A 34-year-old woman with a history of bipolar disorder has been well-controlled on a stable daily dose of lithium carbonate for the past two years. She recently joined a local running club and developed bilateral knee pain. To manage the discomfort, she started taking over-the-counter naproxen regularly. Five days later, she presents to her primary care physician complaining of new-onset nausea, coarse hand tremors, and feeling unsteady when walking. Her vital signs are within normal limits. Laboratory testing reveals a serum lithium concentration of 1.9 mEq/L (therapeutic range: 0.6-1.2 mEq/L). Which of the following pharmacokinetic mechanisms best explains this patient's laboratory findings and clinical presentation?
- Decreased renal excretion of the psychiatric medicationAnswer
- BSuppression of cytochrome P450 activity in the liver
- CCompetitive displacement from circulating plasma proteins
- DUpregulation of hepatic metabolizing enzymes
- EEnhanced systemic absorption from the gastrointestinal tract
Answer
Decreased renal excretion of the psychiatric medication
The correct answer identifies that the interaction between nonsteroidal anti-inflammatory drugs (NSAIDs) and lithium occurs at the level of the kidneys. Lithium is a monovalent cation that is filtered by the glomerulus and extensively reabsorbed in the proximal tubule, closely mirroring the physiological handling of sodium. NSAIDs inhibit the synthesis of renal prostaglandins, which normally help maintain adequate renal blood flow. This inhibition reduces the glomerular filtration rate and triggers a compensatory increase in the proximal tubular reabsorption of sodium and lithium. Consequently, the renal excretion of lithium is significantly decreased, leading to systemic accumulation and toxicity, as seen in this patient's presentation with new-onset tremors and ataxia.
Step-by-Step Solution
Key Concept
Lithium and NSAID Pharmacokinetic Interaction