Thyroid cancer is the unregulated growth of thyroid cells. The thyroid is a butterfly-shaped gland found below the voice box in the front of the neck. As part of the endocrine system, the thyroid releases hormones that regulate metabolism.
Thyroid cancer is most commonly suspected when a person or a healthcare practitioner notices a change in the size or shape of the person's thyroid gland or the practitioner feels an irregularity in the gland during physical examination. Because many other conditions can present with changes in thyroid gland size, shape or texture, further evaluations such as blood tests for assessment of thyroid function, ultrasound to delineate gland structure, or other tests are often required. An enlarged thyroid can be a sign of many conditions besides cancer. Those conditions range from iodine deficiency to inflammation caused by Hashimoto disease.
Many times, further evaluation of the thyroid gland reveals the presence of thyroid nodules. About 90% of thyroid nodules, or tumors, are benign.
Rates of thyroid cancer diagnosis have increased worldwide over the last decade, likely due to more sensitive detection technologies. It is three times more common in women than in men. With about 62,000 new cases each year, thyroid cancer is the 8th most common cancer in the U.S. Most thyroid cancers have a good prognosis and 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 regions of the thyroid and have an impact on 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. These cancers grow slowly and are rarely fatal.
- Follicular thyroid cancer: this is the second most common thyroid cancer—about 10% of cases. 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 cases.
- Medullary thyroid cancer: this accounts for up to 4% of all thyroid cancers. It can develop spontaneously or be inherited. Inherited medullary thyroid cancer is caused by a mutation in the RET gene and is often associated with multiple endocrine neoplasia type 2 (MEN), which is also caused by mutations in the RET gene. This 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, this makes up only 2% of thyroid cancers. It spreads quickly and is hard to treat.
Most thyroid cancers are now found early, when they are treatable. Many people with papillary and follicular thyroid cancer have a good prognosis. According to the American Cancer Society, 97% of people with thyroid cancer are alive 10 years after diagnosis.