MON-407 The Double-Edged Sword of Methimazole: Managing Graves' Disease with a Twist

Abstract Disclosure: V.H. Patel: None. Background:Graves' disease (GD) is a common autoimmune disorder leading to hyperthyroidism, which can occasionally impact hematological parameters, including thrombocytopenia. The mechanisms contributing to thrombocytopenia in GD are multifactorial, involving thyrotoxicosis, autoimmune responses, and drug-induced effects. This case study explores a 40-year-old female with GD who developed symptomatic thrombocytopenia following initiation of methimazole therapy, presenting as an exacerbation of existing thrombocytopenia.Case:A 40-year-old female with no significant medical history presented with a 2-week history of difficulty swallowing, palpitations, weight loss, anxiety, hair loss, and insomnia. Physical examination revealed a moderately enlarged, non-tender thyroid gland with no bruit. Initial laboratory investigations revealed a platelet count of 97,000/µL, TSH of 0.10 µU/mL, free T4 of 6.01 ng/dL, and total T3 of 4.54 ng/mL. Thyroid ultrasound showed bilaterally enlarged thyroid glands with increased vascularity and no discrete nodules. Positive thyroid peroxidase (109 IU/mL) and TSH receptor antibodies (36.60 IU/L) confirmed the diagnosis of Graves' disease (GD). The patient was started on methimazole 10 mg BID and propranolol for symptom control.Two weeks later, she presented to the emergency department with epistaxis and a new bruise on her left anterior thigh. Laboratory findings revealed severe thrombocytopenia with a platelet count of 9,000/µL, alongside signs of improving hyperthyroidism. Additional workups, including vitamin B12, folate, coagulation profile, immunological tests, platelet function tests, and abdominal ultrasound, were unremarkable, ruling out other causes of thrombocytopenia. The patient received a platelet transfusion and was started on IV methylprednisolone, later tapered to oral prednisone. Given the temporal association with methimazole initiation and exclusion of other causes, methimazole-exacerbated immune thrombocytopenia was diagnosed. Endocrinology recommended transitioning to radioactive iodine therapy or surgery for definitive management of GD.Conclusion:This case underscores the rare but serious risk of methimazole-induced thrombocytopenia. While achieving an euthyroid state using medical therapy is generally expected to improve hematologic parameters in hyperthyroid patients, clinicians should remain vigilant for potential drug-induced thrombocytopenia, especially in the presence of new bleeding symptoms. Presentation: Monday, July 14, 2025