RESUMEN
Syndrome of inappropriate antidiuretic hormone secretion (SIADH), the most common cause of euvolemic hyponatremia, results from the inappropriate release of antidiuretic hormone. SIADH may be caused by a variety of malignant tumors, central nervous system (CNS) disorders, intrathoracic disorders, and pharmacological agents. We experienced a case of SIADH associated with sarcoidosis that involved the lungs and mediastinal lymph nodes. A 72-year-old male was admitted to hospital with epigastric and back pain. Laboratory tests showed hyponatremia and low serum osmolality, while the urine sodium concentration and urine osmolality were inappropriately high. A chest x-ray and computed tomography showed mediastinal lymph node enlargement, and a mediastinoscopic lymph node biopsy revealed a noncaseating granuloma. Brain magnetic resonance imaging showed no evidence of CNS sarcoidosis. Systemic corticosteroid therapy improved the observed mediastinal lymph node involvement, and tolvaptan as an SIADH treatment corrected the patient's abnormal sodium level and restored the laboratory findings to normal.