Thyroid stimulating immunoglobulin (TSI): 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.
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