What Is Thyroid Cancer?
The thyroid gland is located 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 diagram).
The thyroid gland contains mainly 2 types of cells -- thyroid follicular cells and C cells (also called parafollicular cells).
The thyroid gland absorbs iodine from the blood and the follicular cells use it to produce thyroid hormone which important for many body functions. Thyroid hormone regulates a person's metabolism. Too much thyroid hormone (a condition called hyperthyroidism) causes a person to have a rapid or irregular heartbeat, be hyperactive, feel nervous, warm, hungry, and often, to lose weight. Too little hormone (called hypothyroidism) causes a person to slow down, feel tired, and gain weight. All this is regulated by the pituitary gland at the base of the brain, which produces a substance called thyroid-stimulating hormone (TSH).
C cells make calcitonin, which helps regulate the body's calcium metabolism.
Different cancers develop from each kind of cell. The differences are important because they determine the seriousness of the cancer and the type of treatment needed.
Many types of tumors can develop in the thyroid gland. Most of these tumors are benignmalignant (cancerous), which means they can spread into nearby tissues and to other parts of the body. (non-cancerous). Others are
Benign Thyroid Enlargement and Nodules
Because the thyroid 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. There are many reasons the thyroid gland might be larger than usual, and most of the time it is not cancer. The medical term for an abnormally large thyroid gland is 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 bumps in it. Diffuse and nodular goiters are usually caused by an imbalance in certain hormones. Not getting enough iodine in the diet can cause these hormonal problems and lead to a goiter, but there are many other reasons.
Lumps or bumps in the thyroid gland are called thyroid nodules. People can develop thyroid nodules at any age, but they are most common in older adults. Thyroid nodules can be either benign or malignant. About 4%-8% of adults have thyroid nodules that can be felt by a doctor. But when people are tested with an ultrasound of the thyroid anywhere from 10% to 40% of people have nodules. Most of these nodules, however, are too small to feel. Often these nodules are cysts filled with fluid or with a stored form of thyroid hormone called colloid. Colloid nodules are one of the most common types of thyroid nodule. Solid nodules have little fluid or colloid.
Some solid nodules may have too many cells, but the cells are not cancer cells. This type of nodule includes hyperplastic nodules and adenomas. Sometimes hyperplastic nodules or adenomas make too much thyroid hormone and cause hyperthyroidism.
Malignant Thyroid Tumors
Only 5% to10% of thyroid nodules are cancerous. There are several types of thyroid cancer, however, papillary carcinoma and follicular carcinoma are the most common. H�rthle cell carcinoma is a subtype of follicular carcinoma. Other types of thyroid cancer, including medullary carcinoma, anaplastic carcinoma, and thyroid lymphoma occur less often.
Differentiated Thyroid Cancers
In differentiated thyroid cancers, the cells appear similar to normal thyroid tissue. This is easily seen under the microscope. Papillary carcinoma: About 70%-80% of thyroid cancers are papillary carcinomas (also called papillary cancer or papillary adenocarcinoma). Papillary carcinomas develop from the thyroid follicle cells and typically grow very slowly. Usually they develop in only one lobe of the thyroid gland, but about 10% of the time they occur in both lobes. Several different variants (subtypes) of papillary carcinoma can be recognized under the microscope. These include the follicular variant, tall cell variant, columnar cell variant, and diffuse sclerosing variant. Of these variants, the follicular variant of papillary carcinoma occurs most often.
The usual form of papillary adenocarcinoma and the follicular variant have the same outlook for survival (prognosis), and treatment is the same for both. The other variants tend to spread more quickly and have a worse prognosis. Even though papillary cancer cells grow slowly, they often spread quickly to the lymph nodes in the neck. Most of the time, however, this can be successfully treated and is rarely fatal.
Follicular carcinoma: Follicular carcinoma is the next most common type of thyroid cancer. It is also sometimes called follicular cancer or follicular adenocarcinoma. Follicular cancer is much less common than papillary thyroid cancer, making up about 20% of thyroid cancers. It is more common in countries where people don�t get enough iodine in their diet. These cancers usually remain in the thyroid gland but some can spread to other parts of the body, such as lungs and bone. Unlike papillary carcinoma, follicular carcinomas spread to lymph nodes less often. The prognosis of follicular carcinoma is probably the same or slightly worse than that of papillary carcinoma.
