PSAT192 Hypercalcemia of Immobility in a Patient with Post-Surgical Hypoparathyroidism

BACKGROUND: Hypercalcemia of immobility is a rare diagnosis in the general population, and thus requires careful evaluation to rule out other causes of non-PTH mediated calcium excess. The pathogenesis is not clearly established, however, likely involves a reduction in mechanical loading stimulus from osteocytes with resultant decreased bone formation and increased bone resorption. Management requires careful assessment to maintain normal calcium. We present a case of hypercalcemia of immobility in a patient with a known history of surgical hypoparathyroidism. CLINICAL CASE: A 64-year-old man with a history of transglottic cancer s/p total laryngectomy and tracheostomy with resulting post-surgical hypoparathyroidism and hypothyroidism presented for a tracheostomy change and was found to be hypercalcemic. Since his laryngectomy four years ago, the patient had a low PTH of 2.9 pg/mL (10.0 - 65.0 pg/mL) with resulting prolonged hypocalcemia from 5.9 to 7.9 mg/dL (8.5-10.5 mg/mL) requiring calcium and daily calcitriol treatment. Four months prior to current hospitalization, he developed avascular necrosis of the hip and became essentially bedbound from severe hip pain. Since immobilization, his calcium was noted to increase, with the highest corrected calcium of 10.9 mg/dL (8.5-10.5 mg/mL), necessitating tapering and eventual discontinuation of calcium supplements and calcitriol. During this admission, corrected calcium was 12 mg/dL (8.5-10.5 mg/mL). PTH-rP was 16 pg/mL (14 - 27 pg/mL), PTH was 11.2 pg/mL (15-65 pg/mL), vitamin D 25-hydroxy total was 14.7 ng/mL (30-60 ng/mL), 1,25 dihydroxy vitamin D was 18.2 pg/mL (19.9 to 79.3 pg/mL), SPEP did not detect monoclonal protein, thyroid function was normal and bone scan showed no evidence of metastasis, thus excluding other causes of hypercalcemia. Bone specific alkaline phosphatase was 11.7 (7.6-14.9 mcg/L). C-telopeptide, a marker of bone resorption, was markedly elevated at 2079 (87-345 pg/mL), suggesting hypercalcemia of immobility. The patient was started on vitamin D supplementation, intravenous fluid hydration and physical therapy was initiated to encourage mobilization. The patient was carefully monitored with regular labs to avoid the risk of hypocalcemia once mobilization improved. His calcium began to decline to 10.5 (8.5-10.5 mg/mL) at time of discharge, eventually requiring calcium and calcitriol supplementation to maintain normocalcemia. CONCLUSION: Hypercalcemia of immobility should be suspected in patients witha history of post-surgical hypoparathyroidism who present with new onset hypercalcemia that persists despite titrating or discontinuing calcium and calcitriol therapy. It is a diagnosis of exclusion and thorough laboratory assessment is needed to rule out other causes of hypercalcemia. These patients are at risk of hypocalcemia once the underlying etiology is corrected i.e restoration of mobility. Therefore, bisphosphonates should be avoided for acute treatment of hypercalcemia unless refractory to intravenous hydration. Once mobilization improves, calcium levels should be carefully monitored, and calcitriol and calcium supplementation should be resumed to maintain normocalcemia. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.