Abstract 13481: A Rare Case of Pembrolizumab (Keytruda®) -Associated Thyrotoxicosis Causing Atrial Dysrhythmias

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Introduction: Immune-Check-Point Inhibitors such as pembrolizumab (Keytruda®) are associated with the development of several immune-related adverse events. Case: 70-year-old malewith history of pleomorphic sarcoma of the right lower extremity with metastases to the lungs was started on Pembrolizumab 3 weeks before presenting to the ER with complaints of generalized weakness and palpitations. The patient was tachycardic and EKG showed atrial tachycardia at 144 beats/minute with right bundle branch block (RBBB) ( FIG 1B ). RBBB had been previously noted on EKG ( FIG 1A ). High-sensitivity-troponins were in the range of 29 to 37 ng/l. The TSH on admission was 0.01 mcu/ml with free T4 of 4.7 ng/dl (> twice the upper limit of normal). Of note, TSH was normal prior to the initiation of Pembrolizumab (2.01 ng/dl). Anti-microsomal anti-thyroid peroxidase (TPO) antibodies were also elevated. US thyroid showed heterogeneous thyroid parenchyma concerning for thyroiditis. The patient received multiple doses of IV metoprolol and IV digoxin for rate control before converting spontaneously to normal sinus rhythm, and was then started on metoprolol succinate to maintain the heart rate. Decision Making: Based on the patient’s recent history of initiating Pembrolizumab therapy and normal thyroid function before, his thyroid dysfunction was attributed to the immunotherapeutic agent leading to atrial dysrhythmias. Endocrinology team was consulted, and prednisone 40 mg daily was started. Repeat free T4 after 5 days came down to 2.5 ng/dl. The patient was discharged home on oral Metoprolol and slow prednisone taper in the next few weeks and outpatient follow-up with endocrinology. Conclusion: Thyroid dysfunction is one of the most common immune-related adverse effects of Pembrolizumab that can cause multisystem dysfunction such as dysrhythmias. Therefore, clinicians should be familiar with their unique adverse effects to minimize morbidity and enable appropriate management.

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