A Case of Denosumab-associated Hyperparathyroidism: Differential Diagnosis Challenge

Abstract

Denosumab is a relatively new medicine that has become the second option in the treatment of bisphosphonate-resistant or intolerant osteoporosis. An 85-year-old female patient was referred to the outpatient clinic of our department with a complaint of waist pain. She stated that her waist pain started 2 weeks before her visit and she had taken the fourth denosumab injection. Her laboratory results revealed normal serum calcium, phosphorus, and vitamin D levels (8.8 mg/dL, 3.1 mg/dL, and 32 ng/L, respectively) before denosumab injection. She had a sensitive point with pain on her lumbar vertebrae. Lower extremity movements were painful and muscle strength was bilaterally reduced. Vital signs and other systemic examinations were normal. Her laboratory tests for serum parathormone (PTH), calcium, phosphorus, and vitamin D levels showed 487 ng/L, 8.6 mg/dL, 0.6 mg/dL, and 30 ng/mL, respectively. She did not have any chronic renal and other bone metabolism diseases, except for osteoporosis. Her X-ray revealed a loss of height in L3 lumbar vertebrae. The spinal magnetic resonance indicated an acute fracture. However, we could not explain hyperparathyroidism in the absence of hypocalcemia and low vitamin D levels. Therefore, we started to investigate primary hyperparathyroidism in the patient. Neck ultrasonography did not show any associated abnormal findings with the parathyroid glands. Parathyroid scintigraphy resulted in normal parathyroid gland activity. During her follow-up, the PTH level decreased with time. Spontaneous regression of PTH led us to suspect an association between denosumab injection and hyperparathyroidism. Mazokopakis (1) reported a similar case in the literature in 2018, reporting a 62-year-old female patient with normal electrolytes and a high PTH level after three months of denosumab injection. The hypocalcemic effect of denosumab combined with inadequate oral calcium and vitamin D intake may have triggered exaggerated PTH secretion.