A 63-year-old woman with a medical history of type 2 diabetes, atrial fibrillation, hypothyroidism, and hypertension was referred for refractory hypomagnesemia. Her blood pressure was 148/68 mm Hg, heart rate 96/min, and weight 106.4 kg. Medications included liraglutide, metformin, apixaban, sotalol, metoprolol, levothyroxine, venlafaxine, lisinopril, ranitidine, and magnesium oxide (400 mg, 2 times/d). Her prior serum magnesium levels ranged from 1.4 mg/dL to 1.6 mg/dL and a recent hemoglobin A1c level was 10.8%. Following baseline blood testing (Table), the patient collected a 24-hour urine sample after discontinuing magnesium supplements for 24 hours.