SAT-599 Propylthiouracil-Induced Anaphylaxis with Subsequent Tolerance of Methimazole

Background: Although the thionamides have been associated with adverse effects such as cutaneous reactions, hepatotoxicity, and agranulocytosis, anaphylaxis is rarely reported (1). We describe the case of a patient with Graves’ disease, who developed anaphylaxis after treatment with propylthiouracil (PTU), and who subsequently tolerated methimazole. Clinical Case: A 38-year-old woman presented to the emergency department with worsening palpitations, weight loss, and tremor. She was found to be severely thyrotoxic with TSH < 0.01 (normal 0.27-4.20 mU/L), free T4 > 100 (normal 12.0-22.0 pmol/L), free T3 44.2 (normal 3.1-6.9 pmol/L), and TSH receptor antibody 4.0 (normal < 1.8 IU/L). Past medical history was significant for beta-thalassemia trait and two previous early pregnancy losses. Treatment options were reviewed, and initial therapy with an antithyroid medication was recommended. The patient was started on propranolol 20 mg PRN and PTU 100 mg PO TID, chosen because she hoped to become pregnant once euthyroid. She was discharged home with outpatient endocrinology follow-up. After the first dose of PTU, she developed acute onset tongue, lip, and facial swelling, with full-body urticaria. She immediately returned to the emergency department, where she became hypotensive and sustained a syncopal episode. She was treated for anaphylaxis with epinephrine, prednisone, diphenhydramine, and ranitidine. The case was reviewed with specialists from allergy, nuclear medicine, and surgery. In general, it is not recommended to use another antithyroid medication after a severe allergic reaction to the initial agent, as there is a chance of cross-reactivity. Thyroidectomy and radioiodine ablation were deemed risky due to severe thyrotoxicosis and the possibility of precipitating thyroid storm. For this reason, she was challenged with gradually increasing doses of methimazole under close monitoring. She tolerated 1 mg and 5 mg of methimazole while in hospital, and was eventually up-titrated to a total daily dose of 20 mg. There was a resultant reduction of free T4 to 29.9 pmol/L and free T3 to 7.83 pmol/L. Radioiodine ablation was arranged with discontinuation of methimazole one week prior. Thyroid uptake and scan showed 24hr uptake of 87.6% (normal 15-35%) and a diffusely enlarged thyroid gland. She was prescribed 370 megabecquerels of I-131, and was counselled to delay pregnancy for a minimum of six months after therapy and until normalization of TSH. Conclusion: This is the first reported case of anaphylaxis secondary to PTU and subsequent tolerance of methimazole. It highlights challenges that can occur in the management of patients with severe thyrotoxicosis, and the importance of multidisciplinary care in this situation. References: (1) Shtessel M, Toh J, Gavrilova T. Anaphylaxis as a delayed reaction of methimazole therapy. Ann Allergy Asthma Immunol. 2015;115:245-246.