Cause

Graves' disease is the most common cause of hyperthyroidism. Graves' disease causes the thyroid gland to make too much thyroid hormone. Graves' disease, like many thyroid problems, often runs in families.

Other common causes include:

• Thyroid nodules. Thyroid nodules are abnormal growths in the thyroid gland that can make too much thyroid hormone. For more information, see the topic Thyroid Nodules.
• Thyroiditis.Thyroiditis occurs when your body makes antibodies that damage your thyroid gland. You can also get thyroiditis from a viral or bacterial infection. At first, thyroiditis may cause your thyroid levels to rise as hormone leaks out from the damaged gland. Later, levels may be low (hypothyroidism) until the gland repairs itself.

Uncommon causes of hyperthyroidism include tumors or eating foods or taking medicines that contain large amounts of iodine.