Central diabetes insipidus: A rare primary manifestation of small-cell lung carcinoma

Diabetes insipidus (DI) is a disorder of water hemostasis that is associated with polyuria-polydipsia syndrome. Central DI (CDI) primarily results from autoimmune destruction, traumatic injury, or anatomical damage caused by neoplasms. Craniopharyngioma, germinoma, and distant metastases are the main neoplastic causes, with pituitary adenomas rarely manifesting as CDI. Pituitary gland metastasis is rare, with the vast majority of cases being asymptomatic. We present a rare case of pituitary metastasis originating from small-cell carcinoma of the lung with CDI and skin swellings as the primary manifestation, without any evidence of the primary malignancy upon initial presentation. A 56-year-old chronic smoker with newly diagnosed type-2 diabetes mellitus presented with a history of polydipsia and polyuria along with soft tissue swellings in the axilla and the chest for the last 3 months. A water deprivation test and a desmopressin challenge test were performed, revealing the presence of CDI. In light of the CDI, a contrast-enhanced magnetic resonance imaging brain was performed, which displayed a loss of pituitary bright spot and four T2 isointense lesions with post-contrast enhancement in the left frontal, parietal, occipital, and right temporal lobes, suggestive of metastatic lesions. Fine needle aspiration cytology of the swelling revealed cytomorphological characteristics indicating the presence of malignancy, specifically favoring carcinoma. Contrast-enhanced computed tomography thorax revealed a right hilar lung mass infiltrating the surrounding structures with multiple regional and distant metastases. A lung biopsy confirmed the presence of small-cell lung carcinoma (SCLC). The final diagnosis was advanced SCLC with multiple distant metastases associated with CDI, and the patient is currently receiving palliative care and inhalational desmopressin. In conclusion, metastatic lesions and lung cancer must be considered early when patients present with polydipsia and polyuria symptoms.