Hyperthyroidism has several causes, including
                
- Graves disease  - thyroid nodules  - thyroiditis, or inflammation of the thyroid  - consuming too much iodine  - overmedicating with synthetic thyroid hormone, which is used to treat underactive thyroid
                
Rarely, hyperthyroidism is caused by a pituitary adenoma, which is a noncancerous tumor of the pituitary gland. In this case, hyperthyroidism is due to too much TSH.
                
Graves Disease
                
Graves disease, also known as toxic diffuse goiter, is the most common cause of hyperthyroidism in the United States. Graves disease is an autoimmune disorder. Normally, the immune system protects the body from infection by identifying and destroying bacteria, viruses, and other potentially harmful foreign substances. But in autoimmune diseases, the immune system attacks the bodys own cells and organs.
                
With Graves disease, the immune system makes an antibody called thyroid stimulating immunoglobulin (TSI) that attaches to thyroid cells. TSI mimics the action of TSH and stimulates the thyroid to make too much thyroid hormone.
                
More information is provided in the NIDDK health topic, Graves' disease.
                
Thyroid Nodules
                
Thyroid nodules, also called adenomas, are lumps in the thyroid. Thyroid nodules are common and usually noncancerous. About 3 to 7 percent of the U.S. population has them.2 However, nodules may become overactive and produce too much hormone.
                
A single overactive nodule is called a toxic adenoma. Multiple overactive nodules are called toxic multinodular goiter. Often found in older adults, toxic multinodular goiter can produce a large amount of excess thyroid hormone.
                
Thyroiditis
                
Thyroiditis is an inflammation of the thyroid that causes stored thyroid hormone to leak out of the thyroid gland. At first, the leakage raises hormone levels in the blood, leading to hyperthyroidism that lasts for 1 or 2 months. Most people then develop hypothyroidismwhen thyroid hormone levels are too lowbefore the thyroid is completely healed.
                
Several types of thyroiditis can cause hyperthyroidism followed by hypothyroidism:
                
- Subacute thyroiditis. This condition involves painful inflammation and enlargement of the thyroid. Experts are not sure what causes subacute thyroiditis, but it may be related to a viral or bacterial infection. The condition usually goes away on its own in a few months.  - Postpartum thyroiditis. This type of thyroiditis develops after a woman gives birth. For more information, see the section titled What happens with pregnancy and thyroid conditions?  - Silent thyroiditis. This type of thyroiditis is called silent because it is painless, as is postpartum thyroiditis, even though the thyroid may be enlarged. Like postpartum thyroiditis, silent thyroiditis is probably an autoimmune condition and sometimes develops into permanent hypothyroidism.
                
Consuming Too Much Iodine
                
The thyroid uses iodine to make thyroid hormone, so the amount of iodine consumed influences the amount of thyroid hormone the thyroid makes. In some people, consuming large amounts of iodine may cause the thyroid to make excess thyroid hormone.
                
Sometimes significant amounts of iodine are contained in medicationssuch as amiodarone, which is used to treat heart problemsor in supplements containing seaweed. Some cough syrups also contain large amounts of iodine. See Eating, Diet, and Nutrition for more information on iodine.
                
Overmedicating with Synthetic Thyroid Hormone
                
Some people who take synthetic thyroid hormone for hypothyroidism may take too much. People who take synthetic thyroid hormone should see their health care provider at least once a year to have their thyroid hormone levels checked and follow the health care providers instructions about the dosage.
                
Some other medications may also interact with synthetic thyroid hormone to raise hormone levels in the blood. People who take synthetic thyroid hormone should ask their health care provider about interactions when starting new medications.