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The thyroid has two halves, or lobes, one on each side of the neck. It wraps around the trachea (windpipe) just under the larynx (Adam's apple). A thin strip of tissue known as the isthmus connects the two halves.
The thyroid is part of the body's endocrine system, a system of glands that controls hormones in the body. It normally weighs less than an ounce, and it cannot be seen or felt in most people; however, it has an important function. The thyroid makes hormones that help regulate the body's heart rate, blood pressure, temperature and metabolism (the breakdown of food to create energy).
What are the different types of thyroid cancer?
Thyroid cancer is categorized based on the type of thyroid cells where the cancer begins and how the cancer cells appear under a microscope.
There are two kinds of cells found in the thyroid.
Follicular cells are the most common. They produce thyroid hormone, which is important for growth, mental function and helping the body create energy. Most thyroid cancers develop from follicular cells.
Parafollicular cells, also known as C cells, produce a small amount of the hormone calcitonin, which helps control calcium metabolism. Most parafollicular cells are in the upper third of each lobe. Medullary thyroid cancer is the only thyroid cancer that develops from parafollicular cells.
Thyroid cancers can also be categorized based on the appearance of their cells. Cancer cells that look most like normal, healthy cells are called well differentiated. Patients with well differentiated thyroid cancers are most likely to be disease-free at the end of treatment. Poorly differentiated and undifferentiated (anaplastic) cancer cells look less and less like healthy cells. These forms of thyroid cancer are usually harder to treat and the outlook for these patients is worse.
Doctors believe most thyroid cancers start as well differentiated. As the cancer grows, its cells can develop additional mutations, changing it into a less differentiated, harder-to-treat type of thyroid cancer.
Follicular cell thyroid cancers
From well differentiated to undifferentiated, the types of thyroid cancer from follicular cells are:
Papillary thyroid cancer: This is typically the least aggressive type of thyroid cancer. It accounts for about 80% of thyroid cancer diagnoses. While papillary thyroid cancer usually occurs in only one lobe of the thyroid gland, it appears in both lobes in 10%-20% of cases. Papillary thyroid cancer is most common in women of childbearing age. Its treatment is successful in most patients.
Follicular thyroid cancer: This accounts for about 10% of thyroid cancers. Though it can be more aggressive than papillary thyroid cancer, follicular thyroid cancer usually grows slowly. Treatment for follicular thyroid cancer is similar to papillary thyroid cancer and is successful for most patients.
Both papillary and follicular thyroid cancer are considered well differentiated cancers. Well differentiated thyroid cancer tends to stay contained within the thyroid gland. When it does spread outside the thyroid, the most common locations of spread, or metastasis, are lymph nodes, lungs, bones and the liver.
Hürthle cell thyroid cancer: Also called oxyphilic cell carcinoma, Hürthle cell carcinoma was considered a type of follicular thyroid cancer until recently. Most patients diagnosed with Hürthle cell carcinoma do well, but the outlook may change based on the extent of disease at the time of diagnosis.
Poorly differentiated thyroid cancer: This is a rare type of thyroid cancer, accounting for less than 5% of all cases. Compared to well differentiated thyroid cancer, it has a higher risk of spreading to the lungs and bones and a higher chance of recurring after treatment. Other than anaplastic thyroid cancer, it is the most aggressive form of the disease.
Anaplastic thyroid cancer: This disease makes up just 1% of thyroid cancer cases. Sometimes called undifferentiated thyroid cancer, it is the most dangerous type of thyroid cancer. Doctors believe it starts as a papillary or follicular cancer that mutates to this aggressive form. Anaplastic thyroid cancer spreads rapidly into areas such as the trachea, often causing breathing difficulties. Due to the specialized and rapid attention these patients need, MD Anderson’s Anaplastic Thyroid Cancer Clinic is dedicated exclusively to treating these patients.
Medullary thyroid cancer
Medullary thyroid cancer (MTC) is the only type of thyroid cancer that develops from the parafollicular cells of the thyroid gland. It accounts for 3% to 10% of thyroid cancers. MTC cells usually produce calcitonin and/or carcinoembryonic antigen (CEA). These proteins can be measured in the blood and used to monitor the response to treatment.
