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Types of Cancer > Head and Neck Cancers > Thyroid Cancer > Overview
Thyroid Cancer: The Basics
Ryan P. Smith, MD
Affiliation:
Abramson Cancer Center of the University of Pennsylvania
Last Modified: July 25, 2003
What is the Thyroid?
The thyroid gland is located on the anterior aspect (front) of the neck. It can be felt just below the thyroid cartilage, or "Adam's apple". It is a butterfly shaped organ that stretches across the midline of the neck just below the Adam's apple, with its "wings" spreading superiorly (towards the head) on either side of the Adam's apple. These "wings" are called the lobes of the thyroid, with the portion extending across midline called the isthmus. As an endocrine gland, the thyroid gland produces and secretes thyroid hormones into the bloodstream. The thyroid does this as a response to a hormone produced by the pituitary gland called thyroid stimulating hormone, or TSH. When the thyroid gland is "turned on" by TSH, it increases its uptake of iodine, which is required to make thyroid hormone, and manufactures and secretes thyroid hormones. These thyroid hormones are very important for many functions ranging from the regulation of our metabolic rate to helping to maintain the function of our heart to regulation of body temperature. An increase in the production (hyperthyroid) or a decrease in the production (hypothyroid) of thyroid hormone can be easily managed, though can cause serious problems if not properly treated. The principal cells of the thyroid are called follicular cells, and are mainly responsible for the production of thyroid hormone.
What is Thyroid cancer?
The definition of a tumor is a mass of abnormally growing cells. Tumors can be either benign or malignant. Benign tumors have uncontrolled cell growth, but without any invasion into normal tissues and without any spread. A malignant tumor is called cancer when these tumor cells gain the propensity to invade tissues and spread locally as well as to distant parts of the body. In this sense, thyroid cancer occurs when cells of the thyroid gland grow uncontrollably to form tumors that can invade the tissues of the neck, spread to the surrounding lymph nodes, or to the bloodstream and then to other parts of the body. The most common types of cancers of the thyroid gland are derived from the cells responsible for thyroid hormone production. The general term for cancers that come from glandular tissue is adenocarcinoma. In the thyroid, the most common types of cancer are papillary adenocarcinoma of the thyroid (75-80%) and follicular adenocarcinoma of the thyroid (˜15%). Papillary thyroid cancer takes on a folded appearance under the microscope, which eases its diagnosis. Follicular thyroid cancer may closely resemble normal thyroid tissue, but as a malignancy, has a propensity to divide uncontrollably and invade and spread. The next most common type of cancer of the thyroid is called medullary thyroid cancer (5%), which is derived from the parafollicular cells of the thyroid. This is often associated with a familial genetic predisposition to develop certain types of cancers (see below). The other major type of thyroid cancer often described is called anaplastic thyroid cancer (2%). This cancer usually affects older people and is very aggressive. Other types of cancers, such as lymphomas (cancer of the lymph gland cells), sarcomas (cancer of soft tissues such as muscle or cartilage cells), or metastases (cancers from other sites that have spread to the thyroid gland) are also seen in the thyroid gland.
Am I at risk for thyroid cancer?
Thyroid cancer is fairly common, as it is found at autopsy in approximately 5% of people with no known thyroid disease. However, death due to thyroid cancer is uncommon, explained by the fact that thyroid cancer is usually an indolent disease, tending to remain localized to the thyroid gland for many years. Most cases of thyroid cancer are sporadic; meaning there is no obvious predisposition or risk factor for development. However, it is more common in women, occurring in a 3:1 ratio. This has prompted studies into the investigation of estrogen as a possible risk factor for thyroid cancer, though this has never been proven. Studies have also shown a preponderance of certain types of thyroid cancer in regions with a high incidence of goiters (enlarged thyroid glands), which occur as a result of a lack of dietary iodine. This is further supported by the decrease of thyroid cancers in population given supplemental iodine.
The most firmly established risk factor for the development of thyroid cancer is exposure to ionizing radiation to the neck region. This is supported by the high incidence of thyroid cancer seen in many populations exposed to radiation. Notably, this includes patients with Graves' disease (a hyperthyroid condition) treated with radiation, Hodgkin's disease patients treated with radiation, survivors of atomic blasts at Nagasaki and Hiroshima, and survivors of the Chernobyl explosion. In fact, thyroid cancer is one of the most common cancers noted in populations exposed to large doses of radiation through accident or war.
A notable genetic predisposition is associated with medullary thyroid cancer, which is associated with a syndrome called multiple endocrine neoplasia (MEN) type 2 syndrome. Patients with MEN type 2 (Sippler's syndrome) have a strong familial history of medullary thyroid cancers and a type of adrenal cancer, called pheochromocytoma.