A 15-year-old female with a history of anorexia nervosa is admitted for medical stabilization. Her Body Mass Index is 15 kg/m². On day 3 of refeeding, she develops shortness of breath, tachycardia, and a new cardiac arrhythmia. Laboratory workup is significant for severe hypophosphatemia. The fluid and electrolyte shifts seen in this patient are primarily driven by the release of which hormone?