Thyroid cancer risks and causes

• A quick guide to what's on this page
• How common thyroid cancer is
• Definite risk factors
• Possible risk factors
• Possible protective factors

Thyroid cancer is quite a rare cancer. About 2,100 cases are diagnosed in the UK each year. Out of every 1,000 cancers diagnosed in the UK, fewer than 10 are thyroid cancers. 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 goes up with age in men until 75 years of age. But for women, the risk stays fairly constant between the ages of 30 and 55 and then falls. Risk seems to be higher in South Asian women. But that is most likely explained by the risk factors below.

Remember - 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
• Family history of thyroid cancer
• A bowel condition called FAP
• Low iodine levels
• Acromegaly

Benign thyroid disease

The two most common thyroid conditions (under- and over-active 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

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. There was an increase in cases of thyroid cancer in the Ukraine, particularly in children, after the Chernobyl nuclear reactor accident. And 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 on people working before 1985 and we don’t know if people working with radiation now are increasing their risk.

Family history of thyroid cancer

It is possible to inherit abnormal genes that increase your risk of medullary thyroid cancer. They cause syndromes called MEN2A and MEN2B. MEN stands for multiple endocrine neoplasia. This means that people who have these abnormal genes are more likely to develop cancers of hormone producing glands at some point in their lives. As many as 1 in 4 people who develop medullary thyroid cancer have an abnormal gene.

If MEN2 runs in your family, you and your relatives may be referred for screening for thyroid cancer. Thyroid cancer can also run in families where there is no sign of MEN2 gene changes, so it is likely that another faulty gene is causing the thyroid cancers. But this gene has not yet been discovered.

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 more about FAP in the bowel cancer section of CancerHelp UK.

Low iodine levels

People who have low iodine levels are more likely to get thyroid cancer than someone who does not have a low iodine level. It is very rare for people in the UK to have low iodine levels but they can sometimes be caused by radiation exposure in the past. Low iodine levels also cause goitre (an enlarged thyroid) and this increases the chance of developing thyroid cancer. If there are low levels of iodine in the soil where you live, there will be low levels in your drinking water and any local milk, meat or vegetables you buy. Pregnancy and smoking can also lower iodine levels slightly.

Acromegaly

An increased risk of thyroid cancer has been found in people with acromegaly, a rare condition where the body over produces growth hormone.

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

• Having had cancer
• Factors related to women and reproduction
• Height and weight
• Smoking
• Poor diet

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 2 years have a slightly increased risk of papillary thyroid cancer. 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. An increased risk has also been shown in women who have taken medicines that stop milk production by the breasts. Do remember though, that thyroid cancer is rare. So an increase in risk is still a very small risk.

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 increased risk after having your womb removed (hysterectomy).

Women whose periods start after the age of 15 may have a small increase in their risk of thyroid cancer when they get older.

Some studies report a slight increase in risk for women taking the oral contraceptive pill but the risk reduces after you’ve stopped taking it.

A drug called clomiphene (Clomid) is used to treat infertility. One study 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.

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.

Smoking

There seems to be a lower risk of thyroid cancers in smokers, particularly if they are still smoking. But it is important to remember that this is one tiny bit of positive medical news that is not balanced by the long list of harms that smoking can do. For example, if you smoke you significantly increase your risk of lung cancer, mouth cancer, bladder cancer, cancer of the food pipe (oesophageal cancer), cervical cancer and stomach cancer. And also of course, heart disease and lung disease.

Poor diet

An unhealthy diet may cause up to 4 out of every 10 thyroid cancer cases. Some research studies show an increased risk with a high intake of butter, cheese and most meats (but not fish), but more studies are needed before we can be sure this is a risk factor. One study has shown that people who eat a lot of refined carbohydrates or sugary foods may have a higher risk of thyroid cancer than people who eat unrefined carbohydrates.

Some studies show that drinking moderate amounts of alcohol can reduce the risk of thyroid cancer. But it can also increase the risk of other cancers.

Eating a lot of vegetables may lower your risk of thyroid cancer. It is not clear why this is the case and we need more research to find out.