Methimazole-Induced Pancytopenia with ANCA Positivity: Diagnostic and Management Challenges

BACKGROUND Pancytopenia is an exceedingly rare adverse effect of antithyroid medications. It can be associated with both propylthiouracil and methimazole. While agranulocytosis is a more common adverse effect, pancytopenia has unique diagnostic and management issues due to its potential severity. CASE REPORT We present the case of a 36-year-old woman who presented to the Emergency Department with pancytopenia 2 months after starting methimazole for Grave's disease. Her case was complicated by severe candidemia and Serratia bacteremia. Laboratory workup revealed positive cytoplasmic and perinuclear anti-neutrophil cytoplasmic antibodies (ANCA) on immunofluorescence testing, with negative results on ELISA for MPO and PR3 antibodies. These findings were attributed to methimazole-induced ANCA positivity rather than primary ANCA-associated vasculitis. Methimazole was discontinued, and the patient was managed with aggressive antimicrobial therapy and supportive care, including blood transfusions and antifungal treatment. Despite the complexity of her condition, she survived after several months of intensive care and showed gradual improvement in blood counts and overall clinical status. CONCLUSIONS This case underscores the importance of recognizing potential iatrogenic causes of pancytopenia in patients receiving antithyroid medications. Early identification, prompt discontinuation of the offending agent, and comprehensive supportive care are essential for positive outcomes. This case also highlights the necessity for vigilant monitoring, patient education, and regular follow-up in individuals on antithyroid medications to prevent severe complications, morbidity, and mortality associated with rare but serious adverse effects like pancytopenia.