Thyroid cancer begins in the thyroid gland, which is located 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. Thyroid cancer accounts for about 1 percent of all cancers, but it is the eighth most common cancer in women and rising in incidence. It is one of the least dangerous cancers in most cases, and the five-year survival rate for all types of thyroid cancer is almost 97 percent.
This year, about 33,550 people will be diagnosed with thyroid cancer in the United States. About 75 percent will be women, making it the eighth-most-common cancer in women. Thyroid cancer is most often seen in adults, with two thirds of the cases occurring between ages 20 and 55.
Thyroid cancer is usually a slow-growing cancer. At least 300,000 people in the United States have completed treatment or are living with the disease.
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The thyroid gland is a small, butterfly-shaped gland in the front of the neck. It normally weighs less than an ounce, and it cannot be seen or felt in most people. But it has an important function in the endocrine system and generates 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, or windpipe, just under the larynx, or Adam's apple. A thin ribbon 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 for the production of 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 very minor role to control calcium metabolism. Most parafollicular cells are in the upper third of each lobe.
Thyroid cancer develops when cells in the thyroid grow uncontrollably. Many papillary and follicular thyroid cancers seem to be caused by mutations in genes. But it is not known why these changes happen in some people and not others.
The development of some thyroid cancers may be affected by a lack of iodine in the diet. The mechanism isn't entirely clear, but an iodine deficiency can lead to an enlarged thyroid gland, called a goiter. Certain types of thyroid cancer are more common in areas with a high incidence of goiters.
Some thyroid cancers are caused by exposure to radiation, including X-rays.
Familial medullary thyroid cancer usually is caused by an inherited mutation in a gene known as the RET gene. If your parent has the mutated gene, you have a 50 percent chance of inheriting this mutation. If you do inherit it, you are likely to develop the cancer.
Thyroid cancer is classified by the type of thyroid cell in which the cancer begins.
Papillary, follicular, and anaplastic thyroid cancer all begin in the follicular cells, which are the most common type of cells in the thyroid. The papillary and follicular cancers—the most common thyroid cancers—are sometimes referred to together as "differentiated thyroid cancer." They have similar treatment. A fourth type of disease, medullary thyroid cancer, begins in the parafollicular cells. These cells, also called C cells, produce calcitonin, a hormone that helps regulate calcium levels in the blood.
Papillary thyroid cancer is the most common type of thyroid cancer, accounting for about 80 percent of thyroid cancers. It generally is slow growing. While papillary thyroid cancer is usually in one lobe of the thyroid gland, it is in both lobes in 10 percent to 20 percent of cases.
Papillary thyroid cancer is most common in women of childbearing age. It is sometimes caused by exposure to radiation, such as during the Chernobyl nuclear plant explosion in 1986.
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 is most often successful.
Follicular thyroid cancer accounts for about 10 percent to 20 percent of thyroid cancers. Like papillary thyroid cancer, follicular thyroid cancer usually grows slowly. Its prognosis is similar to papillary cancer, and its treatment is the same.
Follicular thyroid cancer usually remains 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 as often as papillary thyroid cancer.
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 cancer that is present in about 5 percent of thyroid cancers. Sometimes it has a less favorable prognosis.
Medullary thyroid cancer (MTC) is the only type that develops in the parafollicular cells of the thyroid gland. It accounts for 3 percent to 10 percent of all thyroid cancers.
Medullary 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 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 and familial. The sporadic form is predominant, accounting for 85 percent of medullary thyroid cancers. It is found mostly in older adults and is not passed from parent to child. 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, or MEN 2.
Anaplastic thyroid cancer is the most dangerous form of thyroid cancer. It is also the most rare, making up 1 percent of all thyroid cancers.
It is believed that anaplastic thyroid cancer develops 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 is sometimes called undifferentiated thyroid cancer because the cells are so different from normal thyroid tissue.
Anaplastic thyroid cancer usually is fatal. Most patients die within six months of diagnosis.
The risk factors for thyroid cancer are:
- 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. Estrogen has been studied as a possible risk factor, but a correlation has not been proved. Papillary thyroid cancer is most common in women of childbearing age.
- Exposure to radiation, especially during childhood.
- Genetics. Familial medullary thyroid cancer usually is caused by an inherited mutation in a gene known as the RET gene.
- Iodine deficiency. This is uncommon in the United States, where iodine is often 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.
Last reviewed on 6/4/09
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