From Lethargy to Recovery: Central Diabetes Insipidus from Pituitary Metastasis in Recurrent HER2-Positive Breast Cancer Treated with Fam-Trastuzumab Deruxtecan-Nxki

HER2-positive breast cancer is well known for its aggressive nature and high incidence of central nervous system (CNS) metastasis, often affecting the brain parenchyma or leptomeninges. However, metastasis to the pituitary gland is exceedingly rare. We report a case of ER/PR-negative, HER2-positive breast cancer recurrence presenting with panhypopituitarism (PH) and central diabetes insipidus (CDI), resulting in hypernatremia-induced altered mental status. The patient had completed prior treatment with neoadjuvant chemotherapy, lumpectomy, and trastuzumab. Imaging revealed a temporal lobe mass with suprasellar enhancement. Surgical resection confirmed breast carcinoma metastasis. Hormonal studies confirmed PH and CDI. Management involved DDAVP, corticosteroids, fluid correction, whole-brain radiation, and systemic therapy with fam-trastuzumab deruxtecan-nxki (ENHERTU), leading to significant neurological recovery. This case highlights a rare metastatic pattern and the need for high clinical suspicion and multidisciplinary intervention.