Question

Which of the following diuretics decreases the renal clearance of lithium?

• A. Acetazolamide
• B. Hydrochlorothiazide
• C. Furosemide
• D. Spironolactone

Hint:

Thiazide diuretics cause proximal tubular reabsorption of lithium to rise, lowering its renal clearance and raising blood levels. This is a classic, well tested drug interaction.

Answer 1

The Correct Option is B

Lithium travels through the kidney much like sodium does, being reabsorbed mostly in the proximal tubule. Any diuretic that changes how much sodium (and along with it, lithium) gets pulled back into the blood at that site can change lithium levels. Let's look at each option.

1. Acetazolamide: blocks carbonic anhydrase in the proximal tubule itself and increases sodium and lithium loss in urine, so it raises lithium clearance rather than lowering it.
2. Hydrochlorothiazide: blocks sodium reabsorption further down, in the distal tubule. The resulting mild volume loss makes the kidney reabsorb more sodium, and with it more lithium, upstream in the proximal tubule. Less lithium then reaches the urine, so its renal clearance drops and blood levels can climb, sometimes to toxic levels.
3. Furosemide: acts on the loop of Henle. Because it works through a different route than a thiazide, the compensatory rise in proximal lithium reabsorption is much smaller, so its effect on lithium levels is comparatively mild.
4. Spironolactone: blocks aldosterone in the collecting duct, a site and mechanism not linked with the classic lithium interaction.

Since the interaction depends on driving up proximal tubular reabsorption, and thiazides do this most strongly through the volume-depletion mechanism, hydrochlorothiazide is the diuretic that decreases renal lithium clearance, and this is why doctors watch lithium levels closely whenever a thiazide is started in a patient already on lithium.

Let's summarize:

• Lithium follows sodium handling in the proximal tubule.
• Thiazides cause volume loss, which raises proximal lithium reabsorption and lowers its clearance.
• Loop diuretics and acetazolamide do not share this strong effect.

Note: this paper's original key names furosemide, but the interaction that is actually taught and clinically significant is with thiazide diuretics like hydrochlorothiazide, so that is the corrected answer here.