H'rthle cell carcinoma, also known as oxyphil cell carcinoma, is thought to be a subtype of follicular cancer. This type accounts for about 4% of thyroid cancers. It may have a worse prognosis than typical follicular carcinoma because this subtype of follicular cancer does not absorb radioactive iodine well.
Other Types of Malignant Thyroid Tumors
Medullary thyroid carcinoma: Medullary thyroid carcinoma (MTC : about 3% of thyroid cancers) is the only thyroid cancer that develops from the C cells of the thyroid gland. Sometimes this cancer can spread to lymph nodes, the lungs, or liver even before a thyroid nodule is discovered or a screening test is done. These cancers usually make calcitonin and carcinoembryonic antigen (CEA). Calcitonin, a hormone also produced by normal C cells, helps control the amount of calcium in blood. CEA is a protein produced by certain cancers, such as colorectal cancer and MTC. Both calcitonin and CEA are released into the blood and can be found by blood tests. Because medullary cancer does not absorb or take up radioactive iodine (used for treatment and to find metastases) it has a worse prognosis than differentiated thyroid cancers.
There are 2 types of MTC. The first type, occurring in 85% of 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 thyroid lobe.
The other type of MTC is inherited and can occur in each generation of a family. When MTC is the only type of cancer found in the family, it is called isolated familial medullary thyroid carcinoma (FMTC). The combination of FMTC and tumors of certain other organs is called type 2 multiple endocrine neoplasia (MEN 2). Type 2 MEN has 2 subtypes, MEN 2a and MEN 2b:
- In MEN 2a, MTC occurs with adrenal gland tumors called pheochromocytomas and with parathyroid gland tumors that cause high calcium levels in the blood. The adrenal glands are found next to the upper part of each kidney. Most people have 4 parathyroid glands, 2 behind each lobe of the thyroid.
- In MEN 2b, MTC is associated with pheochromocytoma but not parathyroid gland disease. Instead, MEN 2b includes benign growths of nerve tissues on the tongue and elsewhere called neuromas. In these familial or genetic forms of MTC, the cancers often develop during childhood or early adulthood and can spread early. MTC is most aggressive in the MEN 2b syndrome.
Anaplastic carcinoma: Anaplastic carcinoma is an uncommon (about 2% of all thyroid cancers) form of thyroid cancer. It is believed to develop from an existing papillary or follicular cancer. It is an aggressive cancer that rapidly invades the neck, often spreads to other parts of the body, and is usually fatal. Anaplastic carcinoma is sometimes called undifferentiated thyroid cancer. Undifferentiated thyroid tumors have cells that do not look very much like normal thyroid tissue cells under the microscope.
Thyroid lymphoma: Lymphoma can develop in the thyroid gland but is very uncommon in that location. It does not develop from either thyroid follicular cells or C cells. Rather, lymphomas develop from lymphocytes, the main cell type of the immune system. Most lymphocytes are found in pea-sized collections scattered throughout the body called lymph nodes, and that is where most lymphomas begin. These types of lymphomas are discussed in the American Cancer Society document, Non-Hodgkin Lymphoma.
Parathyroid cancer: Behind, but attached to the thyroid gland are 4 tiny parathyroid glands. The parathyroid gland regulates the body's calcium. Cancers of the parathyroid glands are very rare. There are probably fewer than 100 cases each year in the United States. When they occur, the blood calcium level is almost always elevated. This causes a person to become fatigued, weak, and drowsy. It also blocks the ability of the kidneys to regulate the body's water content and excess water is lost through high volumes of urine. This further complicates the weakness and drowsiness by causing dehydration.
The parathyroid cancer may also be detected as a thyroid nodule if it grows too large. No matter how large the nodule is, the only treatment is to remove it surgically. Unfortunately, parathyroid cancer is more deadly than thyroid cancer and much harder to cure. This document only discusses thyroid cancer.
Do We Know What Causes Thyroid Cancer?
Although scientists have found that thyroid cancer is associated with a number of other conditions, the exact cause of most thyroid cancers is not yet known.
