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Lymphocytic and Hashimoto thyroiditis represent different phases of autoimmune thyroiditis. The initiating factor may be a thyroid-specific defect in suppressor T lymphocytes causing a breakdown in tolerance to self thyroid auto-antigens. There is cellular and antibody-mediated (humoral) immune injury to the gland. The predominant histologic finding is infiltration of the parenchyma by lymphocytes and plasma cells with formation of germinal centers.