Graves' disease

A 45-year-old woman comes to clinic because of a resting hand tremor for the past 2 days. She says she has dealt with palpitations, unintentional loss of 9.1-kg (20-lb), and feelings of increased irritability towards her coworkers over the past 3 months. Her medical, family, and social histories are noncontributory. Her temperature is 36.6°C (98°F), pulse is 110/min, respirations are 15/min, and blood pressure is 120/80 mm Hg. Physical examination shows that the patient appears anxious and diaphoretic. Her eyelids are retracted with evidence of exophthalmos and thyroid is enlarged with no palpable masses. She shows occasional tremors in both hands and has an erythematous, thickened rash on both shins. Laboratory tests show an undetectable amount of thyroid stimulating hormone and elevated concentrations of anti-thyroid peroxidase antibodies. ECG shows tachycardia, but is otherwise within normal limits. Which of the following is the most likely diagnosis?