A 29-year-old woman complains of nervousness and muscle weakness of 6 months in duration. She is intolerant of heat and sweats excessively. She has lost 9 kg (20 lb pounds) over past 6 months, despite increased caloric intake. She frequently finds her hea racing and can feel it pounding in her chest. She also states that she has missed several menstrual periods over past few months. Physical examination reveals warm and moist skin and bulging eyes (exophthalmos). Laboratory studies will likely reveal which of the following endocrine abnormalities in this patient?