When monitoring a patient on Lithium, which level must also be monitored?

Monitoring sodium levels is crucial for a patient on lithium therapy because lithium has a narrow therapeutic window and is heavily influenced by sodium levels in the body. Lithium and sodium ions compete for reabsorption in the renal tubules. If sodium levels decrease (i.e., in situations of dehydration or diuretic use), lithium reabsorption may increase, potentially leading to toxicity. Conversely, if sodium levels are normal, lithium will be appropriately excreted, reducing the risk of accumulation and toxicity. Therefore, close monitoring of sodium levels helps ensure the safety and effectiveness of lithium treatment, making it the key indicator to watch alongside lithium levels themselves.