A 52-year-old man on lithium for bipolar I disorder develops fine resting tremor, polyuria, and polydipsia. His lithium level is 1.0 mEq/L. TSH is elevated. Which complication of long-term lithium use does the elevated TSH indicate?
- A Lithium-induced nephrogenic diabetes insipidus
- B Lithium-induced hypothyroidism ✓
- C Lithium-induced hyperparathyroidism
- D Lithium-induced hyperthyroidism
Explanation
Lithium inhibits thyroid hormone synthesis and release and can cause hypothyroidism in up to 40% of patients on long-term therapy; elevated TSH with normal/low T4 is the earliest indicator. This is more common in women and those with pre-existing autoimmune thyroid disease. The polyuria and polydipsia reflect nephrogenic diabetes insipidus (lithium blocks vasopressin action on collecting duct), which is a separate concurrent complication. The tremor is a dose-related side effect. All three findings (tremor, DI, hypothyroidism) are classic long-term lithium complications.
Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.
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