Thyroid cancer happens when cells in the thyroid develop changes in their DNA. A cell's DNA contains the instructions that tell the cell what to do. The changes, which doctors call mutations, tell the cells to grow and multiply rapidly. The cells go on living when healthy cells would naturally die. The accumulating cells form a mass called a tumor.
The tumor can grow to invade nearby tissue and can spread (metastasize) to the lymph nodes in the neck. Sometimes the cancer cells can spread beyond the neck to the lungs, bones and other parts of the body.
For most thyroid cancers, it's not clear what causes the DNA changes that cause the cancer.
Types of thyroid cancer
Thyroid cancer is classified into types based on the kinds of cells found in the tumor. Your type is determined when a sample of tissue from your cancer is examined under a microscope. The type of thyroid cancer is considered in determining your treatment and prognosis.
Types of thyroid cancer include:
Differentiated thyroid cancers. This broad category includes types of thyroid cancer that start in the cells that produce and store thyroid hormones. These cells are called follicular cells. Differentiated thyroid cancers cells appear similar to healthy cells when viewed under a microscope.
Papillary thyroid cancer. This is the most common type of thyroid cancer. It can happen at any age, but it most often affects people ages 30 to 50. Most papillary thyroid cancers are small and respond well to treatment, even if the cancer cells spread to the lymph nodes in the neck. A small portion of papillary thyroid cancers are aggressive and may grow to involve structures in the neck or spread to other areas of the body.
Follicular thyroid cancer. This rare type of thyroid cancer usually affects people older than 50. Follicular thyroid cancer cells don't often spread to the lymph nodes in the neck. But some large and aggressive cancers may spread to other parts of the body. Follicular thyroid cancer most often spreads to the lungs and bones.
Hurthle cell thyroid cancer. This rare type of thyroid cancer was once considered a type of follicular thyroid cancer. Now it is considered its own type because the cancer cells behave differently and respond to different treatments. Hurthle cell thyroid cancers are aggressive and can grow to involve structures in the neck and spread to other parts of the body.
Poorly differentiated thyroid cancer. This rare type of thyroid cancer is more aggressive than other differentiated thyroid cancers and often doesn't respond to the usual treatments.
Anaplastic thyroid cancer. This rare type of thyroid cancer grows quickly and can be difficult to treat. However, treatments can help slow the progression of the disease. Anaplastic thyroid cancer tends to occur in people older than 60. It can cause severe signs and symptoms, such as neck swelling that worsens very quickly and may lead to difficulty breathing and swallowing.
Medullary thyroid cancer. This rare type of thyroid cancer begins in thyroid cells called C cells, which produce the hormone calcitonin. Elevated levels of calcitonin in the blood can indicate medullary thyroid cancer at a very early stage. Some medullary thyroid cancers are caused by a gene called RET that's passed from parents to children. Changes in the RET gene can cause familial medullary thyroid cancer and multiple endocrine neoplasia, type 2. Familial medullary thyroid cancer increases the risk of thyroid cancer. Multiple endocrine neoplasia, type 2, increases the risk of thyroid cancer, adrenal gland cancer and other types of cancers.
Other rare types. Other very rare types of cancer can start in the thyroid. These include thyroid lymphoma, which begins in the immune system cells of the thyroid, and thyroid sarcoma, which begins in the connective tissue cells of the thyroid.