A 65-year-old woman with a history of multinodular goiter complains of increasing nervousness, insomnia, and hea palpitations. She has lost 9 kg over the past 6 months. There is no evidence of exophthalmos. Laboratory studies show elevated serum levels of free T3 and T4. Serologic tests for antithyroid antibodies are negative. Which of the following is the likely endocrinopathy in this patient?