SUN-329 - Methimazole Induced Angioedema: A Rare Adverse Drug Reaction

Introduction:
Angioedema is a rare adverse drug reaction (ADR) of methimazole use. We present a case of methimazole induced angioedema (MIA) in an elderly patient with toxic multinodular goiter (TMG).

Case Report:
A 92-year-old Hispanic female patient with dementia, hypertension, TMG status post radiation therapy and subclinical hyperthyroidism presented with angioedema after starting methimazole a day prior. On admission, she was tachypneic with facial/tongue swelling and dysarthria. Initial labs indicated TSH 0.01 uIU/mL (0.45-5.33), Free T4 2.53 ng/dL (0.82-1.77), Total T4 19.1 ug/L (6.0-12.0), rest of labs and imaging were normal. She was given diphenhydramine, epinephrine, dexamethasone, and tranexamic acid. Family informed about recent discharge from another hospital for pneumonia where lab work was noted for abnormal thyroid function tests and was prescribed methimazole. She was admitted to ICU for angioedema and airway monitoring. Ultrasound of the thyroid showed 2 cm cystic nodules in the left and right lobe. Thyroid uptake and scan reported a 9% uptake in 24 hours (normal 5-35%), with increase tracer uptake overlying the right thyroid lower pole which may be due to increased thyroid tissue in this region versus a hot nodule. Thyroid antibodies results were negative. During the hospital course, the patient's facial swelling improved and with no clinical symptoms of thyrotoxicosis. Low dose betablockers were advised due to her age and comorbidities, ENT evaluation was considered for definitive treatment, but family deferred decision. Patient was discharged with atenolol with outpatient follow-up.

Discussion:
MIA is a very rare but serious side effect, with only a handful of cases reported in younger adults and pediatric patients. Methimazole disrupts the enzymatic process mediated by thyroid peroxidase and inhibiting hormone production, with reported ADR of pruritus and arthralgias to severe agranulocytosis and hepatotoxicity. Angioedema is characterized by acute, localized, non-pitting swelling of subcutaneous or submucosal tissues, occurring as part of an IgE-mediated type I hypersensitivity reaction, with onset from days to weeks after starting methimazole, but our patient developed symptoms in 24 hours. Treatment includes antihistamines, corticosteroids, epinephrine and close ICU monitoring, with early administration contributing to prevention of airway compromise, an important consideration in the elderly. Current guidelines advise definitive therapy, either radioiodine ablation or thyroidectomy once the patient is stabilized, with beta-blockers used for interim symptom control.

Conclusion:
MIA is a rare but potentially life-threatening adverse reaction. This case highlights the need for prompt recognition and treatment with continuous ICU monitoring, with early detection and management reducing morbidity and hospital length of stay.

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