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 thyroid cartilage (Adam’s apple) in the front part of the neck. In most people, the thyroid cannot be seen or felt. It is butterfly shaped, with 2 lobes — the right lobe and the left lobe — joined by a narrow isthmus (see picture below).
- Follicular cells use iodine from the blood to make thyroid hormones, which help regulate a person’s metabolism. Having too much thyroid hormone (a condition called hyperthyroidism) can cause a rapid or irregular heartbeat, trouble sleeping, nervousness, hunger, weight loss, and a feeling of being too warm. Having too little hormone (called hypothyroidism) causes a person to slow down, feel tired, and gain weight. The amount of thyroid hormone released by the thyroid is regulated by the pituitary gland at the base of the brain, which makes a substance called thyroid-stimulating hormone (TSH).
- 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 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.
Benign thyroid enlargement and nodules
Changes in the thyroid gland’s size and shape can often be felt or even seen by patients or by their doctor.
The medical term for an abnormally large thyroid gland is 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 1 in 20 is cancerous (see the next section). Sometimes these nodules make too much thyroid hormone and cause hyperthyroidism.
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 using ultrasound, many more people are found to have nodules that are too small to feel.
Most nodules are cysts filled with fluid or with a stored form of thyroid hormone called colloid.
Solid nodules have little fluid or colloid. These nodules are more likely to be cancerous than are fluid-filled nodules. 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) as long as they’re not growing or causing symptoms. Others may require some form of treatment.
Malignant (cancerous) thyroid tumors
There are several types of thyroid cancer.
Differentiated thyroid cancers
Most thyroid cancers are differentiated cancers. In these cancers, the cells look a lot like normal thyroid tissue when seen under a microscope. These cancers develop from thyroid follicular cells.
Papillary carcinoma: About 8 out of 10 thyroid cancers are papillary carcinomas (also called papillary cancers or papillary adenocarcinomas). Papillary carcinomas tend to grow very slowly and usually develop in only one lobe of the thyroid gland. Even though they grow slowly, papillary carcinomas often spread to the lymph nodes in the neck. Still, these cancers can often be treated successfully and are rarely fatal.
There are several subtypes of papillary carcinoma. Of these, the follicular subtype (also called mixed papillary-follicular variant) occurs most often. The usual form of papillary carcinoma and the follicular subtype have the same good outlook (prognosis) 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 carcinoma: Follicular carcinoma, also called follicular cancer or follicular adenocarcinoma, 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 carcinoma is not quite as good as that of papillary carcinoma, although it is still very good in most cases.
Hürthle (Hurthle) cell carcinoma, also known as oxyphil cell carcinoma, is actually a variant of follicular carcinoma. It accounts for about 3% of thyroid cancers. The prognosis may not be as good as that of typical follicular carcinoma because this type is harder to find and treat. This is because it is less likely to absorb radioactive iodine, which is used both for treatment and to look for the spread of differentiated thyroid cancer.
Other types of thyroid cancers
These thyroid cancers occur less often than differentiated thyroid cancers.
Medullary thyroid carcinoma: Medullary thyroid carcinoma (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.
Medullary thyroid cancers often release too much calcitonin and a protein called carcinoembryonic antigen (CEA) into the blood. These substances can be detected with blood tests.
Because MTC does not absorb or take up radioactive iodine (used for treatment and to find metastases of differentiated thyroid cancer), the prognosis (outlook) is not quite as good as that for differentiated thyroid cancers. There are 2 types of MTC:
- Sporadic MTC, which accounts for about 8 out of 10 cases of MTC, is not inherited (meaning it does not run in families). It occurs mostly in older adults and affects only one thyroid lobe.
- Familial MTC is inherited and can occur in each generation of a family. These cancers often develop during childhood or early adulthood and can spread early. Patients usually have cancer in several areas of both lobes. Familial MTC is often linked with an increased risk of other types of tumors. This is described in more detail in the section “What are the risk factors for thyroid cancer?”
Anaplastic carcinoma: 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 under the microscope. This cancer often spreads quickly into the neck and to other parts of the body, and is very hard to treat.
Thyroid lymphoma: Lymphoma is very uncommon in the thyroid gland. Lymphomas are cancers that develop from lymphocytes, the main cell type of the immune system. Most lymphocytes are found in lymph nodes, which are pea-sized collections of immune cells scattered throughout the body (including the thyroid gland). Lymphomas are discussed in our separate document, Non-Hodgkin Lymphoma.
Thyroid sarcoma: These rare cancers start in the supporting cells of the thyroid. They are often aggressive and hard to treat. Sarcomas are discussed in our separate document, Sarcoma: Adult Soft Tissue Cancer.
Behind, but attached to, the thyroid gland are 4 tiny glands called the parathyroids. 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 makes 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. Unfortunately, parathyroid cancer is much harder to cure than thyroid cancer.