A 32-year-old woman presents to the clinic for evaluation of symptoms of heat intolerance, palpitations, diarrhea, weakness, and 10 lb weight loss. The symptoms started gradually but are worse over the past 1 week. She has no other medical history and is not taking any medications. Her family history is negative for thyroid diseases.On physical examination, her blood pressure is 90/60 mmHg, heart rate is 110/min, and she has a fine tremor in her hands. The TSH level is suppressed and T3 and T4 are elevated. Which of the following is most likely to precipitate this condition?