Exophthalmos is a feature seen in about 40% of individuals with Graves' disease. The hyperfunctioning thyroid gland leads to an increased T4 level, with positive feedback from the pituitary to decrease thyroid-stimulating hormone (TSH) secretion. There is about 50% concordance of Graves' disease among identical twins. The autoimmune character of this disorder is evidenced by an association with HLA-DR3 and by the presence of an autoantibody against TSH receptor that activates T4 secretion. An increased thyrotropin-releasing hormone (TRH) level would increase the TSH level and increase the T4 , but feedback typically occurs at the level of the pituitary and the hypothalamus, and abnormal increases in TRH are uncommon. Anti-thyroid peroxidase antibodies can be seen in Hashimoto thyroiditis and Graves' disease, but the highest titers occur in Hashimoto thyroiditis. T3 levels are less likely to be elevated than T4 levels.