A 32-year-old woman presents to the emergency department with palpitations, tremor, and anxiety. She reports a 3-month history of heat intolerance and weight loss despite good appetite. On examination, she is tachycardic (HR 110/min), has a fine tremor, and exophthalmos. Thyroid function tests show TSH <0.01 mIU/L and free T4 150 pmol/L (normal 10–20). A thyroid scan confirms diffuse uptake consistent with Graves' disease. Which of the following best explains the mechanism by which thyroid hormones cause tachycardia and tremor in this patient?