In most cases, the exact cause of thyroid cancer is unknown.
However, there are a number of risk factors that can increase your chances of developing the condition, including having another thyroid condition and being exposed to radiation (see below).
How cancer begins
Cancer begins with an alteration to the structure of deoxyribonucleic acid (DNA), found in all human cells. This is known as a genetic mutation. The DNA provides the cells with a basic set of instructions, such as when to grow and reproduce.
The mutation in the DNA changes these instructions, causing the cells to keep growing. This results in them reproducing uncontrollably which causes a lump of tissue (tumour) to form.
How cancer spreads
Left untreated, cancer can spread to the other parts of the body, usually through the lymphatic system. The lymphatic system is similar to the blood circulation system. It is made up of a network of vessels and glands (lymph nodes) distributed throughout the body.
The lymph nodes produce many of the specialised cells needed by your immune system (the body's natural defence system against infection and illness).
Once the cancer reaches your lymphatic system, it is 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 are 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.
The five main risk factors for developing thyroid cancer are:
- thyroid conditions
- radiation exposure
- inherited genetic mutations
These are discussed below.
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).
Around one in five thyroid cancer cases occur in people who have had a previous benign thyroid condition.
Having an overactive thyroid gland (hyperthyroidism) or an underactive thyroid gland (hypothyroidism), does not increase your chances of developing thyroid cancer.
Exposure to radiation during childhood is another risk factor identified with thyroid cancer. Two types of radiation exposure are associated with thyroid cancer. They are:
- nuclear fall-out
- radiation used for medical treatments
Nuclear fall-out is radioactive waste released during a nuclear explosion. It can also occur when a nuclear reactor malfunctions, releasing high levels of nuclear waste into the atmosphere.
Many recently reported cases of thyroid cancer are thought to have been caused by radiation exposure during medical procedures that occurred between 1910-1960. 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. This will hopefully lead to a decrease in the number of thyroid cancer cases in the future.
Inherited genetic mutations
Inherited genetic mutations are responsible for a small number of medullary thyroid carcinomas.
If the instructions carried in genes (the smallest unit of genetic material) are altered, some of the body's processes will not work normally. In medullary thyroid carcinomas, the affected gene is known as the RET gene.
The two types of inherited condition where the mutation occurs are:
- familial medullary thyroid cancer (FMTC) - which affects many family members
- multiple endocrine neoplasia (MEN) syndrome, types 2A and 2B - where family members can develop a number of different endocrine tumours including medullary thyroid cancer
In cases of MEN2A or MEN2B thyroid cancer, the mutations usually develop in childhood or during the teenage years. In FMTC, the mutations usually develop in adulthood.
In FMTC, MEN2A and MEN2B, the gene mutations are passed on through autosomal dominant inheritance. This means that if either of your parents has one of the mutations that causes thyroid cancer, you have a one in two chance of also being born with the affected gene.
If one of your parents has a history of medullary thyroid carcinoma or MEN syndrome, you should consider having a blood test to find whether you have the mutated genes. If your test results are positive, it will usually be recommended that your thyroid gland is surgically removed as a precaution.
If your diet contains low levels of the trace element, iodine, you are at increased risk of developing thyroid cancer.
People exposed to radiation, or those with a history of benign thyroid conditions (see above), 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. You can use the BMI healthy weight calculator to find out what your body mass index is.
Women are around two-and-a-half times more likely to develop thyroid cancer than men. It is thought this may be due to hormones released during a woman's monthly period or during pregnancy. However, there is little scientific evidence to support this theory.