MON-846 - A Hidden Gem: Spontaneous Temporary Resolution of Persistent Primary Hyperparathyroidism

Background: Persistent primary hyperparathyroidism (PHPT) despite repeated cervical parathyroid explorations is uncommon and should prompt consideration of ectopic parathyroid tissue.

Case description: A 34-year-old woman had biochemically confirmed PHPT with hypercalcemia (total calcium 11.9 mg/dL) and elevated PTH (117 pg/mL). She underwent three parathyroid operations without biochemical cure, including resection of bilateral upper parathyroid glands and additional parathyroid tissue from the right cervical thymus; a second exploration after selective venous sampling localized to the right inferior thyroidal vein, with resection of two additional non-hyperplastic parathyroid glands, a right thyroid lobectomy, as well as left transcervical thymectomy, and finally a negative blind left neck exploration. Multiple preoperative imaging studies, including ultrasound, sestamibi SPECT/CT, and 4D CT, were nonlocalizing or negative, and intraoperative PTH failed to decline after all three operations. Nearly five years after the initial diagnosis, she presented with acute chest pain and was found to have spontaneous borderline hypocalcemia (total calcium 8.6 mg/dL), requiring initiation of oral calcium supplementation. PTH at the time had also normalized at 20 pg/mL. MRI and 4D CT revealed a 6 × 3.7 cm multiloculated cystic anterosuperior mediastinal mass with associated hemorrhage. Notably, her previous imaging studies had not extended to the low anterior mediastinum. She was treated with oral calcium for over a month, after which her PTH again rose to 115 pg/mL with increased serum calcium levels. She underwent robotic right transcervical thymectomy with en bloc resection of the mediastinal mass. Intraoperative PTH dropped to undetectable ( < 6 pg/mL). Examination of the specimen demonstrated a multiloculated thymic cyst. The cyst wall was fibrotic, inflamed, associated with hemosiderin deposition, and mostly denuded, save for focal areas lined by squamoid thymic epithelial cells, consistent with prior rupture/infarction. Adherent to the cyst wall was a 0.7 cm cellular parathyroid gland; immunohistochemistry was positive for PTH. Postoperatively, she was treated with calcium and calcitriol and maintained normal calcium levels.

Conclusion: We report a rare case of spontaneous temporary biochemical resolution of persistent PHPT, related to structural injury and transient functional impairment of an ectopic intrathymic mediastinal parathyroid gland in the setting of thymic cyst rupture and hemorrhage. This ectopic gland had been missed on multiple previous parathyroid imaging studies, as well as three parathyroid explorations, which had resulted in removal of all normal parathyroid tissue from the neck. This spontaneous biochemical reversal represents a potential endocrine emergency and should prompt urgent recognition and close biochemical monitoring.

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