source: Graves' Disease & Thyroid Foundation
Hyperthyroidism may be defined as the excess production of thyroid hormones. The most common cause is Graves’ disease. Less common causes include toxic adenoma of the thyroid, the hyperthyroid phase of thyroiditis, and hyperthyroidism due to hydatidiform moles or choriocarcinoma. Rare causes are pituitary thyroidstimulating hormone (TSH) excess, widespread functioning metastatic thyroid carcinoma, and excessive ingestion of thyroid hormone. We shall concern ourselves here primarily with the treatment directed to counteract the thyroidal overactivity.
This disease is considered to be an autoimmune disorder. The most likely possibility is that it results from a defect in immune regulation causing activation and expansion of clones of thyroid-directed lymphocytes (which normally would be suppressed) to survive, interact with an antigen on the thyroid cell membrane, and then produce an antibody (thyroid stimulating immunoglobin) against the TSH receptor. This antibody has the peculiar (if not unique) capacity to be able to attach to the TSH receptor (presumably by an antigen-antibody union), and then to stimulate the thyroid cells in a manner similar to that of TSH.