Grave's disease is the most common cause of-

Graves' disease is the most common cause of hypehyroidism (76%) followed by Toxic multinodular goitre (14%) and Toxic solitary thyroid adenoma (5%). The thyrotoxicosis in Graves's disease results from the production of IgG antibodies directed against the TSH receptors on the thyroid follicular cells which stimulate thyroid hormone production and proliferation of follicular cells. These antibodies are known as thyroid stimulating immunoglobulins or TSH receptor antibodies(TRAb). Reference : page 740 and 747 Davidson's Principles and practice of Medicine 22nd edition