SUN-363 Life-threatening arrhythmias secondary to liothyronine in a myxedema coma patient

Abstract Disclosure: J.P. Scarano Pereira: None. A. Sharma: None. C.M. Abe: None. M. Ramirez: None. Myxedema Coma is a medical emergency that occurs as a complication of severe hypothyroidism. We describe a case of myxedema coma in a 78-year-old female who presented with life-threatening arrhythmias after receiving intravenous liothyronine (synthetic T3). The patient had a long-standing diagnosis of Hashimoto’s disease managed with levothyroxine 50 mcg daily. However, she did not take her levothyroxine for at least 3 months prior to this admission. Due to progressive lethargy and confusion, the patient presented to the emergency room where she was found to be bradycardic (56 beats per minute), hypothermic (35.9 °C), hypotensive (87/47 mmHg), and in volume overload. Transthoracic echocardiogram revealed new anteroseptal akinesis as well as a reduced ejection fraction of 47%. Initial laboratory results were remarkable for an elevated TSH of 78 mIU/L, low free T4 of <0.2 ng/dL and low T3 of 32 pg/dL. The patient was immediately started on stress-dosed corticosteroids, which were later discontinued given a normal random cortisol level of 49.7 mcg/dL before the first hydrocortisone administration. Additionally, 200 mcg of intravenous levothyroxine was given to treat the myxedema coma. Initially, intravenous liothyronine was not administered given that ischemic heart disease had not been excluded. However, after no clear improvement in vital signs after 24 hours of intravenous levothyroxine, treatment with 5mg of intravenous liothyronine every 8 hours was initiated. Immediately after the third dose, the patient started experiencing multiple episodes of atrioventricular nodal reentrant tachycardia leading to severe hypotension which made the medical team discontinue the liothyronine. After three days of levothyroxine replacement, free T4 increased to 0.7 ng/dL, TSH decreased to 37.6 mIU/L and the patient returned to her usual baseline. She was scheduled for a stress test and a cardiac ablation upon discharge. Myxedema coma may be misdiagnosed in patients presenting to the emergency room and may have a mortality rate of up to 60%. There are no specific clinical practice guidelines on how to approach thyroid hormone replacement therapy in cases of myxedema coma. However, in high-risk individuals with cardiac comorbidities, replacement therapy should be used with caution since it may lead to arrhythmias and cardiac arrest. Ideally, intravenous levothyroxine should be prioritized over intravenous liothyronine given that this is a prohormone and the conversion to Triiodothyronine occurs over the course of hours. Presentation: Sunday, July 13, 2025