SUN-771 - Severe Immobilization-Induced Hypercalcemia Following Knee Arthroplasty Despite Partial Postoperative Mobility

Immobilization-induced hypercalcemia is an uncommon diagnosis of exclusion resulting from increased osteoclast-mediated bone resorption due to reduced mechanical loading on the skeleton. Most reported cases occur after prolonged or near-complete immobilization, particularly in patients with spinal cord injury, major trauma, or femoral fractures, whereas reports following elective orthopedic procedures are lacking. We present a 69-year-old woman status post left total knee arthroplasty who was found to have severe hypercalcemia in the setting of partial and short-term postoperative immobilization. The procedure had been performed three weeks earlier; she subsequently presented to the emergency department with persistent nausea and vomiting, during which time her mobility had decreased by more than 50% from baseline. On arrival, she was afebrile, with a heart rate of 124 beats/min and blood pressure of 84/55 mmHg, which improved with intravenous fluids; infectious workup was negative and sepsis was ruled out. Physical examination was notable for dry mucous membranes and a healing left knee surgical site with limited extension. Laboratory evaluation demonstrated severe hypercalcemia with corrected serum calcium of 15.0 mg/dL (8.6–10.2) and ionized calcium of 6.53 mg/dL (4.6–5.3), with suppressed parathyroid hormone at 11 pg/mL (15–65) and a normal thyroid-stimulating hormone level of 0.55 IU/mL (0.45–5.33). The 1,25-dihydroxyvitamin D level was low at 8.8 pg/mL (18–72), while 25-hydroxyvitamin D was normal at 32.4 ng/mL (30–100). She had acute kidney injury with a creatinine of 2.4 mg/dL (baseline 0.6), and 24-hour urine calcium was elevated at >435 mg/24 h (100–300). Parathyroid hormone–related peptide was undetectable, and serum protein electrophoresis showed no monoclonal spike. Neck ultrasound revealed bilateral thyroid nodules, including a TI-RADS 4 right lower-pole nodule. Pan–computed tomography showed no masses, lytic bone lesions, or lymphadenopathy suggestive of malignancy. The patient was treated with intravenous isotonic fluids and calcitonin, resulting in rapid improvement in serum calcium levels. After stabilization, she was discharged home with recommendations to increase mobility and initiate physical therapy. Postoperative skeletal unloading accelerates osteoclast-mediated bone resorption with subsequent calcium release. Although immobilization-induced hypercalcemia is classically linked to prolonged bed rest, this case illustrates that even partial, short-term immobility following knee arthroplasty may be sufficient to precipitate severe hypercalcemia. Early identification allows timely intervention, limits unnecessary diagnostic evaluation, and mitigates renal and metabolic complications, reinforcing the need for early mobilization and targeted metabolic monitoring following orthopedic surgery.

*Unless otherwise noted, all abstracts presented at ENDO must not be released to the press or the public until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.*