The thyroid gland is a small, butterfly-shaped organ located in the front of the throat. It is responsible for releasing hormones which control your body's metabolism. When a person's thyroid produces too much hormone, the result is hyperthyroidism; too little thyroid hormone causes hypothyroidism. Both disorders can cause uncomfortable symptoms and if untreated are potentially dangerous. A person whose thyroid is underactive may notice signs such as fatigue, constipation, sensitivity to cold, elevated blood cholesterol, heavy menstrual periods, dry skin and depression.
Autoimmune Disease
The most common type of hypothyroidism is an inflammatory condition called Hashimoto's thyroiditis, according to the University of Maryland Medical Center website. Hashimoto's disease is an autoimmune disorder, in which a person's immune system attacks healthy cells of the thyroid gland. Women are more likely than men to develop the condition. Doctors are not sure what causes the disease, but both genetic and environmental factors appear to play a role in its development. Hypothyroidism can also result from Riedel's thyroiditis, a rare autoimmune disorder, or thyroid inflammation triggered by pregnancy.
Radiation Therapy
People who suffer from hyperthyroidism, in which the thyroid is overactive, are sometimes treated with either anti-thyroid drugs or radioactive iodine to return thyroid activity to normal, according to the Mayo Clinic website. Sometimes, however, the treatment may result in permanent hypothyroidism. Cancer patients who undergo radiation in the head or neck area can also become hypothyroid as a result of their treatment, sometimes years later.
Thyroid Surgery
Surgical removal of the thyroid gland means that your body can no longer create its own thyroid hormone, and necessitates supplementation with synthetic hormone for the rest of your life. A person who has had only part of his thyroid removed may produce enough thyroid hormone on his own, or he may eventually require medication to prevent hypothyroidism, according to the website EndocrineWeb.
Medications
Some medications can affect the thyroid, causing hypothyroidism. Lithium, used to treat certain psychiatric disorders, causes hypothyroidism in up to half of patients. Amiodarone, a drug used to treat abnormal heart rhythms, and phenytoin and carbamazepinecan, which may be given for epilepsy, can also cause hypothyroidism. Interferon drugs and some chemotherapy drugs can cause or worsen hypothyroid symptoms.
Less Common Causes
Less common causes of an underactive thyroid include congenital disease. One out of every 3,000 babies born in the U.S. each year has an improperly developed thyroid gland. Although uncommon, a person can also become hypothyroid because of a disorder of the pituitary gland, which is responsible for the function of the thyroid gland. Until relatively recently, a dietary deficiency of iodine -- a trace mineral necessary for healthy thyroid function -- was responsible for many cases of hypothyroidism. Iodine deficiency is now rare in the U.S. due to the use of iodized salt.
