Introduction

Primary hyperparathyroidism is most commonly caused by parathyroid adenomas, which typically present with hypercalcemia and modest gland enlargement. These adenomas have a median weight of ~500 mg, compared to normal glands at about 62 mg [1].

Atypical parathyroid tumors represent a distinct entity, characterized by histologic features concerning for carcinoma without definitive evidence of invasion [2]. Distinguishing large benign adenomas from carcinoma is critical, as it directly impacts surgical strategy and postoperative management. We present a rare case of an atypical parathyroid tumor initially concerning for carcinoma. The patient presented with symptomatic hypercalcemia and elevated parathyroid hormone (PTH), with imaging suggesting a large left-sided adenoma. Intraoperatively, the lesion’s size and proximity to the subclavian vessels heightened concern for malignancy. However, after excision, the mass appeared well-circumscribed without invasion. Final pathology confirmed an atypical parathyroid tumor, highlighting the diagnostic challenges and the importance of integrating clinical, biochemical, and intraoperative findings.