Thyroid Cancer Risk Factors

Although the etiology of thyroid cancer is not well understood, there are some exposures which are known to put an individual at higher risk of developing the disease. As with many types of cancer, family history of thyroid cancer is a known risk factor. A second was discovered in the 1960s, when researchers found that exposure of the head and neck to radiation increases the risk of developing thyroid cancer. There is some evidence that iodine may play a role in the prevention of aggressive thyroid cancer. It can also cut the risk of exposure to radiation from nuclear accidents if it is given in advance of the exposure.

Radiation Exposure

In the first half of this century before antibiotics became available, radiation therapy was used to treat several benign diseases, such as acne, scalp ringworm, enlarged tonsils, enlarged thymus, enlarged lymph nodes in the neck as a result of tuberculosis, whooping cough and keloid scars. In fact, over one million young Americans received x-ray treatments to the head or neck between 1920 and 1960. At the time, it was not known that the long term effects of this radiation therapy to the head and neck area would cause thyroid cancer, usually a papillary variant of thyroid cancer. When the association between radiation treatment and thyroid cancer was established, the use of this type of therapy was discontinued. In addition to radiation treatment, exposure to radiation as a result of nuclear weapons or nuclear plant accidents, such as the survivors of Hiroshima, Nagasaki and Chernobyl, also increases the risk for developing thyroid cancer. It is important to note that cancer usually develops very slowly, often several years after the initial exposure. If you have been exposed to radiation, inform your doctor and be sure that your thyroid is examined on a yearly basis.

In addition to increasing risk of thyroid cancer, x-ray treatment particularly for malignant conditions, such as Hodgkin's disease and throat cancer, may also be associated with the development of benign thyroid nodules and hypothyroidism. It is often difficult to distinguish between a benign nodule and a cancerous one because they often appear identical upon initial neck examination; therefore it is important that a nodule in exposed persons be biopsied in order to determine its pathology. In addition, people who were exposed to radiation are more likely to develop thyroid nodules in general. Fortunately, thyroid cancers usually have a very slow growth rate, which allows patients time to consider their treatment alternatives. If a nodule is diagnosed as cancerous, complete cure is often achieved for both exposed and unexposed patients.

Genetic Factors

Family history is a second known risk factor for developing the disease. About 5% of patients who develop papillary thyroid cancer and 20-25% of patients who develop medullary thyroid cancer have a relative who also had thyroid cancer. Medullary thyroid cancer (MTC) is related to a disorder called Multiple Endocrine Neoplasia type 2a (MEN 2a), and is usually the result of a specific genetic mutation (RET proto-oncogene). It is important that patients with MTC, as well as their families be tested for this genetic abnormality (with a simple blood test) to see if they are susceptible to the development of the disease. See types of thyroid cancer for more information.

Types of Cancer

The incidence of thyroid cancer is increasing faster than any other cancer in the United States, and there are approximately 30,000 new cases each year. As terrifying as the thought of cancer is, most thyroid cancers are very treatable and can usually be completely cured with surgery and appropriate therapy. In fact, there are only about 1000 deaths from thyroid cancer per year in the United States, which is less than 1% of all cancer deaths.

There are four major types of thyroid cancer 1) papillary, 2) follicular, 3) medullary and 4) anaplastic. The degree of malignancy of the different types of thyroid cancer is often categorized according to their "differentiation." The more differentiated the thyroid cancer is, the more it resembles the normal thyroid tissue, and therefore the more treatable it is (i.e. the less malignant it is). Papillary and follicular cancers are referred to as "well differentiated" thyroid cancers. These cancers account for about 90% of thyroid cancers and are usually associated with the best outcomes mainly because they tend to grow very slowly particularly in young patients (see the separate sections on papillary and follicular thyroid cancers for additional details). Anaplastic cancer is often called "undifferentiated" because it least resembles the normal thyroid tissue. It is a very rare type of thyroid cancer which is very aggressive and is associated with a poor prognosis (see section on anaplastic thyroid cancer). Thyroid lymphoma is also very uncommon but can be confused with anaplastic cancer because it tends to grow rapidly and cause symptoms like difficulty breathing or swallowing. Fortunately, thyroid lymphoma is very treatable and therefore must be differentiated from anaplastic cancer. It is important to note that medullary thyroid cancer is not classified according to its differentiation because it does not arise from thyroid cells, but rather from "C" cells which are neuroendocrine cells within the thyroid (see section on medullary thyroid cancer). When performed properly, thyroid biopsies are quite accurate at detecting thyroid cancer, except for well differentiated follicular cancer tumors, which can be confused with benign tumors due to the ambiguous nature of the cancer.

While the degree of malignancy is a very important factor for determining the prognosis of the individual, other important factors include the size of the tumor, spread of the tumor into the adjacent structures in the neck, the spread or metastasis of the tumor to other areas of the body, and most importantly, the age of the patient. Various methods have been established to quantify these factors and determine an approximate prognosis for a person with thyroid cancer depending on the specific features of their tumor (see section on prognosis staging).

Below is an overview of the four main types of thyroid cancer, including basic symptoms, diagnostic and treatment methods. Go to the separate sections on the specific types for further details on these types of thyroid cancers.