Hyperthyroidism occurs when the thyroid gland releases too much of the hormone thyroxine (T4). Many disorders are associated with this malfunction of hormone production. These disorders include:

• Graves' disease. The leading cause of hyperthyroidism, accounting for about 70 percent of all cases, according to the American Thyroid Association. Graves' disease is an autoimmune disorder, which means that the body's immune system sends antibodies that stimulate the thyroid to produce excess thyroxine. These antibodies also may attack the tissues behind the eyes and the skin on the lower legs over the shins. The cause of Graves' disease remains unknown, although a genetic predisposition is suspected.
  
  
• Hyperfunctioning thyroid nodules. Occurs when certain areas of the thyroid gland separate from the rest of the gland and produce masses, which are generally noncancerous, called adenomas. The adenomas may be big enough to enlarge the thyroid. They may sometimes produce excess thyroxine.
  
  
• Thyroiditis. Inflammation and temporary destruction of the thyroid gland, which can allow excess hormones to leak into the bloodstream. It can result from a viral infection or problem with the immune system. Thyroiditis may cause hyperthyroidism or hypothyroidism (too little thyroid hormones). Major types include Hashimoto's thyroiditis, subacute granulomatous thyroiditis and silent lymphocytic thyroiditis. Hashimoto's thyroiditis is generally a condition of hypothyroidism but may have a hyperthyroid phase. Most forms of thyroiditis are painless.
  
  Pregnancy can result in postpartum thyroiditis. About 5 to 10 percent of new mothers experience short-term hyperthyroidism, often followed by a few months of hypothyroidism, according to the American Association of Clinical Endocrinologists. Most of these women regain normal thyroid function, but some develop chronic hypothyroidism requiring lifelong treatment.

People with autoimmune disorders, such as type 1 diabetes, have increased risk of Graves' disease, Hashimoto's thyroiditis and other autoimmune conditions.

Other factors that can lead to hyperthyroidism include:

• Ingestion of large amounts of thyroid hormone or iodine.
  
• Tumors of the testes or ovaries.
  
• Thyroid cancer (in rare cases).
  
• Exposure to radioactive iodine. Researchers studying children who were exposed to radiation at the Nevada Test Site in the mid-20th century have found an increased risk of autoimmune thyroiditis and thyroid tumors.

Hyperthyroidism can affect both sexes and all ages, but most often affects women between the ages of 20 and 40. It develops in less than 1 percent of Americans, according to the American Association of Clinical Endocrinologists.