PSAT335 Histopathological changes of the thyroid may be subtle yet important in Amiodarone induced thyrotoxicosis (AIT).

BACKGROUND: Amiodarone is an effective antiarrhythmic for several cardiac arrhythmias. Due to its high iodine content and long half-life with direct toxic effects on the thyroid gland, it is known to cause a spectrum of thyroid dysfunction. AIT type 1, a form of iodine-induced hyperthyroidism, and AIT type 2, a drug induced destructive thyroiditis, pose a diagnostic and therapeutic challenge. Since the physical and psychological consequences of long-term therapy with either condition are too large to ignore, it is crucial to differentiate between the two types. Histopathologic findings in AIT includes disrupted follicles filled with desquamated vacuolated epithelial cells, foamy macrophages, scattered lymphocytes, involution changes and fibrosis. However, these changes may be subtle in patients who have been treated with long term high dose steroids, but helpful in establishing either AIT 1 or 2. CASE: A 56-year-old man with a history of cardiomyopathy with an ejection fraction of 30% and atrial flutter treated previously with six months of amiodarone, presented for a routine appointment found to be in atrial flutter with rapid ventricular rate. The patient was off amiodarone therapy for ∼ 6 months prior to current presentation. The patient was referred to the ER where initial vital signs were remarkable for heart rate in the 140s. Exam was remarkable for mild tremor of the outstretched bilateral hands. Labs were remarkable for TSH of 0.01 ulU/ml (0.27-4.20), Free T4 of 2.2 ng/dl (0.93-1.70), Free T3 of 5.9 pg/ml(2.3-4.2). Thyroid ultrasonography revealed a heterogeneously enlarged thyroid gland with no increased vascularity, and 2.4 cm isthmus nodule of a TI-RAD-3. Thyroid stimulating immunoglobulins (TSI) of 96 (<140%), and thyroid peroxidase (TPO) of 2IU/ml (<9). Thyroid radioactive iodine uptake (RAIU) revealed a severely decreased uptake in the setting of amiodarone use. Mixed AIT was suspected and patient was placed on titrated dose of Methimazole (MMI) and Prednisone. Approximately three weeks later, patient was discharged on MMI 40 mg TID and Prednisone 60 mg daily. He had a close outpatient follow up with further titration of prednisone to 80 mg daily. Due to prolonged nature of disease, he was subsequently referred for total thyroidectomy. Histological examination revealed subtle changes including desquamated follicular epithelial cells and intrafollicular foamy macrophages. The thyroid architecture was maintained and no inflammatory infiltrates of fibrosis were noted. These findings are consistent with AIT type 2. CONCLUSION: AIT is a challenging diagnosis, and in medically refractory cases, total thyroidectomy is a curative option. The pathological findings in a patients treated with prolonged steroids may dampen inflammation of AIT2, and therefore careful histologic examination and monitoring of inflammatory markers may help to consolidate diagnosis. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.