Many types of tumors can develop in the thyroid gland, which is located in the front of the neck. It makes a hormone (the thyroid hormone) that helps control heart rate, blood pressure, body temperature, and weight.
You might develop one or more solid or fluid-filled lumps (nodules) in your thyroid. Most of these are noncancerous (benign) and have no symptoms. But a small percentage are cancerous (malignant).
Most people with thyroid cancer do very well after treatment. The most common types of thyroid cancer can usually be completely removed with surgery. But it is important to be familiar with the symptoms of thyroid cancer and see your doctor as soon as they start. This will make it possible to treat your cancer while it is still at an early stage, when treatment is most likely to be successful.
About 33,500 people will be diagnosed with thyroid cancer in 2007, according to the American Cancer Society.
The Thyroid
The thyroid gland lies in the front of your neck just below your larynx (commonly referred to as your “Adam’s apple” or voice box). In most people, it cannot be seen or felt. It is shaped like a butterfly, with a right lobe and a left lobe on both sides of your windpipe. The two lobes are joined together by a narrow strip of tissue (isthmus). The thyroid gland is attached to the lower part of the larynx and the upper part of the windpipe (trachea).
The thyroid gland makes a hormone (thyroid hormone) that helps control your heart rate, body temperature, and energy level. A hormone is a chemical that circulates in the bloodstream and controls the action of certain cells or organs. Too little thyroid hormone (hypothyroidism) can decrease your energy levels and make you tired. Too much thyroid hormone (hyperthyroidism) speeds up your metabolism, making you lose weight and giving you chest pain, cramps, and a fast or irregular heartbeat.
The production of thyroid hormone requires iodine, a nonmetallic chemical element found in iodized salt. The thyroid gland absorbs iodine from the blood to make thyroid hormone.
A gland in your brain (pituitary gland) constantly monitors the amount of thyroid hormone in your blood. If you do not have enough thyroid hormone, your pituitary releases extra thyroid-stimulating hormone (TSH), which tells your thyroid to absorb iodine from the blood and use it to make more thyroid hormone. Your thyroid gland then makes and releases the hormone directly into your bloodstream. Your pituitary gland senses that the right amount of thyroid hormone is moving through your body, so it slows its production of TSH back to normal.
The thyroid gland contains two main types of cells:
- Follicle cells make and store thyroid hormone. They also make a special thyroid protein (thyroglobulin).
- C cells make another hormone, calcitonin, which helps control the level of calcium in your blood.
A healthy thyroid is a little larger than a quarter, and it cannot usually be felt through the skin. If a mass develops in the thyroid, you will feel it as a lump in your neck. A swollen thyroid gland (goiter) is often caused by too little iodine in the diet. Most Americans receive enough iodine from salt, so goiters in the United States are usually caused by other factors.
Different cancers develop from each kind of cell. These differences are important because they determine the seriousness of the cancer and the type of treatment needed.
How Thyroid Cancer Starts
Thyroid cancer begins when the cells in the thyroid begin to change, grow uncontrollably without dying when they should, and eventually form a tumor. Tumors can be benign (noncancerous) or malignant (cancerous). Thyroid tissue tumors are known as nodules, and 90 percent of all thyroid nodules are benign.
If thyroid cancer spreads (metastasizes) outside the thyroid, cancer cells often travel to nearby lymph nodes (tiny bean-shaped organs throughout the body that help fight infections), nerves, or blood vessels. If the cancer reaches these lymph nodes or the bloodstream, it can also spread to other lymph nodes or other organs, such as the lungs or bones. These are called metastases.
When cancer spreads from its original place to another part of the body, the new tumor has the same kind of abnormal cells and the same name as the original tumor. For example, if thyroid cancer spreads to the lungs, the cancer cells in the lungs are thyroid cancer cells. The disease is metastatic thyroid cancer, not lung cancer, and it is treated as thyroid cancer.
