Diabetes Insipidus as a Rare Complication of Metastatic Breast Cancer With Pituitary Metastases.

Diabetes insipidus (DI) is a rare but significant and life-threatening complication of metastatic breast cancer, typically arising when cancer spreads to the pituitary gland. This condition results from impaired secretion or action of antidiuretic hormone (ADH) or arginine vasopressin (AVP), leading to symptoms including excessive urination and thirst. The posterior lobe of the pituitary and the infundibulum are most susceptible to metastatic involvement due to their direct arterial blood supply, making them more accessible to circulating tumor cells. This phenomenon explains why DI can be a manifestation of pituitary metastases. We report the case of a patient who presented with a five-week history of projectile vomiting, dizziness, blurred vision, headache, insatiable thirst, and nocturia. She reported urinating up to 12 times at night such that she struggled to sleep and would end up having daytime lethargy and fatigue. Biochemical evaluation demonstrated hyperosmolar serum, low urine osmolality, and low urine sodium, consistent with central DI. MRI revealed a pituitary stalk and suprasellar metastases with multiple new cerebral and cerebellar lesions. The patient underwent a desmopressin challenge leading to rapid improvement in polyuria and an increase in urine osmolality. Desmopressin confirmed an intact renal response, establishing the diagnosis of cranial DI secondary to pituitary metastases. Despite symptomatic benefit, her overall condition deteriorated due to progressive intracranial and systemic disease. She subsequently received whole-brain radiotherapy and palliative care but died three months after the diagnosis of DI. This case highlights central DI as a rare but clinically important manifestation of pituitary metastases in metastatic breast cancer. Prompt recognition and endocrine replacement, particularly with desmopressin, can provide meaningful symptomatic relief even when prognosis remains poor. Early multidisciplinary involvement, including endocrinology, oncology, radiology, and palliative care, is essential for optimizing quality of life in such complex cases.