A 48-year-old man with bipolar disorder has been stable on lithium carbonate 1200 mg daily (serum level 0.9 mEq/L) for 5 years. He is now referred for routine monitoring. On examination, he has mild tremor, normal mental status, and no focal neurological signs. Laboratory investigations show: serum creatinine 1.3 mg/dL (baseline 0.8 mg/dL), eGFR 58 mL/min/1.73 m², TSH 6.2 mIU/L (normal <4.5), free T4 8.5 pg/dL (normal 8.5–18), fasting glucose 118 mg/dL, and 24-hour urine output 2.8 L (normal 1–2 L). What is the most appropriate next step in management?

Explanation

## Chronic Lithium Toxicity and Long-Term Monitoring **Key Point:** This patient has developed three well-recognized chronic complications of lithium therapy: (1) **lithium-induced hypothyroidism** (elevated TSH with low-normal free T4), (2) **nephrogenic diabetes insipidus** (polyuria with dilute urine, eGFR decline), and (3) **progressive renal impairment** (creatinine rise, eGFR 58 = stage 3a CKD). ### Chronic Lithium Complications | Complication | Mechanism | Incidence | Management | |---|---|---|---| | **Hypothyroidism** | Inhibits thyroid peroxidase; blocks iodine uptake | 20–30% | Levothyroxine; continue lithium if benefits outweigh risks | | **Nephrogenic DI** | Damages collecting duct; reduces aquaporin-2 expression | 20–40% | Amiloride (blocks lithium entry into collecting duct cells); NSAIDs contraindicated | | **CKD progression** | Direct tubular and glomerular damage | 10–20% | Reduce dose; monitor eGFR; consider switch if eGFR <30 | | **Fine tremor** | Dose-dependent; related to serum level | Common | Reduce dose; propranolol if symptomatic | **High-Yield:** Lithium-induced hypothyroidism and nephrogenic DI are the most common chronic complications. Both are partially reversible if lithium is discontinued early, but TSH elevation often persists even after drug withdrawal. ### Management Strategy 1. **Reduce lithium dose** — from 1200 mg to 900 mg daily to slow renal decline and reduce tremor 2. **Initiate levothyroxine** — for TSH >6 with clinical or biochemical hypothyroidism (free T4 at lower limit) 3. **Evaluate nephrogenic DI** — confirm with water deprivation test if needed; consider amiloride 5 mg daily (blocks lithium reuptake in collecting duct) 4. **Renal monitoring** — repeat eGFR and creatinine every 6–12 months; if eGFR falls below 30 mL/min/1.73 m², consider switching to alternative mood stabilizer 5. **Patient counseling** — avoid NSAIDs, maintain adequate salt and water intake, ensure compliance **Clinical Pearl:** Amiloride is the diuretic of choice in lithium-induced nephrogenic DI because it blocks the epithelial sodium channel (ENaC) in the collecting duct, preventing lithium entry into principal cells. Loop and thiazide diuretics worsen polyuria. ### Why Other Options Are Incorrect | Option | Reason | |---|---| | **Continue same dose + recheck TSH in 6 months** | Ignores active hypothyroidism (TSH 6.2, free T4 at lower limit) and progressive renal impairment; delaying treatment risks further decline. | | **Discontinue lithium + switch to valproate** | eGFR 58 is stage 3a CKD, not an absolute contraindication to lithium; dose reduction and monitoring are appropriate before considering switch. Valproate has its own hepatotoxicity and teratogenicity risks. | | **Increase dose to 1500 mg + add propranolol** | Worsens renal impairment and hypothyroidism; higher lithium levels increase toxicity risk. Propranolol alone does not address underlying complications. | **Mnemonic — Chronic Lithium Complications: THINE** - **T** — Thyroid (hypothyroidism) - **H** — Hyperglycemia (impaired glucose tolerance) - **I** — Insipidus (nephrogenic DI) - **N** — Nephropathy (CKD) - **E** — Encephalopathy (rare, chronic neurotoxicity) ### Monitoring Schedule - **Baseline (before lithium):** TSH, free T4, creatinine, eGFR, urinalysis - **6 months:** TSH, creatinine, eGFR - **Annually:** TSH, free T4, creatinine, eGFR, urinalysis - **If eGFR <45:** Every 3–6 months - **If eGFR <30:** Strongly consider discontinuation or switch [cite:Kaplan & Sadock's Synopsis of Psychiatry 12e Ch 32; Harrison 21e Ch 397]