Coexisting paraneoplastic SIAD and tumour-induced osteomalacia in stage IV small cell lung cancer: a diagnostic challenge

A 70-year-old man with stage IV (extensive-stage) small cell lung carcinoma presented with progressive respiratory symptoms, refractory hypophosphataemia and hyponatraemia. The diagnostic evaluation revealed paraneoplastic arginine vasopressin-mediated syndrome of inappropriate diuresis and fibroblast growth factor 23-mediated renal phosphate wasting, consistent with tumour-induced osteomalacia. This report highlights the diagnostic complexity and reasoning required to identify dual paraneoplastic syndromes in an advanced malignancy, with emphasis on biochemical interpretation and integration of laboratory data.