Which medication's effectiveness is affected by serum sodium levels?

Lithium is a medication that is primarily used to treat bipolar disorder, and its effectiveness and safety are closely linked to serum sodium levels due to its narrow therapeutic range. This connection occurs because lithium and sodium compete for reabsorption in the kidneys. When serum sodium levels are low (a condition known as hyponatremia), the reabsorption of lithium can increase, potentially leading to lithium toxicity. Conversely, when sodium levels are normal or high, lithium reabsorption decreases, which could result in subtherapeutic levels of the medication.

For this reason, monitoring serum sodium levels is crucial for patients taking lithium to ensure that they remain within the therapeutic range and to avoid adverse effects or treatment failures. Understanding the relationship between lithium and sodium is important for managing a patient’s treatment effectively and safely.