Lung neuroendocrine tumors (NETs) are rare clinical condition of pulmonary neoplasm and histologically characterized by neuroendocrine differentiation. Because of neuroendocrine cellular origin, these tumors may produce a biologically active peptide, which results in paraneoplastic syndrome. Among them, the syndrome of inappropriate antidiuresis, characterized by abnormally elevated levels of antidiuretic hormone (ADH), lead to impairment of free water excretion and results in significant electrolyte abnormalities, such as hyponatremia. Here, we report a case of recurrent symptomatic hyponatremia patient who was finally diagnosed metastatic neuroendocrine carcinoma of liver and pleura. We show a direct relationship between neuroendocrine tumor and hyponatremia by immunohistochemical stain for arginine vasopressin. This case emphasizes the importance of early recognition of SIAD, which may be the only initial manifestation of metastatic neuroendocrine carcinoma.