A 27-year-old woman presents to the emergency room with a panic attack. She appears healthy except for tachycardia and a respiratory rate of 30. Electrolytes include calcium 10.0 mg/dL, albumin 4.0 g/dL, phosphorus 0.8 mg/dL, and magnesium 1.5 mEq/L. Arterial blood gases include pH of 7.56, PCO2 21 mm Hg, and PO2 99 mm Hg. Which of the following is the most likely cause of the hypophosphatemia?

Respiratory alkalosis is one of the commonest causes of hypophosphatemia; it results from shift of phosphate from the extracellular to the intracellular space. Hypomagnesemia alone would increase phosphorus by decreasing parathormone effect. Hyperparathyroidism can decrease phosphorus but would cause hypercalcemia. Severe hypophosphatemia is seen with malnutrition, especially during the refeeding stage when carbohydrate intake causes phosphate to shift into the intracellular space. Such patients have clear clinical evidence of malnutrition. In addition, malnutrition almost always causes hypoalbuminemia. Vitamin D deficiency is uncommon in this age group and would be associated with hypocalcemia.