A patient taking lithium is started on a thiazide diuretic. Which outcome is most likely?

• A. Decreased lithium levels
• B. No interaction
• C. Increased mood
• D. Increased risk of lithium toxicity when co-prescribed with thiazide diuretics

Answer: (D)

The important concept here is how certain diuretics affect lithium’s renal handling. Thiazide diuretics promote sodium and water loss and can cause volume depletion. In response, the kidneys increase sodium reabsorption to maintain balance, and lithium tends to follow sodium reabsorption in the kidney tubules. This means more lithium is reabsorbed back into the bloodstream, raising its serum level.

Because lithium has a narrow therapeutic window, even a moderate rise in its level can lead to toxicity. Toxicity can present with tremor, confusion, ataxia, GI symptoms, and in more severe cases seizures or coma. So starting a thiazide with lithium markedly increases the risk of lithium toxicity, making that combination potentially dangerous.

In contrast, an outcome of decreased lithium levels, no interaction, or an improvement in mood due to the diuretic would not fit the pharmacology of this interaction.