Researchers have made great progress in understanding how certain changes in a person's DNA can cause thyroid cells to become cancerous. DNA is the molecule that carries the instructions for nearly everything our cells do. We usually resemble our parents because they are the source of our DNA. However, DNA affects more than our outward appearance. It also can influence our risk for developing certain diseases, such as some kinds of cancer.
Some genes (parts of our DNA) contain instructions for controlling when our cells grow and divide. Genes that can be involved in cancer are called oncogenes. Such genes are often involved in DNA maintenance or repair. Others that slow down cell division or cause cells to die at the appropriate time are called tumor suppressor genes. DNA mutations (defects) that influence the function of these genes can cause cancers.
People inherit 2 copies of each gene -- one from each parent. People can inherit damaged DNA from one or both parents, which accounts for inherited cancers. Many times, though, a person's DNA is damaged by exposure to something in the environment, like smoking or radiation. Sometimes DNA mutates for no apparent reason.
The DNA mutations that cause some forms of papillary thyroid cancer are known to involve over-activation or specific parts of the RET gene. The altered form of this gene, known as the PTC oncogene is found in 10%-30% of papillary thyroid cancers overall, and in a larger percentage of papillary thyroid cancers occurring in children and/or associated with radiation exposure. These RET mutations usually are acquired during a person's lifetime rather than being inherited. They are present only in the cancer cell and are not passed on to the patient's children. Recently scientists have discovered that many (30%-70%) papillary thyroid cancers contain a mutation of the BRAF gene. The BRAF mutation is less common in thyroid cancers in children and in those thought to arise from exposure to radioactivity -- such as around Chernobyl. Both BRAF and RET/PTC changes are thought to cause cells to grow and divide. It is extremely rare for papillary cancers to have changes in both the BRAF and RET/PTC genes. Most cases have either one or the other affected gene, and those with BRAF changes tend to have more aggressive growth and a greater likelihood of spreading to other parts of the body.
Acquired changes in other oncogenes and tumor suppressor genes, such as ras and p53, also have a role in causing follicular anaplastic thyroid cancers, respectively.
Single point mutations in people who have medullary thyroid carcinoma (MTC) involve different parts of the RET gene compared to papillary carcinoma patients. Nearly all patients with the inherited form of MTC and about 1 of every 5 with the sporadic form of MTC have a mutation in the RET gene. Most patients with sporadic MTC have acquired mutations present only in their cancer cells. Those with familial MTC and MEN 2 inherit the RET mutation from a parent. These mutations are present in every cell of the patient's body and can be detected by testing the DNA of blood cells.
Because every person has 2 RET genes but passes only 1 to a child (the child's other RET gene comes from the other parent), the odds that a patient with familial MTC will pass a mutated gene to a child are 1 in 2 (or a 50% chance).
How Is Thyroid Cancer Treated?
This information represents the views of the doctors and nurses serving on the American Cancer Society's Cancer Information Database Editorial Board. These views are based on their interpretation of studies published in medical journals, as well as their own professional experience.
The treatment information in this document is not official policy of the Society and is not intended as medical advice to replace the expertise and judgment of your cancer care team. It is intended to help you and your family make informed decisions, together with your doctor.
Your doctor may have reasons for suggesting a treatment plan different from these general treatment options. Don't hesitate to ask him or her questions about your treatment options.
After thyroid cancer is found, your doctor will discuss treatment options or choices with you. It is important to take the time to consider each of them. In choosing a treatment plan, factors to consider include the type and stage of the cancer and your overall physical health. Sometimes it is a good idea to get a second opinion, and many experienced physicians encourage this. Some insurance companies even require a second opinion before they will agree to pay for certain treatments. A second opinion can provide more information and help you feel confident about the treatment plan you choose.
The methods of treatment for thyroid cancer include surgery, radioactive iodine treatment, thyroid hormone therapy, external beam radiation therapy, and chemotherapy. The best approach to treatment usually uses 2 or more of these methods, and most patients are cured of their thyroid cancer in this way.
If a cure is not possible, the goal may be to remove or destroy as much of the cancer as possible and to prevent the tumor from growing, spreading, or returning for as long as possible. Sometimes treatment is aimed at palliation (relieving symptoms, such as pain or problems with breathing and swallowing).