Mr T, a 40-year-old man with bipolar disorder on lithium carbonate 800 mg daily, recently started ibuprofen for back pain and has reduced water intake. What is the most likely cause of his thirst, polyuria, tremors, and fatigue?

• A. Interaction between lithium and ibuprofen causing elevated lithium levels
• B. Dehydration
• C. Diabetes mellitus
• D. Vitamin deficiency

Answer: (A)

Lithium toxicity is the key idea here. Lithium is eliminated by the kidneys, and its clearance can be reduced by NSAIDs like ibuprofen. NSAIDs decrease renal prostaglandin synthesis, which lowers renal perfusion and the glomerular filtration rate. That makes the kidneys excrete lithium more slowly, so serum levels rise. When lithium levels climb, you get symptoms such as thirst and polyuria from nephrogenic diabetes insipidus-like effects, along with tremors and fatigue from neurotoxic effects. The added factor of reduced water intake compounds the problem by concentrating the lithium further. So, the ibuprofen-lithium interaction causing elevated lithium levels best explains the thirst, polyuria, tremors, and fatigue. Dehydration alone can contribute, but the medication interaction provides the primary mechanism in this scenario. Diabetes mellitus or vitamin deficiency wouldn’t typically produce this exact pattern of acute lithium-associated toxicity.