Paraneoplastic Endocrine Syndromes Associated with Neuroendocrine Neoplasms in Adults: A Case Report and a Contemporary Review.

Neuroendocrine neoplasms (NENs) are distinguished by their remarkable biosynthetic capacity to synthesize and secrete a diverse array of biologically active peptides, amines, and hormones. While a significant proportion of NENs remain clinically non-functional, a subset presents with systemic endocrine manifestations that significantly impact morbidity and mortality. These manifestations range from classic functional syndromes intrinsic to neuroendocrine differentiation to rare, complex paraneoplastic endocrine syndromes driven by ectopic hormone secretion or immune-mediated mechanisms. In this report, we present an extraordinary case of a 33-year-old female with a pancreatic neuroendocrine tumor that demonstrated extreme secretory plasticity through the sequential emergence of three distinct ectopic hormones. The clinical course began with Zollinger-Ellison Syndrome, which progressed into refractory ectopic Cushing's syndrome. The disease further evolved into hypercalcemia of malignancy driven by the simultaneous dual secretion of parathyroid hormone-related protein and 1,25-dihydroxyvitamin D. Despite the implementation of aggressive multimodal therapies, including peptide receptor radionuclide therapy, somatostatin analogs, and targeted systemic agents, the tumor exhibited profound biochemical evolution and clinical aggression. Beyond the case report, this article provides a comprehensive review of the pathophysiology and diagnostic classifications of paraneoplastic endocrine syndromes associated with NENs, while examining the molecular mechanisms that facilitate "secretory switching" and tumor heterogeneity. Advances in functional imaging, such as 68Ga-DOTATATE PET/CT, and molecular characterization have improved the recognition of these syndromes over the last two decades. However, the management of multifaceted ectopic secretion remains a formidable clinical challenge. This review underscores that a high index of clinical suspicion and a coordinated multidisciplinary approach integrating endocrinology, oncology, and nuclear medicine are essential for navigating the prognostic implications and optimizing therapeutic outcomes for patients exhibiting high-grade secretory plasticity.