According to the American Cancer Society, about 45,000 people are diagnosed with thyroid cancer each year in the United States. About 75% of these are women, making it the eighth-most-common cancer in women. Thyroid cancer is seen most often in adults, with two-thirds of the cases occurring between ages 20 and 55.
Although thyroid cancer accounts for about 1% of all cancers, it is becoming more common. At least 450,000 people in the United States have completed treatment or are living with thyroid cancer.
Thyroid cancer is usually a slow-growing cancer. It is one of the least dangerous cancers in most cases, and the five-year survival rate for thyroid cancer is almost 97%.
Thyroid Gland Anatomy
Thyroid cancer begins in the thyroid gland, which is a small butterfly-shaped gland in the front of the neck at the base of the throat. The thyroid is part of the body's endocrine system, a system of glands that control hormones in the body. It normally weighs less than an ounce, and it cannot be seen or felt in most people; however it has an important function. The thyroid makes hormones that help regulate the body's heart rate, blood pressure, temperature and metabolism (the breakdown of food to create energy).
The thyroid has two halves, or lobes, one on each side of the neck. It wraps around the trachea (windpipe) just under the larynx (Adam's apple). A thin strip of tissue known as the isthmus connects the two halves.
Thyroid gland cells are the only cells in the body that absorb and retain iodine. Iodine is needed to make thyroid hormones.
Two kinds of cells are found in the thyroid:
- Follicular cells are the most common. They produce thyroid hormone, which is important for growth, mental function and helping the body create energy.
- Parafollicular cells (also known as C cells) produce a small amount of the hormone calcitonin, which has a minor role to control calcium metabolism. Most parafollicular cells are in the upper third of each lobe.
Types of Thyroid Cancer
Thyroid cancer is grouped by the type of thyroid cells where the cancer begins. Papillary, follicular and anaplastic thyroid cancers begin in the follicular cells. Papillary and follicular cancers—the most common thyroid cancers—are sometimes referred to together as differentiated thyroid cancer. They have similar treatment.
Papillary thyroid cancer is the most common type of thyroid cancer, accounting for about 80% of thyroid cancers. While papillary thyroid cancer typically occurs in only one lobe of the thyroid gland, it may arise in both lobes in up to 10% to 20% of cases. Papillary thyroid cancer is most common in women of childbearing age. It sometimes is caused by exposure to radiation.
Even though papillary thyroid cancer is usually not an aggressive type of cancer, it often metastasizes (spreads) to the lymph nodes in the neck. Papillary thyroid cancer treatment usually is successful.
Follicular thyroid cancer accounts for about 10% of thyroid cancers. Like papillary thyroid cancer, follicular thyroid cancer usually grows slowly. Its outlook is similar to papillary cancer, and its treatment is the same.
Follicular thyroid cancer usually stays in the thyroid gland but sometimes spreads to other parts of the body, such as the lungs or bone. However, it usually does not spread to lymph nodes. It is more common in countries where diets do not contain enough iodine.
Hurthle cell carcinoma, also called oxyphil cell carcinoma, is a type of follicular thyroid cancer. Most patients diagnosed with Hurthle cell cancer do well, but the outlook may change based on the extent of disease at the time of diagnosis.
Medullary thyroid cancer (MTC) is the only type of thyroid cancer that develops in the parafollicular cells of the thyroid gland. It accounts for 3% to 10% of thyroid cancers. Medullary thyroid cancer cells usually make and release into the blood proteins called calcitonin and/or carcinoembryonic antigen, which can be measured and used to follow the response to treatment for the disease.
Sometimes medullary thyroid cancer spreads to the lymph nodes, lungs or liver before a nodule is found or the patient has symptoms. MTC can be treated more successfully if it is diagnosed before it has spread.
There are two types of MTC:
- Sporadic MTC is more common, accounting for 85% of medullary thyroid cancers. It is found mostly in older adults and is not inherited.
- Familial MTC is inherited, and it often develops in childhood or early adulthood. If familial MTC occurs with tumors of certain other endocrine organs (parathyroid and adrenal glands), it is called multiple endocrine neoplasia type 2 (MEN 2). (link to more information). If you have a family history of MTC, it is important for you and your children to be tested for the gene that causes the disease.
Anaplastic thyroid cancer is the most dangerous form of thyroid cancer. It is makes up only 1% of thyroid cancers. It is believed that anaplastic thyroid cancer grows from a papillary or follicular tumor that mutates further to this aggressive form. Anaplastic thyroid cancer spreads rapidly into areas such as the trachea, often causing breathing difficulties.
Anaplastic thyroid cancer sometimes is called undifferentiated thyroid cancer because the cells are so different from normal thyroid tissue.
What causes thyroid cancer?
Although the exact cause of thyroid cancer has not been identified, certain risk factors have been identified. Anything that increases your chance of getting thyroid cancer is a risk factor. They include:
- Age: Two-thirds of thyroid cancer cases occur between ages 20 and 55.
- Gender: Women are three times as likely as men to develop thyroid cancer. Papillary thyroid cancer is found most often in women of childbearing age.
- Exposure to radiation, including X-rays, especially during childhood
- Inherited disorders: Familial medullary thyroid cancer usually is caused by an inherited mutation in the RET gene. If your parent has the gene mutation, you have a 50% chance of having it too. If you inherit the gene, you are likely to develop the cancer. Other types of thyroid cancer also may be caused by diseases that run in families.
- Iodine deficiency: This is uncommon in the United States, where iodine often is added to table salt. In other areas of the world, especially inland regions without fish and shellfish in the diet, iodine levels are sometimes too low.
Not everyone with risk factors gets thyroid cancer. However, if you have risk factors, it’s a good idea to discuss them with your doctor.
Some cases of thyroid cancer can be passed down from one generation to the next. Genetic counseling may be right for you. Learn more about the risk to you and your family on our genetic testing page.