SUN-751 Paget’s Disease of Bone: A Rare Diagnosis and the Clinical Rationale for Excluding Secondary Causes of Osteoporosis

Abstract Disclosure: R. Rani: None. I. Iftikhar: None. H. Luo: None. Introduction-Paget's disease of bone (PDB), known as osteitis deformans, is a benign focal disorder of aging skeleton, characterized by abnormal activation of osteoclasts, resulting in excessive bone resorption and accelerated bone remodelling, leading to osteogenic sclerosis, which increases the risk of osteoarthritis, fractures, nerve impingements, and rarely, osteosarcoma. PDB is typically observed in individuals of European descent due to an autosomal dominant genetic predisposition and is rarely reported in Asian population. Case Summary-A 70-year-old Asian male, with history of multiple fractures was referred in clinic for treatment of osteoporosis. His DEXA scan revealed a T-score of -3.1 at L1-L4 and -4.0 at the left hip. Initial investigations conducted to rule out secondary causes of osteoporosis in a male patient showed low phosphate levels (1.8 mg/dL) and 1,25-dihydroxyvitamin D2 levels (<8 ng/mL), along with elevated ALP (558 U/L) with normal PTH, calcium, free testosterone, TSH, creatine and ALT/AST levels. Despite adequate vitamin D supplementation, his phosphate levels persistently remained low (1.8-2.1 mg/dL), and ALP remained elevated (358-580 U/L). Isoenzyme studies revealed that 83% of the ALP originated from bone, with normal FGF23 levels. He was started on oral phosphate supplementation, and Xray skull and hip was conducted for evaluation of PDB due to persistently elevated ALP levels. Radiography showed thickening of the bilateral iliopectineal lines, and patchy areas of sclerosis in skull, consistent with a diagnosis of PDB. He was treated with a single intravenous infusion of zoledronic acid, which normalized ALP levels to 130 U/L within 3-6months. A repeat DEXA scan one year later showed improvement in bone density, with a T-score of -0.7 at L1-L4 and -3.3 at the left hip. Discussion: - PDB is often asymptomatic and discovered incidentally during imaging or through elevated serum ALP levels. Secondary causes of osteoporosis are more common in men, who also have a higher risk of fractures at similar BMD levels as compared to women. This case report not only presents a rare coincidence of Paget’s disease of bone (PDB) and hypophosphatemia-induced osteoporosis but also emphasizes the importance of ruling out secondary causes before deciding on treatment. Further research is required to determine whether Paget’s disease can be a contributing cause of osteoporosis. Presentation: Sunday, July 13, 2025