Review Question - QID 214904

A 29-year-old woman presents to her primary care physician complaining of anxiety and weight loss. Despite increasing her caloric intake over the last few weeks, she has continued to lose weight. She has experienced palpitations and heat sensitivity. Her menstrual cycles have become irregular as well. Her last menstrual period 2 weeks ago was lighter than normal. She has no other significant medical history and does not take any medications except a daily multivitamin. She does not smoke or drink alcohol. She is not currently sexually active. Her temperature is 98.7°F (37°C), blood pressure is 120/80 mmHg, pulse is 105/min, and respirations are 14/min. Examination of the thyroid reveals a single, non-tender nodule in the thyroid. The thyroid is otherwise mobile and normal in size. Laboratory results are as follows:

Thyroid-stimulating hormone (TSH): 0.1 µU/mL (normal: 0.5-5.0)
Free thyroxine (T4): 45 µg/dL (normal: 5-12)

Radioactive iodine uptake (RAIU) results are shown in Figure A. Which of the following is the most likely diagnosis?