SAT-765 Ectopic Parathyroid Hormone Secretion: An Unusual Cause of Severe Hypercalcemia

Abstract Disclosure: D.R. Vangipuram: None. E. Meyers: None. M. Asadi: None. H. Gnanapragasam: None. A. Dokun: None. E. Bowen: None. Introduction: Severe hypercalcemia in the presence of extremely high parathyroid hormone (PTH) usually indicates a parathyroid carcinoma but might not always be the case. We describe a 70-year-old female presenting with serum calcium 19.3 mg/dL and PTH 830 pg/mL without evidence of a parathyroid tumor. Case Report: The patient is a 70-year-old female with a past medical history of endometrial carcinoma surgically treated in 2016 with no evidence of disease on clinical surveillance. She presented with significant fatigue, confusion, abdominal pain, increased thirst, and polyuria. She was found to have hypercalcemia with serum calcium 19.3 mg/dL (8.3-10.6 mg/dL), PTH 830 pg/mL (15-65 pg/mL), and parathyroid hormone-related peptide 1.1 pmol/L (<4.2 pmol/L). The patient was treated with IV fluids, zoledronic acid, calcitonin, cinacalcet, and subsequently denosumab for refractory hypercalcemia. A 4D-CT of the neck showed no cervical mass, lymphadenopathy, or enlargement of parathyroid glands. PET CT showed multiple hypermetabolic lung, liver, and bone lesions with lymph node enlargement throughout the mediastinum and abdomen. There was no cervical lymph node enlargement and no other lesion in the neck. She underwent a biopsy of the dominant liver mass that showed a poorly differentiated adenocarcinoma. Further extensive immunohistochemical work-up was performed but could not determine the primary site of origin. We concluded that her hypercalcemia was driven by an ectopic source of PTH production. The patient eventually elected to transition to comfort-focused measures and was discharged home with hospice care. Discussion: Ectopic PTH production is rare and accounts for less than 1% of hypercalcemia due to malignancy. Approximately 60 cases have been reported from various primary malignancies, including small cell lung, hepatocellular, ovarian, and endometrial carcinomas among others. These patients had negative localizing studies for parathyroid tumors, and some even underwent surgical neck exploration prior to the discovery of their non-parathyroid malignancy. Conclusion: A significantly elevated PTH, which is usually a feature of parathyroid carcinoma, could also be due to production from a non-parathyroid malignancy. Presentation: Saturday, July 12, 2025