have family members with thyroid cancer are at a higher risk of developing thyroid cancer themselves. But usually the risk is still small because thyroid cancer is rare. The risk of developing papillary or follicular thyroid cancer is estimated to be 4 to 5 times higher for relatives of a person with thyroid cancer than people in the general population.

It is possible to inherit abnormal genes that increase your risk of some types of thyroid cancer. People who have an inherited gene change (mutation) in a gene called the RET oncogene have an increased risk of medullary thyroid cancer. Some people have gene changes that cause syndromes called MEN2A and MEN2B. MEN stands for multiple endocrine neoplasia and people who have these conditions are more likely to develop medullary thyroid cancer and other cancers of hormone producing glands at some point in their lives. If MEN2 runs in your family, you and your relatives may be referred for screening for thyroid cancer. As many as 1 in 4 people who develop medullary thyroid cancer have an abnormal gene. 

A bowel condition called FAP

People who have a bowel condition called FAP (familial adenomatous polyposis) have an increased risk of thyroid cancer. FAP is another condition caused by an inherited gene. There is detailed information about FAP in the bowel cancer section.

Height and weight

A recent analysis of all published studies into body weight and thyroid cancer found that people who are overweight have a slightly higher risk of thyroid cancer. Some studies have shown an increased risk for taller adults, although the researchers couldn’t explain this.

Acromegaly

People who have acromegaly have an increased risk of thyroid cancer. Acromegaly is a rare condition where the body over produces growth hormone.

Previous breast conditions

Women who have had non cancerous (benign) breast conditions have an increased risk of thyroid cancer. The risk is increased by half (50%) when compared to women who have not had benign breast conditions.

Possible risk factors for thyroid cancer have been suggested by some research reports. But there is no convincing proof yet that they do increase the risk of thyroid cancer. They include

• Having had cancer
• Factors related to women and reproduction

Having had cancer

Some studies show that people treated as adults for non Hodgkin's lymphoma, breast cancer, oesophageal cancer, or testicular cancer have an increased risk of thyroid cancer. It is not known if this is due to treatment for these cancers, common risk factors or inherited genetic changes. In the case of oesophageal cancer, it may be because routine checks after treatment pick up the thyroid cancers.

Factors related to women and reproduction

Women who have had a baby in the past 5 years have a slightly increased risk of papillary thyroid cancer. Do remember though, that thyroid cancer is rare. So an increase in risk is still a very small risk. According to 1 study the risk is higher for women who have 2 or more babies in 5 years. This may be because you produce more thyroid stimulating hormone in pregnancy. Having more children does not seem to increase thyroid cancer risk in the long term. 

An increased risk of thyroid cancer has been shown in women who have taken medicines that stop milk production by the breasts. 

Some studies report an increased risk in women who have had miscarriages, but other studies have shown the opposite. If there is an increased risk, it could be because women who have benign thyroid disease have a higher risk of miscarriage. So it could be the benign thyroid disease causing the increase in thyroid cancer, and not the miscarriages.

Women going through the menopause also have a slightly increased risk, compared to women who had their menopause more than 10 years ago. And there are some reports of an increased risk after having your womb removed (hysterectomy) but there is not enough evidence to be sure.

Women whose periods start after the age of 14 may have a small increase in their risk of thyroid cancer when they get older.

Some studies in the past reported a slight increase in thyroid cancer risk for women taking the oral contraceptive pill. But more recent studies have shown that taking the pill does not increase the risk.

A drug called clomiphene (Clomid) is used to treat infertility. Two studies found that it increased thyroid cancer risk but another study showed no increase in risk. So we need more research to find out whether clomiphene does increase risk or not.