Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Thyroid Cancer

What Is It?

Thyroid cancer is the uncontrolled growth of abnormal cells in the thyroid gland. The thyroid gland is shaped like a butterfly and is located under the Adam's apple in the front of the neck. Most cases of thyroid cancer are cured. One of the functions of the thyroid gland is to make thyroid hormone and thyroid hormone requires the use of the element iodine. The thyroid gland possesses specialized functions that can capture and trap the dietary iodine and concentrate it in the gland for production of thyroid hormone. Physicians often times exploit this important function in the treatment of thyroid cancer.

There are two other features in the anatomy of the thyroid gland that are important. First, nestled within the thyroid tissue are the presence of four very small glands called parathyroid glands, which have an important role in regulating the use of calcium in the body. If surgery is done on the thyroid gland, it is important for the surgeon to identify and avoid damaging these small glands.

The nerve that controls the voice box is also very close and related to the thyroid. Thus if surgery is contemplated on the thyroid gland, it is important to recognize this nerve. If it is damaged, permanent hoarseness may result.

The thyroid has two types of cells that produce two hormones that help to regulate body functions.

• Follicular cells in the thyroid produce thyroid hormone called thyroxine, or T-4, which controls the body's rate of metabolism. The metabolism regulated by the thyroid gland is a complex process that may affect the function of many different organs in the body.

• C-cells, also called parafollicular cells, produce calcitonin, a hormone that helps to regulate the level of calcium in the blood.

There are five types of thyroid cancers:

• Papillary carcinoma (also known as papillary adenocarcinoma) — This is the most common type of thyroid cancer, accounting for 75 percent of all thyroid cancers. It develops from follicular cells and usually grows slowly. In most cases, it is in only one of the thyroid gland's two lobes, but it affects both lobes in 10 percent to 20 percent of patients. Papillary carcinoma often spreads to nearby lymph nodes in the neck, but also can spread to areas of the body further away.

• Follicular carcinoma — This is the second most common type of thyroid cancer, and it starts in follicular cells. This type of cancer often involves only the thyroid gland, but it sometimes spreads to other body areas, especially the lungs and bone. Only about one-third of tumors beginning in follicular cells are cancerous. Some thyroid cancers are mixtures of papillary and follicular cells.

• Hürthle cell neoplasm (follicular adenocarcinoma) — This is a poorly understood cancer that is similar to follicular carcinoma.

• Anaplastic carcinoma (undifferentiated thyroid cancer) — This is the rarest form of thyroid cancer, and it has the worst prognosis. Scientists believe that it develops from existing papillary or follicular carcinoma. Anaplastic carcinoma is aggressive and spreads quickly to the neck and other parts of the body. Because of the close proximity of the thyroid to the wind pipe (trachea), patients with this type of cancer may experience acute onset of shortness of breath and may require emergency insertion of a tube into the windpipe to aid in breathing.

• Medullary thyroid carcinoma (MTC) — This is the only type of thyroid cancer that develops from the thyroid's C-cells. It can spread to the lymph nodes, the lungs and the liver even before a thyroid nodule (abnormal lump in the thyroid) has been detected. MTC produces the hormone calcitonin, as well as a protein called carcinoembryonic antigen (CEA). Both of these chemicals are released into the blood. There are two main types of MTC. Sporadic MTC (80 percent of all MTC cases) is not inherited and usually develops in only one thyroid lobe. Familial MTC (20 percent of cases) has two forms, each of which can affect several generations of the same family.

In very rare circumstances, tumors arising from connective tissue (sarcomas) and lymph nodes (lymphomas) also can start in the thyroid gland. These are treated differently from other thyroid cancers.

Although scientists have not identified the exact cause of thyroid cancer, some studies have shown that people exposed to nuclear fallout or nuclear power plant accidents have a higher risk of developing thyroid cancer. In part, this is due to the presence of radioactive iodine that may be a part of nuclear fallout. Because the thyroid has an attraction for iodine, the thyroid tissue accumulates this radioactive substance, which over time may cause cancer. People who have received high-dose radiation for acne or swollen adenoids as a child are at increased risk. The incidence of thyroid cancer also is higher in countries where diets are low in iodine. In addition, people with certain medical conditions, such as Cowden's disease and familial polyposis, have a higher risk of developing thyroid cancer.

Individuals who have received radiation therapy to the body in the chest area (for example, Hodgkin's disease and other childhood cancers) have a definite increase incidence of thyroid abnormalities, including cancer, especially if the thyroid was included in the radiation field. Patients will need life-long follow up to both assess the function of the thyroid as well as the presence of cancer.

Some forms of thyroid cancer are inherited. These occur alone (inherited MTC) or as part of a familial cancer syndrome known as multiple endocrine neoplasia (MEN) type 2. Patients with MEN-2 develop tumors in other organs such as the adrenal gland (pheochromocytomas) and peripheral nervous system (neuromas and ganglioneuromas).

Recent scientific studies also indicate that some forms of thyroid cancer may be caused by changes (mutations) in DNA, the chemical matter that makes you unique.

Thyroid cancer is rare, accounting for only about 1.5 percent of all types of cancer. This cancer strikes about 18,000 people in the United States each year and causes more than 1,200 deaths. The illness is more common in women than in men.