Severe Hyponatremia Masking Central Diabetes Insipidus in a Patient with a Lung Adenocarcinoma

Altered natremia is a common electrolyte disorder in clinical practice and a paraneoplastic manifestation. The syndrome of inappropriate antidiuretic hormone secretion is the first diagnostic suspicion in a patient with cancer and hyponatremia, although entities such as adrenal insufficiency primary or secondary to metastatic involvement must be taken into account. Likewise, immunorelated endrocrinopathies such as hypophysitis have been reported after the introduction of checkpoint inhibitors. A 46-year-old man diagnosed with metastatic adenocarcinoma of the lung with severe hyponatremia (111 mmol/L) consulted due to altered level of consciousness. The initial cranial CT scan did not reveal pituitary brain metastatic involvement; however, an MRI could not be performed due to the patient's clinical situation and subsequent exitus. The water restriction test confirmed the diagnostic suspicion of central diabetes insipidus. Medical treatment with desmopressin was started to avoid fluid depletion with improvement of natremia figures. It represents an exceptional case of central diabetes insipidus masked by severe hyponatremia in a patient with metastatic lung adenocarcinoma without initial evidence of pituitary metastatic involvement by CT imaging in treatment with nivolumab (anti-PD-1 agent). Secondary adrenal insufficiency due to pituitary metastatic involvement and endocrinologic toxicity immunorelated to the new checkpoint inhibitors should be considered as possible etiologic agents of central diabetes insipidus, even with hyponatremia.