A 58-year-old woman on stable-dose lithium carbonate for bipolar I disorder develops gastroenteritis with vomiting and reduced oral intake over 2 days. She presents with coarse tremors, ataxia, confusion, and slurred speech. Which factor best explains this deterioration?

• A Gastroenteritis inducing hepatic cytochrome P450 inhibition
• B Thiazide-associated volume depletion raising proximal tubular lithium reabsorption ✓
• C Reduced dietary sodium causing increased renal lithium excretion
• D Vomiting increasing gastric absorption of lithium twofold

Explanation

Lithium is handled almost entirely by the kidneys, with about 80 percent filtered load reabsorbed proximally along with sodium. Thiazides, and any cause of volume depletion such as diarrhoea and vomiting, increase proximal sodium and lithium reabsorption, raising serum levels into the toxic range. The distractor in option C is wrong because sodium depletion decreases, not increases, lithium excretion; lithium is retained rather than excreted when sodium is scarce.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

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