This relatively common cancer affects three times as many women as men.
Thyroid cancer is a type of cancer — an abnormal, out-of-control growth of cells — that affects the thyroid gland.
The thyroid gland sits at the base of the throat, in the front part of the neck below the voice box.
It's a butterfly-shaped gland, consisting of two lobes connected by a strip of tissue.
The thyroid gland helps regulate metabolism, growth, and maturation by releasing hormones into the bloodstream, specifically triiodothyronine (T3), tetraiodothyronine (thyroxine, or T4), and calcitonin.
Types of Thyroid Cancer
There are four main types of thyroid cancer:
- Papillary carcinoma
- Follicular carcinoma, including a special form called Hürthle cell carcinoma
- Anaplastic carcinoma
- Medullary carcinoma
Papillary and follicular carcinoma both develop in thyroid follicular cells.
Unlike anaplastic and medullary carcinoma, these two are considered "differentiated cancers," meaning that they look like normal thyroid tissue when viewed under a microscope.
They’re the two most common types of thyroid cancer, with papillary carcinoma accounting for about 80 percent of thyroid cancer cases, and follicular carcinoma accounting for about 10 percent, according to the American Cancer Society (ACS).
Papillary carcinoma grows very slowly, typically only in one lobe of the thyroid gland. They often spread to the neck's lymph nodes over time.
Despite this spreading, they are usually easily treated, and rarely fatal.
Follicular carcinoma often spreads to other parts of the body, such as the lungs and bones, rather than the lymph nodes.
The outlook for this cancer is worse than for papillary carcinoma, but is still quite good overall.
According to the ACS, Hürthle cell carcinoma accounts for about 3 percent of all thyroid cancers and is a bit harder to find and treat than other follicular carcinomas.
Anaplastic carcinoma, though rare — it makes up only 2 percent of thyroid cancers — is the most dangerous type of thyroid cancer, according to the National Institutes of Health. It often spreads to other parts of the body and is difficult to treat.
Medullary carcinoma affects thyroid "C" cells (also called parafollicular cells), which produce the calcium-regulating hormone calcitonin. It makes up about 4 percent of all thyroid cancers, according to the ACS.
Thyroid cancer is fairly common compared with most other cancers, ranking as the 8th most common cancer in the United States, according to the National Cancer Institute (NCI).
It makes up nearly 4 percent of all new cancer cases and is expected to affect more than 62,000 new people in the United States in 2015.
What's more, the number of new cases has been rising by an average of 5 percent each year for the past 10 years, due in large part to more frequent detection of the disease through imaging tests such as CT scans.
Women are about three times as likely as men to get thyroid cancer, and more than two-thirds of new cases occur in people between the ages of 20 and 65.
The disease is also more common in people of Asian descent than in other ethnic groups, according to the NCI.
Thyroid Cancer Risk Factors
Aside from age, gender, and race, other risk factors for thyroid cancer include:
- Radiation therapy in the neck, especially during childhood
- Exposure to radiation from a nuclear bomb or nuclear power plant malfunction
- Personal history of goiters (swelling caused by an enlarged thyroid)
- Family history of thyroid disease or thyroid cancer
- Certain genetic or hereditary conditions, such as familial medullary thyroid cancer and multiple endocrine neoplasia type 2A or 2B syndrome
- A low-iodine diet
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Editorial Sources and Fact-Checking
- How does the thyroid work? PubMed Health.
- Thyroid cancer; MedlinePlus/NIH.
- General Information About Thyroid Cancer; National Cancer Institute.
- What are the risk factors for thyroid cancer? American Cancer Society.
- SEER Stat Fact Sheets: Thyroid Cancer; National Cancer Institute.
- What is thyroid cancer? American Cancer Society.