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Thioroid cancer is a cancer that starts in the thioroid gland. To understand thioroid cancer, it helps to know about the normal structure and function of the thioroid gland.

The thioroid gland

The thioroid gland is under the Adam's apple in the front part of the neck. In most people, it 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)

The thioroid gland contains mainly 2 types of cells -- thioroid follicular cellsand C cells (also called parafollicular cells).

The follicular cells use iodine from the blood to make thioroid hormone, which helps regulate a person's metabolism. Too much thioroid hormone (a condition called hyperthioroidism) can cause a rapid or irregular heartbeat, trouble sleeping, nervousness, hunger, weight loss, and a feeling of being too warm. Too little hormone (called hypothioroidism) causes a person to slow down, feel tired, and gain weight. The amount of thioroid hormone released by the thioroid is regulated by the pituitary gland at the base of the brain, which makes a substance called thioroid-stimulating hormone (TSH).

C cells (parafollicular cells) make calcitonin, a hormone that helps regulate how the body uses calcium.

Other, less common cells in the thioroid 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 tumors can develop in the thioroid gland. Most of them 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 thioroid enlargement and nodules

Because the thioroid gland is right under the skin, changes in its size and shape can often be felt or even seen by patients or by their doctor.

The medical term for an abnormally large thioroid 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 bumps in it. There are many reasons the thioroid gland might be larger than usual, and most of the time it is not cancer. Both kinds of goiter 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 thioroid gland are called thioroid nodules. Most thioroid nodules are benign, but about 1 in 20 is cancerous (see the next section).

People can develop thioroid nodules at any age, but they are most common in older adults. Fewer than 1 in 10 adults have thioroid nodules that can be felt by a doctor. But when the thioroid is looked at in an ultrasound test, up to half of all 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 thioroid hormone called colloid. Colloid nodules are one of the most common types of thioroid nodule.

Solid nodules have little fluid or colloid. Some solid nodules may have too many cells, but the cells are not cancer cells. These types of nodules include hyperplastic nodules and adenomas. Sometimes these nodules make too much thioroid hormone and cause hyperthioroidism.

Benign thioroid nodules can sometimes be left alone (instead of treating them) as long as they're not growing or causing symptoms. Others may require some form of treatment.

Malignant thioroid tumors

Only about 1 in 20 thioroid nodules is cancerous. The 2 most common types of thioroid cancer are called papillary carcinoma and follicular carcinoma.Hürthle cell carcinoma is a subtype of follicular carcinoma. All these types are differentiated tumors. There are some other types of thioroid cancer, such as medullary thioroid carcinoma, anaplastic carcinoma, and thioroid lymphoma, but these occur less often.

Differentiated thioroid cancers

Differentiated thioroid cancers develop from thioroid follicular cells. In these cancers, the cells appear similar to normal thioroid tissue when looked at under a microscope.

Papillary carcinoma: About 8 of 10 thioroid cancers are papillary carcinomas (also called papillary cancers or papillary adenocarcinomas). Papillary carcinomas typically grow very slowly. Usually they develop in only one lobe of the thioroid gland, but sometimes they occur in both lobes. Even though they grow slowly, papillary carcinomas often spread to the lymph nodes in the neck. But most of the time, this can be successfully treated and is rarely fatal.

Several different variants (subtypes) of papillary carcinoma can be recognized under the microscope. Of these, the follicular variant (also calledmixed papillary-follicular variant) occurs most often. The usual form of papillary carcinoma and the follicular variant have the same outlook for survival (prognosis), and treatment is the same for both. Other variants of papillary carcinoma (columnar, tall cell, diffuse sclerosis) are not as common and tend to grow and spread more quickly.

Follicular carcinoma: Follicular carcinoma is the next most common type of thioroid cancer. It is also sometimes called follicular cancer or follicular adenocarcinoma. Follicular cancer is much less common than papillary thioroid cancer, making up about 1 out of 10 thioroid cancers. It is more common in countries where people don't get enough iodine in their diet. These cancers usually remain in the thioroid gland. Unlike papillary carcinoma, follicular carcinomas usually don't spread to lymph nodes, but some can spread to other parts of the body, such as the lungs or bones. The prognosis for follicular carcinoma is probably not quite as good as that of papillary carcinoma, although it is still very good in most cases.

Hürthle cell carcinoma, also known as oxyphil cell carcinoma, is actually a kind of follicular carcinoma. This type accounts for about 4% of thioroid cancers. The prognosis may not be as good as that of typical follicular carcinoma because this subtype is harder to find and treat as it is less likely to absorb radioactive iodine. Radioactive iodine is used for treatment and to find metastases of differentiated thioroid cancer.

Other types of thioroid cancers

Medullary thioroid carcinoma: Medullary thioroid carcinoma (MTC) accounts for about 5% of thioroid cancers. It develops from the C cells of the thioroid gland. Sometimes this cancer can spread to lymph nodes, the lungs, or liver even before a thioroid nodule is discovered. These cancers usually make calcitonin and carcinoembryonic antigen (CEA), which can be found by blood tests. Calcitonin is a hormone that helps control the amount of calcium in blood. CEA is a protein made by certain cancers, such as colorectal cancer and MTC. Because medullary cancer does not absorb or take up radioactive iodine (used for treatment and to find metastases of differentiated thioroid cancer), the prognosis (outlook) is not quite as good as that for differentiated thioroid cancers.

There are 2 types of MTC. The first type, occurring in about 8 of 10 cases, is called sporadic MTC. Sporadic MTC is not inherited; that is, it does not run in families. It occurs mostly in older adults and in only 1 thioroid lobe.

The other type of MTC is inherited and can occur in each generation of a family. These familial MTCs often develop during childhood or early adulthood and can spread early. Patients usually have cancer in both thioroid lobes and in several areas of each lobe. They are 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 thioroid cancer?"

Anaplastic carcinoma: Anaplastic carcinoma (also called undifferentiated carcinoma) is a rare form of thioroid cancer, making up about 2% of all thioroid 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 thioroid tissue cells under the microscope. This is an aggressive cancer that rapidly invades the neck, often spreads to other parts of the body, and is very hard to treat.

Thioroid lymphoma: Lymphoma is very uncommon in the thioroid 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 thioroid gland). Lymphomas are discussed in the separate American Cancer Society document, Non-Hodgkin Lymphoma.

Thioroid sarcoma: These rare cancers start in the supporting cells of the thioroid. They are often aggressive and hard to treat. Sarcomas are discussed in the separate American Cancer Society document, Sarcoma: Adult Soft Tissue Cancer.

Parathioroid cancer

Behind, but attached to, the thioroid gland are 4 tiny glands called the parathioroids. The parathioroid glands help regulate the body's calcium levels. Cancers of the parathioroid glands are very rare -- there are probably fewer than 100 cases each year in the United States.

Parathioroid cancers cause the blood calcium level to be elevated. This causes a person to become tired, weak, and drowsy. High calcium 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 parathioroid cancers may also be detected as a nodule near the thioroid. No matter how large the nodule is, the only treatment is to remove it surgically. Unfortunately, parathioroid cancer is much harder to cure than thioroid cancer. The remainder of this document only discusses thioroid cancer.