A 65-year-old woman with a history of multinodular goiter complains of increasing nervousness, insomnia, and heart palpitations. She has lost 9 kg (20 lb) over the past 6 months. Physical examination reveals a diffusely enlarged thyroid. There is no evidence of exophthalmos. Laboratory studies show elevated serum levels of T3 and T4. Serologic tests for antithyroid antibodies are negative. Which of the following is an important complication of this patient's endocrinopathy?