A Rare Case of Hypokalemia and Hypomagnesemia

An 18-year-old adolescent male presented to our emergency department complaining of chest pain that started about 2 days earlier and remained unchanged. Chest x-rays revealed a right apical pneumothorax. The patient did not use any medication. Two months earlier he had presented to the emergency department with a similar episode that resolved spontaneously. Blood testing performed at the time of presentation showed severe hypokalemia with a potassium concentration of 2.5 mmol/L. He was admitted to our respiratory unit. He was 180 cm (70.9 in) tall and weighed 64 kg (141 lb, body max index 19.8 kg/m2). He was in good general health, and the physical examination was unremarkable. A renal ultrasound was normal. He noted an unintentional weight loss of 7 kg (15.4 lb) over the last 36 months. Furthermore, he reported fatigue and muscle weakness. His personal history and family history were unremarkable. He was delivered naturally at term and developed normally during childhood and puberty. Further blood tests revealed moderate hypomagnesemia of 1.2 mg/dL [0.5 mmol/L; reference interval 1.7–2.2 mg/dL (0.7–0.9 mmol/L)]. Arterial blood gas analysis showed metabolic alkalosis (pH 7.46, bicarbonate 28.9 mmol/L). Sodium, chloride, and calcium were within reference intervals. Renal function, fasting blood glucose, and a complete blood count were normal. Table 1 provides a selection of relevant laboratory results at presentation and during follow-up. View this table: Table 1. Laboratory results at presentation and on further investigation.a As recommended by the British Thoracic Society, the patient was scheduled for video-assisted thoracoscopic surgery (1). Before surgery, an endocrine specialist was consulted to further investigate the cause of the patient's hypokalemia and hypomagnesemia. The spontaneous pneumothorax was unrelated to the abnormal electrolyte results and is not further discussed here. ### QUESTIONS 1. What are the most common causes of hypokalemia? 2. Which laboratory tests can differentiate between renal and nonrenal causes of potassium …