What is Thyroid Cancer?

Thyroid cancer is a cancer that starts in the thyroid gland. To understand thyroid cancer, it helps to know about the normal structure and function of the thyroid gland.

The thyroid gland: 

The thyroid gland is below the Adam's apple in the front part of the neck. It is butterfly shaped, with 2 lobes — the right lobe and the left lobe — joined by a narrow structure called the isthmus (see picture below).

The thyroid gland has 2 main types of cells: 

Follicular cells use iodine from the blood to make thyroid hormone, which helps control how the body uses energy. Too much thyroid hormone (a condition called hyperthyroidism) can cause a rapid or abnormal heartbeat, trouble sleeping, nervousness, hunger, weight loss, and a feeling of being too warm. Too little hormone (called hypothyroidism) causes a person to slow down, feel tired, and gain weight.

C cells (parafollicular cells) make calcitonin, a hormone that helps control how the body uses calcium.

Other, less common cells in the thyroid gland include immune system cells (lymphocytes) and supportive (stromal) cells.

Different cancers can start from each kind of cell. The type of cell where the cancer starts is important because it affects how serious the cancer is and what type of treatment is needed.

Many types of tumors can start in the thyroid gland. Most of them are benign (non-cancerous) but others are malignant (cancerous), which means they can spread into nearby tissues and to other parts of the body. The information here covers only cancerous tumors of the thyroid.

Malignant (cancerous) Thyroid Tumors:

The 2 most common types of thyroid cancer are papillary carcinoma and follicular carcinoma. These types are differentiated tumors, which develop from thyroid follicular cells. In these cancers, the cells look a lot like normal thyroid tissue when seen under a microscope. There are other types of thyroid cancer, which are rare.

Differentiated Thyroid Cancers:

  • Papillary carcinoma: About 8 of 10 thyroid cancers are papillary carcinomas. Most often they grow very slowly. Often they grow in only one lobe of the thyroid gland, but sometimes they happen in both lobes. Even though they grow slowly, they often spread to the lymph nodes in the neck. But most of the time, these cancers can be treated well and are rarely fatal.
  • Follicular carcinoma: Follicular carcinoma is the next most common type of thyroid cancer. This cancer is much less common than papillary thyroid cancer, making up about 1 out of 10 thyroid cancers. These cancers usually remain in the thyroid gland. They usually don't spread to lymph nodes, but some can spread to other parts of the body, such as the lungs or bones. The outlook for follicular carcinoma may not be quite as good as that of papillary carcinoma, although it is still very good in most ases.
  • Hürthle cell carcinoma is a kind of follicular carcinoma. It accounts for a very small number of thyroid cancers. The outlook may not be as good as that of typical follicular carcinoma because this type is harder to find and treat.

Other types of thyroid cancers

  • Medullary thyroid carcinoma (MTC): This accounts for about 4% of thyroid cancers. It starts in the C cells of the thyroid gland. In some cases MTC can run in families. Sometimes these cancers can spread to other parts of the body even before a lump in the thyroid is found. The outlook for these cancers is not quite as good as that for differentiated thyroid cancers.
  • Anaplastic carcinoma: This is a rare form of thyroid cancer, making up about 2% of all thyroid cancers. This cancer is also called undifferentiated because the cancer cells do not look very much like normal thyroid cells under the microscope. It is a fast-growing cancer that often spreads to other parts of the body and is very hard to treat.
  • Parathyroid cancer: Behind, but attached to, the thyroid gland are 4 tiny glands called the parathyroids. The parathyroid glands help control the body's calcium levels. Cancers of the parathyroid glands are very rare — there are probably fewer than 100 cases each year in the United States.

Parathyroid cancers are often found because they cause a person’s blood calcium level to be high. The person can become tired, weak, and drowsy. High calcium also makes you urinate (pee) a lot. Larger parathyroid cancers may also be found as a lump (nodule) near the thyroid. Parathyroid cancer is treated with surgery, but it is much harder to cure than thyroid cancer.

Last Medical Review: 08/05/2011

Last Revised: 08/05/2011