Abstract
Hypercalcaemia of malignancy (HCM) is a paraneoplastic syndrome that often portends a poor prognosis. We present an extremely rare (<1%) case of HCM due to extrarenal calcitriol (1,25-(OH)2D) production in a patient with splenic marginal zone lymphoma. A man in his 80s presented with a 3-week history of fatigue, unsteadiness and abdominal pain, and new findings of anaemia, kidney injury and hypercalcaemia. Laboratory evaluation, bone marrow biopsy and positron emission tomography/computed tomography (PET/CT) confirmed the diagnosis of splenic marginal zone lymphoma which produced calcitriol (1,25-(OH)2D3), causing the patient’s hypercalcaemia.