The thyroid gland is an important part of the body's endocrine system. Located in the front of the neck just below the thyroid cartilage (Adam's apple), the thyroid produces hormones which are important for regulating metabolism, stimulating protein synthesis, increasing the heart rate and force of contraction, and assisting in the production of red blood cells and of Vitamin A.  Dysfunction of the thyroid gland can result in the under- or over-production of thyroid hormones. Those with hypothyroidism may complain of such symptoms as weight gain, cold intolerance, depression, fatigue, and difficulty sleeping and concentrating. Those with hyperthyroidism may complain of weight loss, heat intolerance, nervousness, jitteriness, irritability, and difficulty concentrating. Physically, thyroid dysfunction may manifest itself through changes in the skin, hair and nails, changes in weight, goiter (an enlarged thyroid), and--rarely--protrusion of the eyes called exophthalmos. The most common causes of thyroid dysfunction are autoimmune, in which a person's own antibodies attack the thyroid directly (Hashimoto's thyroiditis) or stimulate thyroid hormone receptors to over-produce thyroid hormones (Grave's disease). If a patient is suspected of having thyroid dysfunction, a simple blood test can measure the level of thyroid hormones and look for the presence of antibodies. Treatment can then be started based on these findings and may include medical treatment alone or in combination with either radiation or surgery. In many cases, it is necessary to take daily medication and have periodic thyroid hormone level checks for the remainder of one's life. Rarely, a malignant tumor can arise within the thyroid gland. Thyroid cancer accounts for about 1% of all cancers. In 1997, the estimated number of new cases of thyroid cancer was 16,100 with females more than twice as likely to be affected as males. (Skin, lung, prostate, breast, and colon cancers are far more common.) The greatest risk factors for thyroid cancer include a history of radiation therapy to the neck and a family history of thyroid cancer. Patients may notice changes in their voices or difficulty swallowing but more commonly have no symptoms at all. Usually, a lump or mass is discovered in the neck either by the patient or by the physician on routine physical exam. Several tests are available to evaluate a suspected thyroid mass. These include ultrasound, fine needle aspiration biopsy, a radionuclide scan, and a blood test to measure the level of circulating thyroid hormones and antibodies. The test of choice for the diagnosis of a thyroid mass is the fine needle aspiration biopsy (FNA). In this procedure, a thin needle is inserted into the mass to draw out a tissue sample. The sample is then examined under a microscope to see if it contains cancer cells. There are five major types of thyroid cancer. The most common type, papillary carcinoma, accounts for about 80% of all thyroid cancers. It also has the best prognosis with a ten-year survival rate of 95%. The other types of thyroid cancers include follicular (~10%), medullary (~5%), Hurthle cell (~4%), and anaplastic (~1%) carcinomas. overall the ten-year survival rates for these others cancers ranges from very good to poor depending on how advanced they are at the time of diagnosis. If the sample is shown to be cancerous, it is necessary to remove the mass surgically. If there is suspicion that the cancer has spread to other areas of the neck, it is also necessary to dissect the involved areas to examine the lymph nodes. All or part of the thyroid gland may have to be removed depending on the size of the mass. If all of the gland is removed, the patient will need to take thyroid hormone replacement medication for the rest of one's life. Thyroid surgery is done under general anesthesia and is usually performed on an inpatient basis. The surgery usually takes less than three hours. The surgery is very safe and complications are rare. Greatest care is taken to avoid injury to the laryngeal nerves which run near the thyroid gland. These nerves supply the vocal cords and damage to the nerves can cause permanent hoarseness. Care is also taken to avoid injury to or resection of the parathyroid glands which sit just behind the thyroid gland. These glands regulate blood calcium levels. If their blood supply is compromised, the parathyroid glands can be transplanted to other areas in the body and they will continue to function normally. Following surgery, it is often necessary to have follow-up studies performed to ensure that the cancerous growth has been completely removed. Radio-labeled iodine is injected into the blood and a scanner measures the amount of uptake by the thyroid gland. If there is evidence of residual cancer growth or metastasis, then radioactive iodine therapy or external beam radiation can be used on the affected areas. Overall, the survival rate for thyroid cancer is very good, with 94% of patients living beyond five years. More information on thyroid cancer can be obtained by writing to the American Cancer Society or calling toll-free (800) ACS-2345, or by connecting to http://endocrineweb.com.