A 52-year-old man with bipolar disorder has been on lithium 900 mg daily for 3 years with good symptom control. He presents to the psychiatry clinic for routine monitoring. Which is the most common cause of lithium toxicity in chronic users on stable dosing?

Explanation

## Lithium Toxicity in Chronic Users **Key Point:** In patients on stable lithium therapy, the most common cause of toxicity is **dehydration and reduced renal clearance**, not acute overdose or drug interactions. ### Mechanism of Toxicity Lithium is a small monovalent cation that: 1. Is filtered freely by the glomerulus 2. Is reabsorbed in the proximal tubule (competing with sodium reabsorption) 3. Has no metabolism — entirely dependent on renal excretion 4. Has a narrow therapeutic window (0.6–1.2 mEq/L) Any reduction in glomerular filtration rate (GFR) or increased proximal tubular reabsorption → lithium accumulation → toxicity. ### Why Dehydration Is Most Common | Scenario | Frequency in Chronic Users | Mechanism | |----------|---------------------------|----------| | **Dehydration / volume depletion** | Most common | ↑ proximal tubular reabsorption of lithium (sodium-lithium exchanger) | | Drug interactions (NSAIDs, ACE-I, thiazides) | Common but preventable | ↓ GFR or ↑ reabsorption | | Acute infection / fever | Less common | Fluid loss + reduced intake | | Intentional overdose | Rare in compliance-adherent patients | Acute event, not chronic toxicity | **Clinical Pearl:** Dehydration from any cause—diarrhea, vomiting, excessive sweating, reduced fluid intake in summer heat, or diuretic use—is the single most common precipitant of lithium toxicity in stable chronic users. **High-Yield:** Counsel all lithium patients: maintain **consistent sodium and fluid intake** (≥2–3 L/day). Even mild dehydration can raise serum lithium levels by 0.2–0.5 mEq/L. ### Monitoring Strategy **Mnemonic: SAFE LITHIUM** — factors to monitor: - **S**odium intake (must be consistent) - **A**cute illness (fever, diarrhea, vomiting) - **F**luid intake (≥2–3 L/day) - **E**lectrolytes (renal function, Na^+^, K^+^) - **L**evel (serum lithium: check at 5 days, then 6-monthly) - **I**ntake of NSAIDs / ACE inhibitors - **T**herapy adherence - **H**ypertension drugs (thiazides ↑ lithium) - **I**nfections (fever → dehydration) - **U**rine output (polyuria is a sign of chronic toxicity) - **M**edication review (drug interactions) [cite:KD Tripathi 8e Ch 12]