* papillary carcinoma – this is the most common type, accounting for 4 out 5 of all cases; it usually affects people under 40 years of age, especially women
* follicular carcinoma – this accounts for around 1 in 10 cases and tends to affects older adults
* medullary thyroid carcinoma – this accounts for around 1 in 20 cases; unlike the other types of thyroid cancer, cases of medullary thyroid carcinoma can run in families
* anaplastic thyroid carcinoma – this is the least common and most aggressive type of thyroid cancer, accounting for 1 in a 100 cases; most cases of anaplastic thyroid carcinoma affect older people who are between 60 to 80 years of age
In biological terms, papillary and follicular carcinomas have a lot in common so they are collectively known as differentiated thyroid cancers (DTCs).
The cause of thyroid cancer is largely unclear. The most important risk factor is exposure to ionising radiation, particularly if the exposure occurred during childhood. However, this cause accounts for a tiny minority of cases.
Ionising radiation is the more powerful type of radiation, as opposed to non-ionising radiation, which is the safer type of radiation released by devices such as mobile phones.
Most people will only experience exposure to significant amounts of ionising radiation if they undergo medical procedures such as X-rays or radiotherapy. Even then the amounts should be low enough not to pose a major risk.
However, before the link between ionising radiation and thyroid cancer was properly understood, many children were exposed to potentially dangerous levels of radiation to treat often harmless conditions, such as acne.
Historically, there are two major events that led to people being exposed to high levels of ionising radiation and a subsequent rise in thyroid cancer cases:
* the detonation of atomic bombs over Hiroshima and Nagasaki in 1945
* the accident at the Chernobyl nuclear reactor in Ukraine in 1986
See the Health A-Z topic about Radiation for more information.
Differentiated Thyroid Cancers
The outlook for the differentiated thyroid cancers (DTCs) is very good. These types of thyroid cancer tend to be both very slow growing and relatively straightforward to treat.
Treatment involves a two-step procedure:
* surgery is used to remove most or all of the thyroid gland
* a radioactive chemical called iodine is then used to kill off any remaining cancerous cells
Of the people with DTCs, 80-90% will live a normal lifespan.
Medullary Thyroid Carcinomas
The outlook for cases of medullary thyroid carcinoma is less favourable, as this type of thyroid cancer does not respond to iodine treatment so it can be difficult to remove all of the cancerous cells. However, the outlook is still a lot better than for most other types of cancer.
Survival rates for medullary thyroid carcinoma depend on what stage the cancer has advanced to when it is diagnosed (see Thyroid cancer – diagnosis for more information about the stages of thyroid cancer). If it is diagnosed in its early stages then the outlook is excellent, with almost all (97%) people living at least five years after a diagnosis.
If medullary thyroid carcinoma is only diagnosed when it has spread to other parts of the body then the outlook is poor, with only 1 in 4 people living at least five years after diagnosis.
However, the outlook for advanced medullary thyroid carcinoma may well improve in the future. A number of clinical trials ((a type of research that tests one treatment against another) looking at new types of medication known as targeted therapies have reported good result in terms of extending survival rates. For more information see Thyroid cancer – treatment and Thyroid cancer – clinical trials.