The thyroid gland is located low in the front of the neck, below the Adam's apple. The gland is shaped like a butterfly and wraps around the windpipe or trachea.
The two wings or lobes on either side of the windpipe are joined together by a bridge, called the isthmus, which crosses over the front of the windpipe.
Thyroid cancer accounts for nearly 2% of all new cancers diagnosed in the United States every year, according to the U.S. Surveillance, Epidemiology, and End Results cancer registry (SEER) predictions. Over the last 10 years, thyroid cancer has had a larger increase in incidence than any other type of cancer.
Thyroid cancer occurs in all age groups, although its incidence increases with age, especially after 30 years of age. More aggressive forms of thyroid cancer are found in older patients. Thyroid cancer occurs three times more frequently in women than in men. Thyroid cancer originates from one of two different types of thyroid cells: follicular cells or so-called parafollicular, or C cells.
There are four major types of thyroid cancers, listed below in order of decreasing frequency:
Papillary (includes follicular variant papillary thyroid carcinoma)
Papillary thyroid cancer (PTC) is the most common type of thyroid cancer and accounts for more than two-thirds of all thyroid cancers. There is a higher risk of developing this tumor in persons who have had previous head and neckradiation.
Most patients will not die from papillary thyroid cancer.
They are considered low risk if:
- They have small tumors.
- There is no invasion of surrounding structures and no metastasis (distant spread).
The spread of papillary thyroid cancer to lymph nodes may indicate recurrence, but it is not associated with a higher chance of death.
Follicular variant papillary thyroid cancer is a type of papillary thyroid cancer that has a survival rate similar to that of papillary thyroid cancer. Overall, papillary thyroid cancer is associated with a high survival rate.
Follicular (includes Hurthle cell and insular carcinoma)
Follicular thyroid cancer (FTC) occurs more in older patients compared to papillary thyroid cancer. The diagnosis of "malignancy" depends on the spread to local tissue and blood vessels. Like papillary thyroid cancer, the patient's age, size of tumor, and the extent that the tumor has spread can predict severity of disease.
Like papillary cancer, follicular cancer develops from the follicular cells and tends to grow slowly.
Variants of follicular thyroid cancer include insular carcinoma and Hurthle cell carcinoma of the thyroid.
Medullary carcinoma of the thyroid originates from the thyroid parafollicular, or C cells. C cells produce a hormone called calcitonin, which can be measured and used as a marker of medullary carcinoma. Medullary carcinoma can occur "sporadically" with no association, with other endocrine diseases, or may have agenetic basis when associated with familial medullary carcinoma or the multiple endocrine neoplasia syndromes (MEN).
Multiple endocrine neoplasia syndromes are a group of endocrine diseases that result from an inherited gene mutation. With multiple endocrine neoplasia syndromes, the adrenal glands, the parathyroid glands, and the surface of the mouth may be affected in addition to the thyroid.
Medullary carcinoma can include multiple tumors in both lobes of the thyroid and frequently spreads to local lymph nodes.
Anaplastic thyroid cancer
Anaplastic thyroid cancer is a rare and fast-growing variant.
Some genetic mutations are related to some thyroid cancers. Damage to DNA can cause these gene mutations due to changes that occur during the natural aging process, radiation exposure, or radiation treatments (as used in the past for skin conditions and head and neck conditions).
Anaplastic thyroid cancer typically occurs in older patients and accounts for less than 5% of all thyroid cancers. One-fifth of patients may have a current history of another cancer, including a more common form of thyroid cancer. Anaplastic cancer is the most aggressive thyroid cancer. Local and distant spreading occurs rapidly to other sites, including lymph nodes and the lungs.
Thyroid cancer typically presents as a thyroid nodule, or lump, that sometimes can be felt in the front of the throat. Most thyroid nodules arebenign; less than 5% are cancerous.
A thyroid nodule larger than 1 cm that is found to have decreased iodine uptake on a nuclear medicine thyroid scan needs to be evaluated with a fine needle aspiration biopsy.
Early thyroid cancer often does not cause symptoms.
But as the cancer grows, symptoms may include:
- A lump, or nodule, in the front of the neck near the Adam's apple;
- Hoarseness or difficulty speaking in a normal voice;
- Swollen lymph nodes, especially in the neck;
- Difficulty swallowing or breathing;
- Pain in the throat or neck.
These symptoms are not sure signs of thyroid cancer. An infection, a benign goiter, or another problem also could cause these symptoms. Anyone with these symptoms should see a doctor as soon as possible. Only a doctor can diagnose and treat the problem.
No one knows the exact causes of thyroid cancer. Doctors can seldom explain why one person gets this disease and another does not. However, it is clear that thyroid cancer is not contagious. No one can "catch" cancer from another person.
Research has shown that people with certain risk factors are more likely than others to develop thyroid cancer. A risk factor is anything that increases a person's chance of developing a disease.
