The thyroid is a small, butterfly-shaped gland located at the base of the neck that produces thyroid hormones, primarily thyroxine (T4) and some triiodothyronine (T3). These hormones travel throughout the body and regulate metabolism by telling the cells in the body how fast to use energy and produce proteins. The thyroid gland also makes calcitonin, a hormone that helps to regulate calcium levels in the blood by preventing the breakdown (reabsorption) of bone and increasing calcium elimination from the kidneys.
The body has an elaborate feedback system to control the amount of T4 and T3 in the blood:
- When blood levels of the hormones decrease, the hypothalamus releases thyrotropin-releasing hormone, which in turn causes the pituitary gland to release thyroid-stimulating hormone (TSH), also called thyrotropin. TSH stimulates the thyroid gland to produce and release T4 (primarily) and T3.
- As thyroid hormone levels increase in the blood, the pituitary gland produces less TSH and the thyroid produces less T4 and T3.
Under normal circumstances, this feedback system regulates thyroid activity to maintain relatively stable levels of thyroid hormones in the blood.
Thyroid cancer is the unregulated growth of thyroid cells. Thyroid cancer is most commonly suspected when your healthcare practitioner notices a change in the size or shape of your thyroid gland, or you or your practitioner feels a lump or swelling in the neck during a physical exam.
Because many other conditions can cause changes in thyroid gland size, shape or texture, further evaluations such as blood tests for thyroid function, ultrasound imaging, or other tests are often required. An enlarged thyroid can be a sign of many conditions besides cancer. Those conditions range from low iodine levels to inflammation caused by Hashimoto disease.
Rates of thyroid cancer diagnosis have increased worldwide over the last decade, likely due to tests that are more sensitive in detecting the disease. Thyroid cancer is three times more common in women than in men. Each year, about 53,000 new cases of thyroid cancer are diagnosed and roughly 2,000 people die of the disease.
Most thyroid cancers are found early, when they are most treatable, and have a good outlook (prognosis). Often, they can be cured, especially if they have not spread to other areas of the body. This is particularly true of the most common types, papillary and follicular thyroid cancers (see below).
There are four main types of thyroid cancer. They develop from different areas of the thyroid, which impacts treatment options and prognosis. The main types of thyroid cancer include:
- Papillary thyroid cancer: this is the most common form of thyroid cancer—about 80% of cases are papillary. The cancer cells develop from cells that line the follicles in the thyroid, which are the cells that make T4 and T3. These cancers grow slowly and are rarely fatal.
- Follicular thyroid cancer: this is the second most common thyroid cancer—about 10% of cases. These cancer cells also develop from cells that line the follicles. It is most common in countries where diets lack iodine. Hürthle cell carcinoma is a form of follicular thyroid cancer and accounts for about 3% of all thyroid cancer cases. Follicular and papillary thyroid cancer are sometimes referred to as differentiated thyroid cancer.
- Medullary thyroid cancer: this accounts for up to 4% of all thyroid cancers. The cancer cells develop from other cells in the thyroid called C cells, which make the hormone calcitonin. It can be sporadic or be inherited. Inherited medullary thyroid cancer, called familial medullary thyroid cancer (FMTC), is caused by a variant (mutation) in the RET gene. This type may be associated with multiple endocrine neoplasia type 2 (MEN2), which is also caused by variants in the RET gene. Medullary thyroid cancer may require more than one type of treatment and is harder to cure than more common thyroid cancers.
- Anaplastic thyroid cancer: also called undifferentiated carcinoma; makes up only 2% of thyroid cancers. It spreads quickly and is hard to treat.