MON-402 Atypical Parathyroid Tumor Presenting with Normocalcemia and Cytology Features of Follicular Thyroid Neoplasm

Abstract Disclosure: S. Ntelis: None. A. Karachaliou: None. S.K. Majumdar: None. Background: Atypical parathyroid tumor (APT) is a rare histological diagnosis with uncertain malignant potential. Clinical presentation typically includes hypercalcemia in setting of hyperparathyroidism. We report a case of APT that presented with normocalcemia and preoperative fine-needle aspiration (FNA) biopsy suggesting a follicular thyroid neoplasm. Clinical Case: A 48-year-old man, with history of hypertension and no personal or family history of endocrine malignancies, presented after a syncopal episode. Carotid duplex revealed a 1.6 cm left lower lobe thyroid nodule with internal vascularity. Ultrasound-guided FNA was performed and cytology demonstrated features of follicular neoplasm/Hurthle cell neoplasm. Gene expression analysis showed adequate but low thyroid cell content, and was negative for gene mutations/fusions, copy number alterations, as well as parathyroid markers. Left hemithyroidectomy was performed per patient’s preference. Surgical pathology demonstrated a 1.6 cm atypical parathyroid neoplasm associated with thymic tissue. No lymphovascular or perineural invasion, or extension outside the parathyroid gland were identified. There was near complete loss of parafibromin nuclear expression. Preoperative calcium was 9.4 mg/dL and remained normal (9.2 mg/dL) after surgery. Parathyroid hormone (PTH) was tested after the pathology report and returned at 20 pg/mL (normal range: 16-77 pg/mL). PET scan showed no signs of metastases. Our patient remained asymptomatic with no jaw abnormalities on exam. Given the loss of parafibromin expression, genetic testing for HRPT2/CDC73 mutations was sent to assess the risk for parathyroid and other tumors and determine the surveillance plan. Clinical Lessons: Parathyroid tumors are very rare and affect 0.1%-0.3% of the general population (1). APT is a unique histological diagnosis with uncertain malignant potential. 1.2-1.3% of primary hyperparathyroidism is attributed to APT and that is typically the initial presentation. Interestingly, our patient had normal calcium levels. Normocalcemic hyperparathyroidism has a reported prevalence of 0.1-0.7% (2). There were also no findings on cytology or gene expression analysis supporting the diagnosis of a parathyroid tumor. In setting of normal calcium levels, differentiation between APT and thyroid tumors could be difficult. Pathology is required for definitive diagnosis (3). References: (1) Erickson et al. Overview of the 2022 WHO Classification of Parathyroid Tumors. Endocr Pathol 33, 64-89 (2022). (2) Cusano NE, Cetani F. Normocalcemic primary hyperparathyroidism. Arch Endocrinol Metab. 2022;66(5):666-677. (3) Cetani F et al. Atypical parathyroid adenomas: challenging lesions in the differential diagnosis of endocrine tumors. Endocrine-Related Cancer. 2019;26(7), R441-R464. Presentation: Monday, July 14, 2025