The thyroid stimulating hormone receptor (TSHR) is expressed in the membrane of thyroid follicular cells. It is involved in regulating thyrocyte cell growth and function. TSHR is also the target autoantigen in thyroid autoimmune diseases. Autoantibodies to TSHR that act as agonists are responsible for the hyperthyroidism of Graves" Disease. Another class of autoantibodies that block the binding of TSH to TSHR may mediate the hypothyroidism associated with Hashimoto"s thyroiditis, primary myxoedema, and neonatal hypothyroidism.