MON-769 Parathyroid Plot Twist: When Tumors Don't Follow the Script

Abstract Disclosure: M.F. Hassan: None. M. Manolas: None. A. Manavalan: None. Introduction: Atypical parathyroid adenomas (APAs) comprise 1% of parathyroid tumors. APAs are hypercellular lesions with high-risk features such as solid or trabecular growth, fibrous bands or cellular atypia without the cardinal characteristics of malignancy such as local invasion or metastasis. However, concerns about their malignant potential have been raised. We report on such a tumor presenting in a very young woman. Clinical Case:A 24-year-old woman with a history of schizoaffective disorder was admitted for agitation and psychosis. Labs revealed calcium 13.2mg/dl (8.5-10.5mg/dl), phosphate 3 mg/dl (2.5-4.5mg/dl), intact PTH 1056 pg/ml (20-80 pg/ml), Vitamin D 25 hydroxy 10 ng/ml (30-60 ng/ml). She was first noted to have PTH mediated hypercalcemia on a previous psychiatric admission six months earlier during which she eloped prior to further work up. She did not provide us with family history or allow for a neck examination or 24-hour urinary calcium testing. She had no known fractures or renal stones and GFR was normal. Her calcium (peak 13.2 mg/dl) responded to fluid replacement and cinacalcet which was titrated up to 120 mg twice a day to maintain near normal calcium levels. Antiresorptives were not employed. MENIN gene and plasma metanephrines were negative. CT imaging revealed a large heterogeneous enhancing left parathyroid mass measuring 2.6 x 4.2 cm. The patient was unable to tolerate a sestamibi scan. Almost 2 months after her initial presentation, a court-ordered surgery was performed. PTH levels declined from 1886 preoperatively to < 5 ng/ml post operatively. Pathology revealed a parathyroid lesion weighing 10 gms and measuring 3.9 x 2.6 x 1.1 cm. The specimen contained areas of fibrosis but lacked invasion and high mitotic rate, consistent with an atypical parathyroid tumor. The patient had an uncomplicated postoperative course and was discharged with endocrine follow up for further genetic testing. Conclusion: Atypical parathyroid adenomas are rare tumors that usually present in the 4th - 6th decade of life but may present earlier when a familial syndrome is involved. They are usually associated with PTH elevation > 1000 pg/ml and moderate hypercalcemia. They may be associated with genetic variations in CDC73, a tumor suppressor gene that codes for parafibromin, in which case they are associated with a higher malignancy risk. The WHO has recently reclassified them as atypical parathyroid tumors to highlight this possibility. Some suggest staining to evaluate for parafibromin loss for prognostic purposes. While no clear guidelines exist, individuals diagnosed with these tumors should undergo genetic testing and may benefit from being followed for rising calcium or PTH levels. The two most striking aspects about this case are the age at presentation, which is 2-4 decades younger than studies report and the ease with which calcium levels were managed despite delayed surgery. Presentation: Monday, July 14, 2025