A 28-year-old woman with a 6-year history of bipolar II disorder (recurrent hypomanic episodes and major depressive episodes) is being evaluated for treatment optimization. She has been on lithium for 2 years. Which investigation is most appropriate to assess long-term safety and guide ongoing therapy?

Explanation

## Long-Term Monitoring in Bipolar II Disorder on Lithium **Key Point:** Lithium is a narrow-therapeutic-window drug with significant organ toxicity potential. Regular monitoring of serum levels, renal function, and thyroid function is mandatory for safe long-term use. ### Lithium Monitoring Protocol **High-Yield:** The classic triad of lithium monitoring: | Parameter | Frequency | Target Range | Clinical Significance | |-----------|-----------|---------------|----------------------| | Serum lithium level | Baseline, 5 days after initiation, then every 3–6 months | 0.6–1.2 mEq/L (therapeutic); >1.5 = toxicity | Narrow window; toxicity risk with dehydration, NSAIDs, ACE inhibitors | | Renal function (creatinine, eGFR) | Baseline, 6 months, then annually | eGFR >60 mL/min/1.73m² | Lithium nephrotoxicity; chronic use → polyuric nephrogenic DI | | Thyroid function (TSH, free T4) | Baseline, 6 months, then annually | TSH 0.5–5 mIU/L | Lithium-induced hypothyroidism in 20–30% of patients | **Clinical Pearl:** In India, where iodine deficiency is endemic, lithium-induced hypothyroidism is particularly common. Baseline and annual thyroid screening is essential. ### Why Other Investigations Are Not Indicated ```mermaid flowchart TD A[Bipolar II on Lithium ≥2 years]:::outcome A --> B{Routine monitoring needed?}:::decision B -->|Yes| C[Serum Li level + Renal + Thyroid]:::action C --> D{Any abnormality?}:::decision D -->|Elevated Li level| E[Adjust dose or frequency]:::action D -->|eGFR decline| F[Consider alternative mood stabilizer]:::action D -->|TSH elevation| G[Add levothyroxine or switch agent]:::action D -->|Normal| H[Continue lithium, repeat in 3-6 months]:::action B -->|PET/Genetic/PSG?| I[Not indicated for routine monitoring]:::urgent ``` **Mnemonic:** **LITHIUM MONITORING = SLEET** - **S** = Serum level (0.6–1.2 mEq/L) - **L** = Lithium toxicity signs (tremor, confusion, ataxia) - **E** = Electrolytes & renal function (creatinine, eGFR) - **E** = Endocrine (TSH, free T4) - **T** = Timing (baseline, 5 days, then 3–6 monthly) **Warning:** Do not confuse bipolar II (hypomania + depression) with bipolar I (mania + depression). Both require the same lithium monitoring, but bipolar II has shorter hypomanic episodes (<4 days) and often more depressive burden.