Overview of Thyroid Cancer
Thyroid cancer is a disease in which cancerous (malignant) cells develop in thyroid gland tissue. The thyroid is a small, butterfly-shaped gland that produces hormones that regulate metabolism and affect body temperature, energy level, and heart rate. It is located just below the Adam's apple at the front of the throat and wraps around the windpipe (trachea).
Healthy function of the thyroid gland is dependent on the nutrient iodine, which is present in seafood, seaweed, and iodized salt. When the thyroid produces too much hormone (called hyperthyroidism), nervousness, excessive sweating, tremor, and weight loss can occur. When the thyroid produces too little hormone (called hypothyroidism), fatigue, depression, weight gain, and sleepiness may occur.
Within the thyroid gland, there are two types of cells: follicle cells and C cells. If one of these types of cells grows out of control, a lump in the thyroid (called a thyroid nodule) or thyroid cancer can result. About 90 percent of thyroid nodules are benign—not cancerous.
Types of Thyroid Cancer
There are several different types of thyroid cancer. The most common type, called papillary thyroid cancer, or papillary carcinoma, occurs in the follicle cells, accounts for about 80 percent of all thyroid cancers, and may spread (metastasize) to the lymph nodes. This type is slow growing and has the highest rate for successful treatment. In most cases, papillary carcinoma affects only one side of the thyroid gland.
Follicular thyroid cancer, or follicular carcinoma, also occurs in the follicle cells and is fairly slow growing. Unlike papillary carcinoma, this type rarely spreads to the lymph nodes. However, it can spread to the lungs and bone, and is more challenging to treat if this occurs. This type of thyroid cancer is more common in areas of the world where there is an inadequate amount of iodine in the diet (e.g., inland desert areas).
Medullary thyroid carcinoma, or medullary thyroid cancer, occurs in the C cells of the thyroid. This type, which accounts for less than 5 percent of all thyroid cancers, often can be successfully treated if it is found before it has spread to other parts of the body.
One sub-type of medullary carcinoma is hereditary, or runs in families. In families with this type of thyroid cancer, special blood chemistry tests and genetic tests are available to help evaluate risk.
Anaplastic thyroid cancer is the least common form, accounting for 1–2 percent of all cases. This type is more difficult to treat due to its rapid growth rate. Anaplastic thyroid cancer is twice as common in men as it is in women, has a much lower survival rate, and occurs mainly in people over age 65.
Incidence and Prevalence of Thyroid Cancer
In the United States, about 20,000–30,000 cases of thyroid cancer are diagnosed each year. Women are 2 to 3 times more likely to develop thyroid cancer than men, and the disease is most common in patients between the ages of 20 and 55 (except in the case of anaplastic thyroid cancer). In areas of the world where diets are low in iodine (e.g., inland desert areas), rates of papillary and follicular thyroid cancers are generally higher.
Causes and Risk Factors for Thyroid Cancer
In most cases, the cause for thyroid cancer is unknown. Risk factors include the following:
- Gender (except for anaplastic thyroid cancer, the disease is more common in women)
- Heredity (some types are genetic, that is, run in families)
- Inadequate amount of iodine (found in seafood, seaweed, iodized salt) in the diet
- Radiation exposure during childhood
Between the 1920s and 1950s, radiation was used in the treatment of several childhood medical conditions (e.g., recurrent tonsillitis, severe acne). This treatment increases the risk for developing thyroid cancer later in life. The use of low-dose radiation to treat Hodgkin's disease in children is also linked to an increased risk for thyroid cancer.
About 20 percent of medullary thyroid carcinoma cases are hereditary. Blood chemistry tests and genetic tests can help determine the risk in members of families in which this disease occurs.