The following risk factors are associated with an increased chance of developing thyroid cancer:
- Radiation. People exposed to high levels of radiation are much more likely than others to develop papillary or follicular thyroid cancer. One important source of radiation exposure is treatment with x-rays. Between the 1920s and the 1950s, doctors used high-dose x-rays to treat children who had enlarged tonsils, acne, and other problems affecting the head and neck. Later, scientists found that some people who had received this kind of treatment developed thyroid cancer. (Routine diagnostic x-rays—such as dental x-rays orchest x-rays—use very small doses of radiation. Their benefits nearly always outweigh their risks. However, repeated exposure could be harmful, so it is a good idea for people to talk with their dentist and doctor about the need for each x-ray and to ask about the use of shields to protect other parts of the body). Another source of radiation is radioactive fallout. This includes fallout from atomic weapons testing (such as the testing in the United States and elsewhere in the world, mainly in the 1950s and 1960s), nuclear power plant accidents (such as the Chornobyl [also called Chernobyl] accident in 1986), and releases from atomic weapons production plants (such as the Hanford facility in Washington state in the late 1940s). Such radioactive fallout contains radioactive iodine (I-131). People who were exposed to one or more sources of I-131, especially if they were children at the time of their exposure, may have an increased risk for thyroid diseases. People who are concerned about their exposure to radiation from medical treatments or radioactive fallout may wish to ask the Cancer Information Service at 1-800-4-CANCER about additional sources of information.
- Family history. Medullary thyroid cancer can be caused by a change, or alteration, in a gene called RET. The altered RET gene can be passed from parent to child. Nearly everyone with the altered RET gene will develop medullary thyroid cancer. A blood test can detect an altered RET gene. If the abnormal gene is found in a person with medullary thyroid cancer, the doctor may suggest that family members be tested. For those found to carry the altered RET gene, the doctor may recommend frequent lab tests or surgery to remove the thyroid before cancer develops. When medullary thyroid cancer runs in a family, the doctor may call this "familial medullary thyroid cancer" or "multiple endocrine neoplasia (MEN) syndrome". People with the MEN syndrome tend to develop certain other types of cancer. A small number of people with a family history of goiter or certain precancerous polyps in the colon are at risk for developing papillary thyroid cancer.
- Being female. In the United States, women are two to three times more likely than men to develop thyroid cancer.
- Age. Most patients with thyroid cancer are more than 40 years old. People with anaplastic thyroid cancer are usually more than 65 years old.
- Race. In the United States, white people are more likely than African Americans to be diagnosed with thyroid cancer.
- Not enough iodine in the diet. The thyroid needs iodine to make thyroid hormone. In the United States, iodine is added to salt to protect people from thyroid problems. Thyroid cancer seems to be less common in the United States than in countries where iodine is not part of the diet.
Most people who have known risk factors do not get thyroid cancer. On the other hand, many who do get the disease have none of these risk factors. People who think they may be at risk for thyroid cancer should discuss this concern with their doctor. The doctor may suggest ways to reduce the risk and can plan an appropriate schedule for checkups.
Surgery to remove all cancer in the neck and any cancerous lymph nodes is the initial therapy for most thyroid cancers. Complications are rare when the procedure is performed by an experienced thyroid surgeon.
Radioactive Iodine using I-131 is typically used as a follow-up to surgery, or "adjuvant" treatment in papillary and follicular thyroid cancers. This treatment is usually given two to six weeks following thyroid surgery. It involves giving high doses of I-131 in a liquid or pill form.
Patients undergoing this treatment must restrict their dietary intake of iodine for approximately five to14 days before the treatment and must restrict their contact with children and pregnant women for three to seven days after treatment. The goals of this treatment include destruction of any remaining thyroid tissue in the neck, a reduction in cancer recurrence rate, and improved survival.
Radiation treatment, known as external-beam radiation therapy, is used in patients with cancer that cannot be treated with surgery or is unresponsive to radioactive iodine, as well as for older patients with cancer that has distant spread. Radiation is sometimes combined with chemotherapy.
Chemotherapy is sometimes useful for progressive diseases unresponsive to radioactive iodine or radiation.
Treatments for the four thyroid cancer types
- Papillary thyroid cancer responds to treatment with surgery and radioactive iodine.
- Follicular thyroid cancer responds to treatment with surgery and radioactive iodine treatment.
- Medullary thyroid cancer must be treated with surgical removal of the entire thyroid gland in addition to complete removal of all neck lymph nodes and fatty tissue. This type of cancer does not respond to radioactive iodine therapy and has a much lower cure rate than either papillary or follicular thyroid cancer. After surgery, patients should be followed every six to 12 months with a blood calcitonin level to watch for recurrence.
- Anaplastic thyroid cancer often cannot be cured with surgery by the time of diagnosis (due to spread of the disease). This cancer is not responsive to radioactive iodine and may require radiation and chemotherapy.