A form of immunoglobulin G (IgG) that can bind to thyrotropin (TSH) receptors on the thyroid gland. TSIs mimic the action of TSH, causing excess secretion of thyroxine and triiodothyronine. The TSI level is abnormally high in persons with hyperthyroidism due to Graves' disease. 

Graves' disease is a classic form of hyperthyroid disease, affecting approximately 0.4% of the population of the United States. It is caused by IgG immunoglobulins, collectively known as thyroid stimulating immunoglobulins (TSI). 

TSIs can bind to tissues in the eyeballs and beneath the skin and contribute to the development of exophthalmos (bulging eyes) and pretibial myxedema (skin thickening on the front of the lower legs). Graves’ disease is a type of autoimmune problem that causes the thyroid gland to produce too much thyroid hormone, which is called hyperthyroidism. Graves’ disease is often the underlying cause of hyperthyroidism.

In Graves’ disease, your immune system creates antibodies that cause the thyroid to grow and make more thyroid hormone than your body needs. These antibodies are called thyroid-stimulating immunoglobulins (TSIs). The TSIs bind to thyroid cell receptors, which are normally “docking stations” for thyroid-stimulating hormone (TSH—the hormone responsible for telling the thyroid to produce hormones). The TSIs then trick the thyroid into growing and producing too much thyroid hormone, leading to hyperthyroidism.