# Refractory hyponatremia in small cell lung cancer: a case report and literature review

## TL;DR

A 76-year-old man with hyponatremia was diagnosed with small cell lung cancer causing SIADH, and his sodium levels normalized after cancer treatment.

## Contribution

This case report highlights the rare presentation of small cell lung cancer as SIADH and emphasizes the importance of early diagnosis and management.

## Key findings

- Hyponatremia in the patient was attributed to SIADH caused by small cell lung cancer.
- Treatment of the underlying cancer led to normalization of serum sodium levels.
- The case underscores the diagnostic approach and management strategies for SIADH in oncology.

## Abstract

The clinical manifestations of lung cancer are diverse. Presentation initially as the Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is relatively uncommon. This article reports the diagnosis and treatment process of a 76-year-old male patient with hyponatremia, ultimately diagnosed with SIADH attributed to small cell lung cancer (SCLC). The patient presented with fatigue and significant hyponatremia. Comprehensive investigation excluded adrenal, thyroid, and other functional abnormalities. Combined with features including high urinary sodium, euvolemic status, and normal renal function, the findings were consistent with a diagnosis of SIADH. Subsequent imaging and pathological examinations revealed the etiology to be SCLC. Management included fluid restriction, sodium supplementation, and the use of tolvaptan. Subsequent chemotherapy and radiotherapy for the SCLC led to the gradual normalization of serum sodium. This article reviews the literature in conjunction with this case, summarizing the diagnostic approach to hyponatremia, as well as the diagnostic workup, pathophysiological mechanisms, and treatment strategies for SIADH, aiming to enhance the early recognition and comprehensive management of this condition.