Severe Hyponatremia Following Nitrofurantoin Therapy: A Case of Suspected Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH).

Nitrofurantoin is a widely prescribed antibiotic for the treatment of uncomplicated urinary tract infections, but it is an uncommon cause of the syndrome of inappropriate antidiuretic hormone secretion (SIADH). We report the case of a previously healthy 68-year-old woman who developed acute, severe hypotonic hyponatremia due to SIADH shortly after initiating nitrofurantoin therapy for a presumed urinary tract infection. She presented to the emergency department with persistent nausea, vomiting, dizziness, and generalized weakness two days after starting nitrofurantoin. Laboratory investigations revealed profound hyponatremia with inappropriately concentrated urine, normal renal, thyroid, and adrenal function, and no evidence of infection or malignancy, consistent with SIADH. Nitrofurantoin was promptly discontinued, and the patient was managed in the intensive care unit with strict fluid restriction and close biochemical monitoring. Her serum sodium levels gradually normalized over several days, with complete resolution of symptoms. This case highlights nitrofurantoin as a rare but clinically significant trigger of SIADH, particularly in older adults. Early recognition, prompt withdrawal of the offending medication, and appropriate management are essential to prevent potentially life-threatening complications. Increased clinical awareness of antibiotic-induced SIADH may facilitate earlier diagnosis and safer prescribing practices.