Lithium has been known as a second-line treatment for hyperthyroidism. However, it has a narrow therapeutic range especially in patients with impaired renal function. Toxicity can cause neurological, gastrointestinal, cardiovascular, and renal symptoms, including rare cases of renal failure needing renal replacement therapy. This case report highlights a rare instance of acute lithium toxicity in a patient with multinodular goiter and chronic kidney disease, following methimazole-induced agranulocytosis.

A 55-year-old Filipino woman with chronic kidney disease and multinodular toxic goiter who developed methimazole-induced agranulocytosis presented with altered mental status after pre-treatment with lithium. Laboratory tests confirmed elevated lithium levels, consistent with acute lithium toxicity. She developed acute kidney injury, necessitating urgent hemodialysis with hemoperfusion. After two sessions, her neurological status and renal function improved. The patient resumed pre-treatment with carbimazole and subsequently underwent successful RAI then maintained on levothyroxine for long-term thyroid management

Lithium toxicity is rare but can cause life-threatening complications, particularly in patients with pre-existing kidney disease. Hemodialysis remains the treatment of choice for severe toxicity, significantly improving patient outcomes. Lithium toxicity can occur even within therapeutic levels, emphasizing the need for careful monitoring. This case highlights the importance of clinical vigilance when using lithium.