抗利尿激素分泌异常综合征（SIADH）在肺癌患者中很常见。在此，我们报告一例持续性低钠血症，提示恶性 SIADH，有助于小细胞肺癌 (SCLC) 的早期诊断。联合放化疗使 SIADH 得到部分缓解和消退。早期复发表现为严重低钠血症复发和和肽素水平升高，这些和肽素水平被用作抗利尿激素（ADH）的替代标志物。由于姑息性免疫化疗结合液体限制和溶质替代无法控制低钠血症，因此开始使用 ADH V2 受体拮抗剂托伐普坦治疗。随着时间的推移，托伐普坦的剂量需要增加，同时和肽素水平呈指数增加。总而言之，这是托伐普坦继发性失败的罕见病例，具有和肽素水平升高的独特文献证据。这一观察结果支持了这样的假设：极高的 ADH 水平可能会导致托伐普坦从 V2 受体上竞争性取代。版权所有 © 2024 Menzi、Jaramillo、Pfister、Schefer 和 Jehle。

The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is frequent in lung cancer patients. Here, we report a case with persistent hyponatremia, which suggested malignant SIADH and facilitated an early diagnosis of small cell lung cancer (SCLC). A combined radio-chemotherapy led to a partial remission and resolution of SIADH. An early relapse was indicated by reoccurring severe hyponatremia and increased copeptin levels, which were used as surrogate markers for the antidiuretic hormone (ADH). As palliative immunochemotherapy, together with fluid restriction and solute substitution, were unable to control hyponatremia, treatment with the ADH V2-receptor antagonist tolvaptan was initiated. Over time, the dose of tolvaptan needed to be increased, paralleled by a well-documented exponential increase of copeptin levels. In summary and conclusion, this is a rare case of a secondary failure to tolvaptan with unique documentary evidence of increasing copeptin levels. This observation supports the hypothesis that exceedingly high ADH levels may lead to competitive displacement of tolvaptan from the V2 receptor.Copyright © 2024 Menzi, Jaramillo, Pfister, Schefer and Jehle.