SUN-385 Not All That Mimics Cancer Is Cancer: A Case Of Methimazole-induced Pancytopenia

Abstract Disclosure: A.S. Alvelo Aviles: None. M. Gines Rosario: None. A.C. Franjul: None. Hyperthyroidism affects up to 1.4% of the global population, with Graves’ disease accounting for a 2% in women and 0.5% in men. Agranulocytosis is a rare adverse effect of antithyroid medications, while pancytopenia is even more uncommon. The prevalence of these hematologic complications is estimated to range from 0.1% to 0.5% among patients receiving antithyroid treatment. This can lead to life-threatening conditions, particularly in the setting of severe infections. Methimazole is a widely used medication in the management of hyperthyroidism, including Graves’ disease. We report a case in which an initially highly suspected hematologic malignancy, upon further evaluation, raised the possibility of pancytopenia induced by antithyroid medication.A 56-year-old female with a medical history of systemic lupus erythematosus and Graves’ disease previously on methimazole therapy, presents with fever, shortness of breath, and progressively worsening productive cough. She appeared critically ill—tachycardic, tachypneic, and requiring oxygen supplementation. Initial laboratory results were notable for pancytopenia: WBC count of 0.8 with an absolute neutrophil count (ANC) of 0, consistent with severe neutropenia; severe thrombocytopenia of 4,000/μL; and anemia of 8.0 g/dL. Thyroid function tests were within normal limits. Physical examination revealed generalized hematomas, and several non-painful, non-bleeding oral mucosa ulcers. Broad-spectrum antibiotics initiated and admitted to the ICU with a diagnosis of neutropenic fever and community-acquired pneumonia. ENT evaluation for glossodynia included nasal endoscopy biopsy confirming fungal infection. Further history revealed an ongoing workup for suspected medication-induced pancytopenia versus lymphoma. Bone marrow biopsy showed marked hypocellularity with plasma cell predominance, without evidence of malignancy—findings seen in methimazole-induced pancytopenia. Treatment with granulocyte colony-stimulating factor (G-CSF) initiated; however, slow bone marrow response, developing severe infectious complications, including bacteremia. Pancytopenia induced by antithyroid medications is a rare complication of an otherwise commonly used treatment. It carries a significant mortality risk and should be promptly considered and ruled out in patients presenting with new-onset pancytopenia. The underlying pathogenesis remains unclear, however current research points to an autoimmune mechanism involving interleukins. At present no clear consensus has been reached due its rarity. Furthermore, additional research into the pathophysiology of this condition is necessary to guide effective treatment. Presentation: Sunday, July 13, 2025