SAT-369 Parathyroid Microadenomas as a Cause of Normocalcemic Primary Hyperparathyroidism (PHPT) and a Surgical Challenge

Background: Patients with normocalcemic primary hyperparathyroidism (PHPT) share similar risks of osteoporosis and nephrolithiasis with patients having hypercalcemic PHPT. The prevalence and natural history of normocalcemic PHPT is not well defined. Parathyroid microadenomas weighing < 100 mg may present as special clinical challenges in both the diagnosis and surgical resection. Clinical Case: A 55-year-old woman presented for an evaluation of osteoporosis following a non-traumatic L1 compression fracture at the age of 50 years. Noting an initial serum calcium of 10 mg/dL, we diagnosed her with normocalcemic PHPT based on a concurrent parathyroid hormone (PTH) level of 112 pg/mL (12-88 pg/mL) and a lack of other causes for an elevated PTH, including a normal eGFR, serum 25-hydroxyvitamin D, 24-hour urine calcium, and autoantibodies for celiac disease. Initial dual-energy x-ray absorptiometry (DXA) scan from 2015 showed normal bone mineral density (BMD) and T-scores in the L-spine, total hip, and femoral neck. Neck imagings, including ultrasound, sestamibi scan, and 4D CT, failed to localize a parathyroid adenoma. In the absence of a target and indications for parathyroidectomy other than osteoporosis, she was treated with bisphosphonates. While her serum calcium remained normal (9.6-10.2 mg/dL) despite an elevated PTH level (92-116 pg/mL) over a two-year period, subsequent DXA scans showed a significant loss in BMD on the bisphosphonates, prompting a peripheral DXA scan, which revealed severe osteoporosis at the distal third of the forearm (BMD 0.450 g/cm2, T-score -3.7), consistent with PTH-associated bone resorption. Following a 4-gland exploration with removal of 2 parathyroid microadenomas (63 and 100 mg), intra- and post-operative PTH levels normalized. She now awaits a repeat DXA scan at 1 year after a successful parathyroidectomy. Conclusion: Although many patients with normocalcemic PHPT may progress to develop hypercalcemia, some remain normocalcemic over an extended period. We report a patient who maintained normocalcemia for over 2 years. To correctly establish the diagnosis of normocalcemic PHPT, a comprehensive evaluation must be performed to exclude secondary causes of an elevated PTH (renal insufficiency, hypovitaminosis D, hypercalciuria, and malabsorption). Since significant osteoporosis may be preferentially seen in the cortical bone, BMD at the distal 1/3 of forearm should be evaluated in addition to the L-spine, hips and femoral necks. When a parathyroid adenoma cannot be localized in patients with normocalcemic PHPT, a careful 4-gland exploration for parathyroid microadenoma(s), paying special attention to the size, texture, and shape of the glands, and guided by intraoperative PTH levels, ensures the successful removal of the culprit gland(s).