PSUN76 Massive Four-Gland Parathyroid Hyperplasia: The Case of the Missing Glands

INTRODUCTION: Parathyroid adenoma (PA) and parathyroid hyperplasia (PH) are common causes of primary hyperparathyroidism (PHPT) for which the only definitive treatment is surgical removal. Surgery is often preceded by imaging for localization, including by ultrasound (US), Sestamibi scan (MIBI), positron emission tomography/computer tomography (PET/CT), and four-dimensional computed tomography (4D-CT). While it is common for parathyroid pathology to not be visualized on imaging, non-visualization is more typical of adenomas and hyperplasia of smaller size. CLINICAL CASE: A 65-year-old male without history of kidney dysfunction or family history of parathyroid disease was referred to surgery for parathyroidectomy to manage longstanding PHPT. Serology showed a parathyroid hormone (PTH) level of 581 pg/mL (reference range: 12-65 pg/mL) and calcium of 11.8 mg/dL (reference range: 8.6-10.5 mg/dL). Preoperative parathyroid localization US revealed a large well-defined 1.5 cm PA. The patient underwent parathyroidectomy, which detected a large right inferior parathyroid gland, confirmed with pathology to be an atypical parathyroid adenoma weighing 1800 mg. Postoperative PTH level was 550 pg/mL and calcium was 12.6 mg/dL. MIBI did not display any definite parathyroid adenoma or ectopic parathyroid tissue. Further evaluation with PET/CT did not demonstrate any evidence of metabolically active residual or ectopic parathyroid adenoma or metastatic disease, and repeat US showed did not reveal evidence of any abnormal parathyroid tissue. Ultimately, 4D-CT detected a multiglandular parathyroid with a 1.8×1.4×3.0 cm right superior adenoma weighing approximately 3931 mg, 1.2×0.8×2.1cm left superior adenoma weighing approximately 1048 mg, and a 0.3×0.4×0.4cm left superior adenoma weighing approximately 25 mg. The patient underwent re-exploration parathyroidectomy with removal of the right superior adenoma weighing 3105 mg and left superior adenoma weighing 1100 mg. The remaining left inferior gland was biopsied and the majority of it was implanted into the arm. Repeat serology one week post-operatively showed improvement of PTH level to 5 pg/mL and of calcium to 8.9 mg/dL and serology one year post-operatively showed a PTH level of 82 pg/mL and a calcium of 9.4 mg/dL. CONCLUSION: We present a patient with massive four-gland PH who initially presented with findings more typical of PA, but ultimately had additional hyperplasic glands that were identified after removal of the adenomatous gland. These glands were unusually large to be seen in PH and yet still be undetected on multiple imaging modalities; no previously reported non-ectopic hyperplastic glands of this size have been documented that were not detected on US or MIBI. Presentation: Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m.