Sometimes MTC spreads to the lymph nodes, lungs or liver before a thyroid nodule is found or symptoms develop. MTC can be treated most successfully if it is diagnosed before it has spread.
There are two types of MTC:
- Sporadic MTC accounts for 85% of medullary thyroid cancers. It occurs mostly in older adults and is not inherited.
- Familial MTC is inherited and often seen in childhood or early adulthood. If familial MTC occurs in association with tumors of certain other endocrine organs (such as the parathyroid and adrenal glands), it is called multiple endocrine neoplasia type 2 (MEN 2). Parents, siblings and children of people with MTC should undergo genetic testing to learn if they have inherited the gene that causes familial MTC.
Thyroid cancer statistics
According to the National Cancer Institute, about 52,000 people are diagnosed with thyroid cancer each year in the United States. About 75% of these are women. Thyroid cancer is seen most often in adults, with more than half of the cases occurring between ages 20 and 55.
Thyroid cancer risk factors
Although the exact cause of thyroid cancer is unknown, certain risk factors have been identified. They include:
- Age: Two-thirds of thyroid cancer cases occur between ages 20 and 55.
- Gender: Women are three times as likely as men to develop thyroid cancer. Papillary thyroid cancer, the most common type of the disease, is found most often in women of childbearing age.
- Exposure to high levels of radiation, especially during childhood: This exposure often is due to radiation treatment to the head and neck area for a childhood cancer. Occasional X-rays or scans that are part of routine medical procedures (such as a dental exam) have very low levels of radiation and are not risk factors.
- Inherited disorders: Familial medullary thyroid cancer is usually caused by an inherited mutation in the RET gene. Children of a person with this gene have a 50% chance of having this mutation. People who inherit the gene are likely to develop medullary thyroid cancer. Other types of thyroid cancer also may be caused by diseases that run in families. People with a history of thyroid cancer in their family should consider genetic counseling to learn and manage their risk.
- Iodine deficiency: The thyroid uses iodine to make hormones. People with low iodine levels have a higher risk of developing thyroid cancer. This risk factor is uncommon in the United States, where iodine often is added to table salt. In other parts of the world, especially inland regions without fish and shellfish in the diet, iodine levels can be low enough to become a common risk factor for thyroid cancer.
Not everyone with risk factors gets thyroid cancer. However, if you have risk factors it is a good idea to discuss them with your doctor.
Some cases of thyroid cancer can be passed down from one generation to the next. Genetic counseling may be right for you. Learn more about the risk to you and your family on our genetic testing page.
Why choose MD Anderson for thyroid cancer treatment?
At MD Anderson’s Endocrine Center, you receive customized care for thyroid cancer from a team of renowned experts. Their level of experience and expertise in treating every type of cancer is among the most impressive in the nation. This increases your chance for successful treatment.
Your personal medical team is made up of experts from several specialties. They work together, communicating and collaborating with each other and with you, to ensure you receive seamless, coordinated care.
If your treatment for thyroid cancer includes surgery, our surgeons use the most advanced techniques that are proven to have good results. These surgeons are skilled in performing complex surgeries, including procedures for residual cancers, recurrent cancers and cancers that impact critical structures in the head and neck area.
The Endocrine Center also specializes in treating patients with rare and aggressive thyroid cancers. This includes anaplastic thyroid cancer patients, who get rapid, specialized care in our Anaplastic Thyroid Cancer Clinic.
We are at the forefront of research on how to better treat and prevent thyroid cancer. MD Anderson doctors led an international study that showed patients with certain types of papillary or follicular thyroid cancer do best when they are treated with surgery, radioactive iodine and thyroid hormone suppression therapy. In addition, the first drug to get FDA approval for anaplastic thyroid cancer treatment was developed through MD Anderson research.
As leaders in thyroid cancer research, we also are able to offer a number of clinical trials of innovative therapies, including ones studying the use of therapies to shrink tumors before surgery and the expanded use of chemotherapy for some patients.
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