Types of Thyroid Cancer
There are several types of thyroid cancer. The most common types are papillary carcinoma and follicular carcinoma. Some doctors call both papillary and follicular carcinoma “differentiated thyroid cancers” or “well-differentiated thyroid cancers.” Differentiated thyroid cancers tend to grow slowly and most can be treated successfully if they are found early.
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Papillary carcinoma (papillary cancer or papillary adenocarcinoma) - Papillary carcinomas start in the thyroid follicle cells and usually grow very slowly. In most cases, they start in only one lobe of the thyroid gland, but in rare cases both lobes are involved. Several different variants (subtypes) of papillary carcinoma can be identified under the microscope:
- Follicular variant (most common variant)
- Tall cell variant
- Columnar cell variant
- Diffuse sclerosing variant
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Follicular carcinoma is the next most common type of thyroid cancer, but it is much less common than papillary thyroid cancer, accounting for about 10 percent of thyroid cancers. It is more common in countries where people do not eat enough iodine. These cancers start in the thyroid follicle cells and stay in the thyroid gland. However, they can spread to other parts of the body, such as the lungs and bone.
- Hürthle cell carcinoma is a subtype of follicular cancer that accounts for about 4 percent of thyroid cancers.
Other types of thyroid cancer are less common:
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Medullary thyroid carcinoma (MTC) - MTCs account for about 5 percent to 10 percent of thyroid cancers and are the only ones that start in the C cells of the thyroid gland. Sometimes these cancers can spread to lymph nodes, the lungs, or liver even before a thyroid nodule is discovered or a screening test is done. MTCs are easier to control if they are found and treated before they spread to other parts of the body. These cancers usually make calcitonin (which helps control the amount of calcium in the blood) and carcinoembryonic antigen (CEA, a protein produced by certain cancers). Both calcitonin and CEA are released into the blood and can be found by blood tests.
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Sporadic - About 85 percent of cases of MTC are sporadic, which means that they are not inherited and do not run in families. Sporadic MTC usually occurs in older adults and in only one thyroid lobe.
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Familial (isolated familial medullary thyroid carcinoma) - Familial MTCs are inherited and are usually found in people who are in their 40s and 50s. Unlike multiple endocrine neoplasia—type II (MEN 2) cancer, where tumors are present not only in the thyroid gland but in other endocrine glands as well—familial MTC affects only the thyroid gland.
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Multiple endocrine neoplasia, type II (MEN 2) - MEN 2 medullary cancers are passed from one generation to the next and usually start much earlier in life than other thyroid cancers. People with MEN 2 have thyroid tumors and tumors in other endocrine glands, such as the adrenal or parathyroid glands. There are two types of MEN 2:
- MEN 2a - People with MEN 2a also have adrenal gland tumors (pheochromocytomas) and parathyroid gland tumors that cause high blood calcium levels. The parathyroid glands produce a hormone that helps the levels of calcium and phosphorus in the body.
- MEN 2b - People with MEN 2b also have pheochromocytomas but not parathyroid gland disease. Instead, they typically have benign growths of nerve tissues (neuromas) on the tongue and in other parts of the body. They also have thick lips and eyelids. These cancers often start during childhood or early adulthood and can spread early.
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Anaplastic carcinoma (undifferentiated thyroid cancer) - Only about 1 percent to 2 percent of thyroid cancers are anaplastic carcinomas, which start in the thyroid follicle cells. Scientists believe that this type of cancer starts in an existing papillary or follicular cancer. Anaplastic carcinoma is an aggressive cancer that rapidly spreads throughout the neck and often to other parts of the body.
Key Statistics
According to the American Cancer Society, of the 33,500 new cases of thyroid cancer expected in the United States in 2007, about 25,480 will occur in women and 8,070 in men.
Thyroid cancer is one of the least deadly cancers. The 5-year survival for all cases is nearly 97 percent. Thyroid cancer is also different from other kinds of cancer in that it mainly affects younger people. Nearly two-thirds of people are diagnosed between the ages of 20 and 55.
This content has been reviewed and approved by Myo Thant, MD.