The thyroid is a gland located in your neck, just beneath the Adam's apple. It releases hormones - thyroxine (T4) and triiodothyronine (T3) - that increase the body's metabolic rate.
A metabolic rate is the rate of chemical processes occurring within the body that are necessary to maintain life. In metabolism, some substances are broken down to provide energy for vital processes while other substances are made.
The thyroid gland is self-regulated by thyroid-stimulating hormone (TSH), made by the pituitary gland (sometimes called the master gland) in the brain. TSH stimulates the thyroid gland to produce thyroid hormones (T4 and T3). When thyroid hormone levels in the body are high, TSH production will "switch off," stopping the thyroid from making more T4 and T3.
Hyperthyroidism occurs when the thyroid gland becomes overactive, making an excessive amount of thyroid hormones. It is much less common than hypothyroidism (underactive thyroid gland) and has a variety of causes.
Hyperthyroidism is 5 to 10 times more likely to occur in women than men. Risk factors for developing hyperthyroidism include having other autoimmune diseases (e.g., type 1 diabetes, pernicious anemia), a family history, eating large amounts of iodine, female gender, and previous goiter.
Thyroid overactivity sometimes results from inappropriately elevated TSH levels, or may be due to problems in the thyroid gland itself.
Graves' disease occurs when the immune system produces an abnormal antibody that acts like TSH to stimulate the entire thyroid gland. This leads to an overactive thyroid and higher levels of thyroid hormones. Graves' disease is the most common cause of hyperthyroidism, accounting for up to 80% of all hyperthyroid cases. The abnormal antibody may also stimulate inflammation in the eyes, and less commonly the skin.
Another form of hyperthyroidism is called toxic nodular goiter. Abnormal tissue or "nodules" in the thyroid (called adenomas) become autonomous; that is, they stop working under the control of TSH and are constantly "on" in terms of producing thyroid hormones.
When the thyroid gland becomes inflamed, you can develop a condition called thyroiditis. Inflammation causes the thyroid to release stored thyroid hormone for a brief period (usually no more than a few weeks). Long-term damage from inflammation will usually lead to hypothyroidism.
Hyperthyroidism can also be caused by taking too much thyroid hormone, or by medications such as amiodarone* that contain high amounts of iodine.
In rare cases, secondary hyperthyroidism is caused by a tumor of the pituitary gland making too much TSH, leading to constant stimulation of the thyroid gland. More rarely, the pituitary gland becomes insensitive to thyroid hormones, no longer knowing when levels are high and that it should "switch off" the production of TSH.