What is the expected interaction when a thiazide diuretic is prescribed to a patient taking lithium?

When a thiazide diuretic is prescribed to a patient taking lithium, the expected interaction is an increased lithium level in the bloodstream. Thiazide diuretics can reduce renal clearance of lithium, leading to higher serum concentrations. This occurs because thiazides decrease the ability of the kidneys to excrete lithium, primarily due to their effects on sodium reabsorption in the distal convoluted tubule. As sodium levels drop due to the diuretic, the kidneys will hold onto lithium, increasing its serum concentration.

This interaction is clinically significant as it may increase the risk of lithium toxicity, which can manifest as symptoms such as nausea, vomiting, diarrhea, and, in severe cases, may lead to seizures or renal failure. Therefore, monitoring lithium levels is important when starting a thiazide diuretic in a patient already taking lithium, and dosage adjustments may be necessary to avoid toxicity.

In contrast, the other options do not relate to the recognized pharmacokinetic interactions between thiazide diuretics and lithium. Clozapine levels and valproic acid levels are not directly affected by thiazide diuretics in the same way, making the correct answer specifically relevant to lithium levels.