A Diagnostic Odyssey: Post-Surgical Polyuria and Hyponatremia Unmasking Multiple Myeloma-Associated SIADH-A Case Report

Hyponatremia and polyuria are common postoperative complications, often posing significant diagnostic challenges.This case report details a complex scenario of a 56-year-old male with hypertension and diabetes who developed polyuria post-femur cyst evacuation, initially misinterpreted as diabetes insipidus and treated with desmopressin.Subsequent severe symptomatic hyponatremia necessitated a comprehensive re-evaluation, revealing a shift towards syndrome of inappropriate antidiuretic hormone secretion (SIADH).Further investigation, particularly through fractional excretion studies, led to the diagnosis of SIADH secondary to underlying multiple myeloma, a rare etiology.The patient's condition stabilized with targeted treatment for SIADH and subsequent chemotherapy for multiple myeloma.This case highlights diagnostic challenges in electrolyte imbalances, the use of fractional excretion studies to differentiate hyponatremia etiologies, and the importance of considering rare paraneoplastic causes, such as multiple myeloma, in SIADH.