Thyroid-stimulating immunoglobulins, or TSI's, are IgG antibodies 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.
Which of the following is an autoantibody that binds to TSH receptor sites on thyroid cell membranes preventing thyroid-stimulating hormone binding?
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