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Difficulty: EasyPharmacologic Interventions

A 45-year-old woman presents to the primary care clinic for follow-up of elevated blood pressure. Over her last three visits, her blood pressure has averaged 152/94 mmHg, and a diagnosis of primary hypertension is established. Her medical history is notable for bipolar I disorder, which has been well controlled on a stable dose of lithium for the past four years. Her recent serum lithium level was within the target therapeutic range, and her comprehensive metabolic panel shows normal renal function. The physician decides to initiate pharmacologic therapy for her hypertension. Which of the following antihypertensive medications is most likely to decrease the patient's renal clearance of lithium and precipitate drug toxicity?

  1. HydrochlorothiazideAnswer
  2. B
    Amlodipine
  3. C
    Metoprolol
  4. D
    Prazosin
  5. E
    Clonidine

Answer

Hydrochlorothiazide
Lithium is an ion that is filtered by the glomerulus and reabsorbed primarily in the proximal tubule, much like sodium. Hydrochlorothiazide, a thiazide diuretic, causes initial sodium loss and volume depletion. This triggers a compensatory mechanism in the kidneys to increase sodium reabsorption in the proximal tubule. Because the proximal tubule cannot distinguish well between lithium and sodium, lithium reabsorption is concurrently increased, which can rapidly elevate serum lithium to toxic levels. For patients on lithium who require antihypertensive therapy, calcium channel blockers (like amlodipine) or beta-blockers are generally preferred.

Step-by-Step Solution

1
Identify the patient's underlying condition and current critical medication.
The patient has bipolar I disorder and is taking lithium.
Lithium has a narrow therapeutic index and its clearance is heavily dependent on renal function and sodium balance.
2
Evaluate the planned intervention and its potential interactions.
The goal is to initiate an antihypertensive agent while avoiding drug-drug interactions with lithium.
Certain antihypertensives alter renal hemodynamics or tubular reabsorption, inadvertently affecting lithium levels.
3
Select the medication known to cause clinically significant lithium retention.
Hydrochlorothiazide is identified as the interacting drug.
Thiazide diuretics cause mild volume depletion, triggering increased proximal tubular reabsorption of sodium. Because lithium is handled similarly to sodium in the proximal tubule, lithium reabsorption also increases, leading to toxicity.

Key Concept

Drug-drug interactions causing lithium toxicity (thiazide diuretics, NSAIDs, ACE inhibitors/ARBs)
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Topics

Pharmacologic Interventions

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