Thyroid cancer is a type of cancer that starts in the thyroid gland. Cancer starts when cells begin to grow out of control. (To learn more about how cancers start and spread, see What Is Cancer?)
The thyroid gland makes hormones that help regulate your metabolism, heart rate, blood pressure, and body temperature.
The thyroid gland is in the front part of the neck, below the thyroid cartilage (Adam’s apple). In most people, the thyroid cannot be seen or felt. It is shaped like a butterfly, with 2 lobes — the right lobe and the left lobe — joined by a narrow piece of gland called the isthmus (see picture below).
The thyroid gland has 2 main types of cells:
Other, less common cells in the thyroid gland include immune system cells (lymphocytes) and supportive (stromal) cells.
Different cancers develop from each kind of cell. The differences are important because they affect how serious the cancer is and what type of treatment is needed.
Many types of growths and tumors can develop in the thyroid gland. Most of these are benign (non-cancerous) but others are malignant (cancerous), which means they can spread into nearby tissues and to other parts of the body.
Changes in the thyroid gland’s size and shape can often be felt or even seen by patients or by their doctor.
An abnormally large thyroid gland is sometimes called a goiter. Some goiters are diffuse, meaning that the whole gland is large. Other goiters are nodular, meaning that the gland is large and has one or more nodules (bumps) in it. There are many reasons the thyroid gland might be larger than usual, and most of the time it is not cancer. Both diffuse and nodular goiters are usually caused by an imbalance in certain hormones. For example, not getting enough iodine in the diet can cause changes in hormone levels and lead to a goiter.
Lumps or bumps in the thyroid gland are called thyroid nodules. Most thyroid nodules are benign, but about 2 or 3 in 20 are cancerous. Sometimes these nodules make too much thyroid hormone and cause hyperthyroidism. Nodules that produce too much thyroid hormone are almost always benign.
People can develop thyroid nodules at any age, but they occur most commonly in older adults. Fewer than 1 in 10 adults have thyroid nodules that can be felt by a doctor. But when the thyroid is looked at with an ultrasound, many more people are found to have nodules that are too small to feel and most of them are benign.
Most nodules are cysts filled with fluid or with a stored form of thyroid hormone called colloid. Solid nodules have little fluid or colloid and are more likely to be cancerous. Still, most solid nodules are not cancer. Some types of solid nodules, such as hyperplastic nodules and adenomas, have too many cells, but the cells are not cancer cells.
Benign thyroid nodules sometimes can be left alone (not treated) and watched closely as long as they’re not growing or causing symptoms. Others may require some form of treatment.
The main types of thyroid cancer are:
Most thyroid cancers are differentiated cancers. The cells in these cancers look a lot like normal thyroid tissue when seen in the lab. These cancers develop from thyroid follicular cells.
Papillary cancer (also called papillary carcinomas or papillary adenocarcinomas): About 8 out of 10 thyroid cancers are papillary cancers. These cancers tend to grow very slowly and usually develop in only one lobe of the thyroid gland. Even though they grow slowly, papillary cancers often spread to the lymph nodes in the neck. Even when these cancers have spread to the lymph nodes, they can often be treated successfully and are rarely fatal.
There are several subtypes of papillary cancers. Of these, the follicular subtype (also called mixed papillary-follicular variant) is most common. It has the same good outlook (prognosis) as the standard type of papillary cancer when found early, and they are treated the same way. Other subtypes of papillary carcinoma (columnar, tall cell, insular, and diffuse sclerosing) are not as common and tend to grow and spread more quickly.
Follicular cancer (also called follicular carcinoma or follicular adenocarcinoma): Follicular cancer is the next most common type, making up about 1 out of 10 thyroid cancers. It is more common in countries where people don’t get enough iodine in their diet. These cancers usually do not spread to lymph nodes, but they can spread to other parts of the body, such as the lungs or bones. The outlook (prognosis) for follicular cancer is not quite as good as that of papillary cancer, although it is still very good in most cases.
Hürthle (Hurthle) cell cancer (also called oxyphil cell carcinoma): About 3% of thyroid cancers are this type. It is harder to find and to treat.
Medullary thyroid cancer (MTC) accounts for about 4% of thyroid cancers. It develops from the C cells of the thyroid gland, which normally make calcitonin, a hormone that helps control the amount of calcium in blood. Sometimes this cancer can spread to lymph nodes, the lungs, or liver even before a thyroid nodule is discovered.
This type of thyroid cancer is more difficult to find and treat, There are 2 types of MTC:
Anaplastic carcinoma (also called undifferentiated carcinoma) is a rare form of thyroid cancer, making up about 2% of all thyroid cancers. It is thought to sometimes develop from an existing papillary or follicular cancer. This cancer is called undifferentiated because the cancer cells do not look very much like normal thyroid cells. This cancer often spreads quickly into the neck and to other parts of the body, and is very hard to treat.
Less than 4% of cancers found in the thyroid are thyroid lymphomas, thyroid sarcomas, or other rare tumors.
The parathyroids are 4 tiny glands behind, but attached to, the thyroid gland.The parathyroid glands help regulate 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 high blood calcium levels. This makes a person tired, weak, and drowsy. It can also make you urinate (pee) a lot, causing dehydration, which can make the weakness and drowsiness worse. Other symptoms include bone pain and fractures, pain from kidney stones, depression, and constipation.
Larger parathyroid cancers may also be found as a nodule near the thyroid. No matter how large the nodule is, the only treatment is to remove it surgically. Parathyroid cancer is much harder to cure than thyroid cancer.
Our information on thyroid cancer does not cover parathyroid cancer.
The American Cancer Society medical and editorial content team
Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.
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Last Revised: March 14, 2019
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