A 4-year-old girl is brought to the emergency department by her parents, who report she has been increasingly lethargic and vomiting for the past 24 hours. Her parents noted she has been drinking more water than usual and waking up several times at night to urinate over the past two weeks. She has lost approximately 2 kg (4.4 lbs) during this time despite a normal appetite until yesterday. There is no history of fever, diarrhea, or recent travel. Her past medical history is unremarkable, and she takes no medications. There is a family history of autoimmune thyroid disease in her mother. She appears lethargic, ill, and moderately dehydrated with deep, rapid, sighing respirations. She is afebrile. Her blood pressure is 85/55 mmHg, her heart rate is 145 bpm, her respiratory rate is 40 breaths per minute, and her oxygen saturation is 98% on room air.