低血鈉症發生在慢性肺疾病病人的身上可以有很多不同造成的原因，但是一些可治療、可治瘉的的疾病可能會被遺漏。我們報告一位七十五歲男性病人，他有慢性阻塞性肺疾病而且發生暈厥的表現。他除了使用支氣管擴張劑外，並無使用利尿劑及類固醇等藥物。他的血壓及體液狀態是正常的。最值得注意的是血鈉值是122 mmol/L並且符合了抗利尿激素不適當分泌症候群 (SIADH) 的診斷標率。即使在良好的控制慢性阻塞性肺疾病之後，低血鈉仍然存在著。荷爾蒙的檢查顯示有腦下垂體功能性低下的情形，頭部核磁共振影像學檢查發現在腦下垂體處有一巨大腺瘤。在經蝶骨內視鏡手術移除腫瘤及住院中短期使用類固醇和甲狀腺素後，此後並不須其他治療，病人的血鈉仍然維持正常。在慢性肺疾病的病人身上發生低血鈉症的鑑別診斷，無功能性的腦下垂體腺瘤應該被列為是其中之一，特別是在老年人族群。

Hyponatremia in patients with chronic lung disease may be caused by a variety of disorders, but treatable, curable disorders may be overlooked. A 75-year-old man with chronic obstructive pulmonary disease (COPD) presented with an episode of syncope. He denied the use of diuretics or systemic steroids. He used oral bronchodilators for his lung disease. His blood pressure and volume status were normal. He had hyponatremia with a serum sodium level of 122 mmol/L. This was initially attributed to the very common syndrome of inappropriate secretion of antidiuretic hormone (SIADH). The hyponatremia persisted despite good control of the COPD. Measurement of hormones indicated hypopituitarism. Magnetic resonance imaging of the brain showed a pituitary macroadenoma in the sella. The tumor was removed by transsphenoidal surgery and the patient was treated with glucocorticoid and thyroid hormone replacement temporarily. His serum sodium normalized without further treatment. Nonfunctional pituitary adenoma with hypopituitarism should be kept in mind as a cause of hyponatremia in pulmonary diseases, especially in the elderly patients.