A 16-year-old woman presented with clinical manifestations of hyperthyroidism several weeks after receiving her third hepatitis B immunization. An endocrine assessment revealed persistently elevated free triodothyronine (FT3) levels and multiple small nodules on thyroid ultrasound. A MRI of the brain, TSH response to thyrotropin releasing hormone, blood levels of TSH, T3, T4, FT4, and assays for thyroid autoantibodies were normal. She responded to Sertraline, which blocks the conversion of T4 to T3 and FT3 in peripheral tissues. She was shown to be genetically prone to developing autoimmune disease, raising the prospect that her illness was caused by pituitary thyrotroph T3 receptor autoantibody produced in response to hepatitis B immunization.