Which drug requires monitoring of lithium level and sodium?

• A. Desvenlafaxine
• B. Amitriptyline
• C. Phenelzine
• D. Lithium

Answer: (D)

Lithium requires monitoring of both its blood level and the body's sodium because it has a very narrow therapeutic window and its excretion in the kidneys is intertwined with sodium handling. The therapeutic range is roughly 0.6–1.2 mEq/L; levels above this range raise the risk of serious toxicity, which can present with tremor, confusion, ataxia, and in severe cases seizures or coma. Sodium balance directly affects lithium reabsorption in the renal tubules: when sodium is low (hyponatremia or dehydration), the kidneys reabsorb more lithium, raising blood levels; when sodium is high, lithium is excreted more, lowering its blood level. So clinicians check both the lithium concentration and sodium status (along with renal function) to keep treatment effective and safe, adjusting dose or hydration as needed. The other drugs listed do not require lithium-level monitoring; they have different safety and monitoring considerations that don’t involve lithium or sodium measurements.