Normocalcemic Presentation of a Giant Polycystic Parathyroid Carcinoma

A 70-year-old woman presented with a large anterior cervical mass. Computed tomography and ultrasonography revealed a 70-mm polycystic structure adjacent to the right lobe of the thyroid gland, extending to its caudal aspect. Serum calcium and TSH levels were normal, whereas the serum intact PTH level was slightly elevated. Fine-needle aspiration showed cytology findings consistent with nodular goiter. After 20 months, the tumor was smaller on ultrasound but was completely solid. Serum PTH levels were markedly elevated, and Tc-99m sestamibi scintigraphy revealed striking hyperactivity of the mass. The patient underwent right hemithyroidectomy with en bloc tumor resection. Pathological analysis revealed capsular, thyroid, and venous invasion. Immunohistochemical staining was positive for PTH and galectin-3, and a high mitotic index was observed. Based on these findings, parathyroid carcinoma was diagnosed. Parafibromin staining was positive, resulting in the exclusion of etiology associated with <i>CDC73</i> pathogenic variants. After 10 months, the patient was clinically free of recurrent disease. This case illustrates an atypical presentation of parathyroid carcinoma with early normocalcemia, followed by overt hyperparathyroidism, possibly because of tumor transformation.