Early parathyroid hormone elevation after denosumab therapy following parathyroidectomy: a diagnostic pitfall

Persistently elevated parathyroid hormone (PTH) after parathyroidectomy is often attributed to residual disease but may reflect altered calcium (Ca) homeostasis. The impact of denosumab on perioperative PTH interpretation remains unclear. A 77-year-old woman presented with a hypercalcemic crisis and was initially managed at another hospital. During treatment with hydration and elcatonin (40 units twice daily), denosumab (60 mg) was administered, resulting in a rapid decline in corrected Ca (c-Ca). Eleven days later, at presentation to our institution, her c-Ca was 9.9 mg/dL (SI: 2.48 mmol/L; reference range, 8.8-10.1 mg/dL [SI: 2.20-2.53 mmol/L]), and intact PTH was 195 pg/mL (SI: 20.7 pmol/L; reference range, 10-65 pg/mL [SI: 1.1-6.9 pmol/L]). At admission, c-Ca decreased further while PTH increased. The patient had chronic kidney disease and vitamin D deficiency. A focused parathyroidectomy was performed for a single adenoma. Thirty minutes after excision, PTH remained above the surgical success threshold and increased further the morning after surgery, while c-Ca levels decreased. Calcium and vitamin D supplementation subsequently normalized both parameters. Denosumab exposure, together with vitamin D deficiency and renal impairment, may cause transient postoperative PTH elevation despite successful parathyroidectomy. Persistent disease should not be assumed when Ca declines despite elevated PTH.