Non-Parathyroid Hyperparathyroidism: Rare Paraneoplastic Manifestation of GI Malignancy: A Case Report

Abstract

Hypercalcemia is a frequent metabolic complication in malignancy, but the mechanisms are typically mediated through parathyroid hormone–related peptide (PTHrP), calcitriol production, or osteolytic metastases. True ectopic secretion of intact parathyroid hormone (PTH) by non-parathyroid tumors is exceedingly rare, with only a few cases documented in the literature. We present the case of a 73-year-old female with chronic kidney disease, hypertension, diabetes mellitus, and heart failure who presented in the ED with progressive altered mental status. Her laboratory evaluation revealed persistent hypercalcemia with elevated intact PTH levels. A parathyroid nuclear scan excluded primary hyperparathyroidism, while further workup ruled out PTHrP-mediated, vitamin D - related, and osteolytic causes. Her imaging revealed a retroperitoneal mass, and biopsy confirmed metastatic gastrointestinal adenocarcinoma. Considering the biochemical profile and negative parathyroid imaging, ectopic secretion of intact PTH from the malignancy was strongly suspected. Unfortunately, further confirmatory testing could not be performed due to her transition to the hospice. This case report highlights the diagnostic challenges posed by ectopic PTH production, a rare paraneoplastic manifestation, and also emphasises the importance of considering this rare etiology in patients with unexplained hypercalcemia and elevated PTH levels despite a negative parathyroid evaluation. Early recognition may enable timely management and prevent prolonged morbidity.