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What Is It?

Thyroid cancer is the uncontrolled growth of abnormal cells in the thyroid gland. The thyroid gland is shaped like a butterfly. It is located under the Adam's apple in the front of the neck. Most cases of thyroid cancer can be cured.

One of the functions of the thyroid gland is to make thyroid hormone, which requires iodine. The gland collects iodine from foods, concentrates it, and produces thyroid hormone. Doctors often exploit this important function when treating thyroid cancer.

Thyroid hormone helps regulate the body's metabolism and energy level. An overactive thyroid can lead to hyperactivity, the "jitters," and an irregular heart rhythm; an underactive thyroid, fatigue and sluggishness. Cancer can affect the thyroid and cause these changes.

Nestled against the thyroid gland are four very small glands called parathyroid glands. They play a role in regulating the body's use of calcium. The nerve that controls the voice box is also very close to the thyroid. If you need a thyroid operation, your surgeon needs to identify and avoid damaging these structures. If the voice box nerve is damaged, for example, your voice may sound hoarse permanently.

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

  • Follicular cells produce a thyroid hormone called thyroxine, or T-4. It controls the body's metabolism and may affect the function of various organs.

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

There are five types of thyroid cancers:

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

  • Follicular carcinoma — The second-most-common type of thyroid cancer, follicular carcinoma starts in follicular cells. It often affects just the thyroid gland, but it can spread to other parts of the body, 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 poorly understood cancer seems 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, spreading quickly to other parts of the body. Because the thyroid is so close to the wind pipe (trachea), patients with this type of cancer may experience sudden shortness of breath. They may need to have a tube inserted into the windpipe to help them breathe.

  • Medullary thyroid carcinoma (MTC) — This is the only type of thyroid cancer that develops from the C-cells. It can spread to the lymph nodes, the lungs, and the liver before an abnormal lump in the thyroid has even 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. It usually develops in only one thyroid lobe. Familial MTC (20 percent of cases) can affect several generations of the same family.

Rarely, tumors arising from connective tissue (sarcomas) and lymph nodes (lymphomas) can start in the thyroid gland. They are treated differently than 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. Because the thyroid has an attraction for iodine, the thyroid tissue accumulates this radioactive substance. Over time, it may cause cancer.

Others who have a higher risk of thyroid cancer include people who

  • received high-dose radiation for acne or swollen adenoids as a child

  • have a diet that's very low in iodine

  • have certain medical conditions, such as Cowden's disease and familial polyposis.

Individuals who have received radiation therapy to the chest (to treat Hodgkin's disease, for example) have an increased incidence of thyroid abnormalities, including cancer. This is even more likely if the thyroid was included in the radiation field. Such people will need life-long follow up to assess thyroid function and check for 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 parts of the body, such as the adrenal gland and peripheral nervous system.

Some forms of thyroid cancer may be caused by genetic changes (mutations) that occur after birth.

Thyroid cancer is rare, accounting for only a small percentage of all cancers. However, it does strike more women than men.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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