A 37-year-old woman has had difficulty swallowing and a feeling of fullness in the anterior neck for the past week. She is recovering from a mild upper respiratory tract infection 1 month ago. On physical examination, her temperature is 37.4degC, pulse is 74/min, respirations are 16/min, and blood pressure is 122/80 mm Hg. Palpation of her diffusely enlarged thyroid elicits pain. Laboratory studies show an increased serum T4 level and a decreased TSH level. Two months later, she no longer has these complaints. The T4 the level is now normal. Which of the following conditions is most likely to have produced these findings?