USMLE Step 1 Practice Test 1

A 34-year-old woman comes to the physician because of a 3-month history of palpitations, heat intolerance, and a 5-kg (11-lb) weight loss despite an increased appetite. Temperature is 37.4°C (99.3°F), pulse is 112/min, respirations are 16/min, and blood pressure is 138/62 mm Hg. Examination shows bilateral proptosis with lid retraction, a diffusely enlarged, nontender thyroid gland with a bruit, and nonpitting, waxy thickening of the skin over both shins. Laboratory studies are shown. Which of the following is the most likely mechanism of this patient's hyperthyroidism?