SAT-755 Beyond Hyperparathyroidism: A Surprising Discovery of Thyroid Cancer in Surgery

Abstract Disclosure: H. Akbari: None. R. Devbhandari: None. C. Garrison: None. R. Jhingan: None. Background: Primary hyperparathyroidism (PHPT) is a common endocrine disorder characterized by excessive parathyroid hormone (PTH) secretion, leading to hypercalcemia and systemic effects. The diagnosis is established through biochemical markers and imaging. While PHPT is primarily due to parathyroid adenomas, its coexistence with unexpected malignancies, such as thyroid cancer, is rare. This case highlights an incidental diagnosis of encapsulated cystic follicular variant of papillary thyroid carcinoma (FVPTC) in a patient undergoing surgery for PHPT, despite a benign preoperative ultrasound assessment. Case Presentation: A 62-year-old woman, employed as an accountant, presented with PHPT. She had a history of elevated calcium for several years and persistently high-normal PTH. Her latest labs included calcium of 10.3 mg/dL (normal: 8.5-10.2 mg/dL) and PTH of 93 pg/mL (normal: 15-65 pg/mL). She reported nocturia (3-4 times per night), indigestion, constipation, fatigue, and decreased mental acuity. Additionally, she experienced bony pain, and a DEXA scan revealed osteopenia. She had no history of nephrolithiasis.Serial calcium and PTH measurements showed persistently elevated calcium levels (10.1-10.9 mg/dL) and intermittently high PTH. A parathyroid nuclear scan identified mild radiotracer uptake in the posterior left thyroid lobe, suggestive of possible parathyroid adenomas. A preoperative thyroid ultrasound showed a 3.5 cm predominantly cystic nodule in the inferior left thyroid lobe, classified as TI-RADS 1, with no concerning features.The patient underwent a parathyroidectomy with the excision of a 15 x 8 mm left upper parathyroid adenoma. Intraoperatively, a soft, cystic thyroid lesion adjacent to the left parathyroid gland was also removed. During dissection, the cyst ruptured, revealing internal solid components. The entire cyst wall was excised and sent for pathology. Surprisingly, the lesion was identified as an encapsulated cystic FVPTC with capsular invasion but no lymphovascular invasion. The surgical team confirmed the complete removal of the lesion, with no need for additional surgery. Postoperatively, the patient’s PTH and calcium levels normalized, and she remained asymptomatic at follow-up. Conclusion: This case emphasizes the importance of careful intraoperative assessment and histopathology in thyroid lesions, even when imaging suggests benignity. Surgical exploration of adjacent thyroid lesions during parathyroidectomy may be warranted, especially when near the target gland. Recognizing the potential for unexpected malignancies in endocrine surgery ensures timely intervention and improved patient outcomes. Presentation: Saturday, July 12, 2025