A Hidden Risk of Levodopa: A Case Report of Drug-Induced Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) and Hyponatraemia

Hyponatraemia is a common electrolyte imbalance in hospitalised patients, particularly among the elderly. While often mild, asymptomatic, and largely self-resolving, severe cases can lead to significant neurological complications. We report a 67-year-old woman with Parkinson's disease and chronic hyponatremia who developed acute symptomatic hyponatraemia after starting levodopa/carbidopa. Her sodium levels dropped to 108 mmol/L, with associated confusion. Investigations revealed euvolemic hyponatraemia consistent with SIADH, with normal adrenal and thyroid function. Levodopa/carbidopa was suspected as the trigger and discontinued. She was transitioned to rotigotine, resulting in gradual correction of sodium levels and full neurological recovery. This case highlights a rare but important adverse effect of levodopa, emphasising the need for clinicians to consider dopaminergic medications as a potential cause of hyponatraemia. Early recognition and appropriate management are crucial to prevent serious complications, particularly in older adults with pre-existing electrolyte disturbances.