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.
- C cells (also called 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 growths and tumors can start in the thyroid gland. Most are benign (non-cancerous) but some 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
There are several types of thyroid cancer.
Differentiated thyroid cancers
Most thyroid cancers are differentiated cancers. These cancers start in thyroid follicular cells. In these cancers, the cells look a lot like normal thyroid tissue when seen under a microscope.
Papillary thyroid cancer: About 8 of 10 thyroid cancers are papillary cancers. Most often they grow very slowly. Often they grow in only one lobe of the thyroid gland. Even though they grow slowly, they often spread to the lymph nodes in the neck. But most of the time, these cancers can be cured and are rarely fatal.
Follicular thyroid cancer: This is the next most common type of thyroid cancer. It is much less common than papillary thyroid cancer, making up about 1 out of 10 thyroid cancers. Follicular cancers usually stay 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 cancer may not be quite as good as that of papillary cancer, but it is still very good in most cases.
Hürthle cell cancer is a kind of follicular cancer. It accounts for a very small number of thyroid cancers. The outlook may not be as good as that of typical follicular cancer because this type is harder to find and treat.
Rarer types of thyroid cancers
These cancers occur less often than differentiated thyroid cancers.
Medullary thyroid cancer (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 is found in the thyroid. The outlook for these cancers is not quite as good as that for differentiated thyroid cancers.
Anaplastic thyroid cancer: This is a rare type 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 quickly into the neck and to other parts of the body and is very hard to treat.
There are also other types of thyroid cancers that occur even more rarely. To learn more about rare types of thyroid cancers, see our more detailed document Thyroid Cancer.
The rest of this document only covers differentiated thyroid cancer.
Last Medical Review: 02/27/2014
Last Revised: 02/13/2015