ABSTRACT
Antithymocyte globulin (ATG) is used as an immunosuppressive treatment (IST) to deplete clonal suppressor T cells in patients with severe aplastic anemia (SAA). The depletion of suppressor T cells by ATG may affect the activation of B cells, which results in an increased risk for autoimmune conditions. A 12-year-old boy was diagnosed with idiopathic SAA. As he did not have an human leukocyte antigen-matched sibling, he was treated with rabbit ATG (3.5 mg/kg/day for 5 days) and cyclosporine. Five months later, he became transfusion independent. However, 23 months after IST, he complained of mild hand tremors, sweating, weight loss, palpitations, and goiter. Results of thyroid function tests revealed hyperthyroidism (free thyroxine, 3.42 ng/dL; thyroid stimulating hormone [TSH], <0.01 nIU/mL; triiodothyronine, 3.99 ng/mL). Results of tests for autoantibodies were positive for the antimicrosome antibody and TSH-binding inhibitory immunoglobulin, but negative for the antithyroglobulin antibody and antinuclear antibody. He was treated with methimazole, and his symptoms improved. The patient has been disease free for 39 months after IST and 9 months after methimazole treatment. This case report suggests that although rare, rabbit ATG may have implications in the pathogenesis of autoimmune hyperthyroidism. Our findings suggest that thyroid function tests should be incorporated in the routine follow-up of SAA patients treated with ATG.