Lithium — Clinical Use & Monitoring
Lithium's therapeutic range is one of the narrowest in psychiatry — a single missed lab draw or a patient's decision to skip water on a hot day can shift the balance from treatment to crisis.
Core Concept
Lithium is the gold-standard mood stabilizer for bipolar I disorder, effective for treating acute mania and preventing both manic and depressive episodes. It works by modulating neurotransmitter release and enhancing serotonin function, though its precise mechanism remains incompletely understood. The therapeutic serum level is 0.6–1.2 mEq/L, drawn 8–12 hours after the last dose (trough level). During acute mania, levels closer to 1.0–1.2 mEq/L are targeted; for maintenance, 0.6–1.0 mEq/L. Monitoring is intensive at first: serum lithium levels are drawn every 1–2 weeks during initiation and dose changes, then every 2–3 months once stable. Baseline and periodic labs include thyroid function (TSH, T3/T4), renal function (BUN, creatinine), and electrolytes — lithium is cleared almost entirely by the kidneys and directly affects thyroid activity. Adequate sodium and fluid intake are non-negotiable because lithium and sodium share renal reabsorption pathways. When sodium drops (from dehydration, low-sodium diets, or diuretics), the kidneys retain lithium in its place, driving levels up. This pharmacokinetic reality is the single most testable concept for this drug.
Watch Out For
Don't confuse therapeutic monitoring labs (thyroid, renal, lithium trough) with toxicity signs — monitoring belongs here, toxicity recognition belongs in the sibling atom. Students mix up lithium's trough timing: the draw is 8–12 hours after the LAST dose, not 8–12 hours after the first morning dose. Lithium is not an antipsychotic — it doesn't treat psychosis directly and isn't grouped with typical or atypical agents.
Clinical Pearl
Lithium follows sodium: when sodium goes down, lithium goes up. Think of them as roommates competing for the same seat on the renal bus.
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