MON-783 From Hypocalcemia Prevention To Hypercalcemia Crisis: A Cautionary Tale After Parathyroidectomy

Abstract Disclosure: J. Tatchou: None. J. Bautista: None. D. Umana De Goenz: None. M. Ezer: None. Introduction: Hypocalcemia is a common concern after parathyroidectomy, especially in cases of hungry bone syndrome or transient hypoparathyroidism. Routine postoperative calcium and vitamin D supplementation is often employed; however, in rare cases, this can lead to hypercalcemia. We present a unique case of severe hypercalcemia one week after partial parathyroidectomy, likely triggered by routine supplementation and concurrent thiazide diuretic use. Case summary: A 67-year-old woman with primary hyperparathyroidism, non-toxic multinodular goiter, hypertension, and hyperlipidemia presented with one week of worsening generalized weakness, nausea, vomiting, and anorexia. She had undergone right upper parathyroid lobectomy and left thyroid lobectomy one week prior and was discharged on calcium carbonate 500 mg-vitamin D3 400 unit,1 tablet every six hours. Preoperative PTH was 72.5 pg/mL (n=15-65 pg/mL), which appropriately dropped to 19.9 pg/mL ten minutes post-excision. Relevant medications included triamterene-hydrochlorothiazide 37.5-50 mg PO daily. Examination revealed dry mucous membranes and decreased muscle strength (4/5) in all extremities. CT head ruled out intracranial hemorrhage. Labs showed calcium 19.9 mg/dL (n=8.5-10.1 mg/dL), ionized calcium 2.1 mmol/L (n=1.15-1.35 mmol/L), PTH 7.3 pg/mL, vitamin D 62 ng/mL (n=30-100 ng/mL), magnesium 1.1 mg/dL (n=1.8-2.4 mg/dL), potassium 2.6 mmol/L (n=3.5-5.1 mmol/L), and creatinine 1.7 mg/dL (baseline 0.9). Management included aggressive IV fluid resuscitation, electrolyte repletion, and ICU monitoring. Calcium/vitamin D supplementation and thiazide diuretics were discontinued. By hospital day three, her calcium normalized to 10.1 mg/dL, with resolution of symptoms. At discharge, calcium was 9.1 mg/dL with improved renal function. She was advised against restarting supplements or thiazides. Conclusion: Hypercalcemia occurring one week after parathyroidectomy—especially when intraoperative PTH drops appropriately per Miami criteria—is uncommon. This case highlights the importance of maintaining a broad differential for postoperative hypercalcemia, including supplementation overcorrection, vitamin D toxicity, and medication effects such as thiazides. Thorough review of all supplements and medications is essential. More importantly, this case underscores the need for close postoperative calcium and PTH monitoring to promptly identify and manage unexpected complications. Presentation: Monday, July 14, 2025