A 62-year-old male patient with obesity, hypothyroidism, and type 1 diabetes, with a body mass index of 31.2 (calculated as weight in kilograms divided by height in meters squared), and weight of 93 kg presented to the emergency department with palpitations, excessive sweating, confusion, fever, and hand tremors. The results of an electrocardiogram showed atrial fibrillation, and the patient received immediate treatment. Medical history included autoimmune hypothyroidism, obesity, and type 1 diabetes, which had been treated with 200-渭g levothyroxine daily, 10-mg tirzepatide weekly, and multiple daily insulin injections, respectively. At presentation, thyrotropin level was 0.001 mIU/L, and free thyroxine level was 7.26 ng/dL. At a recent physician visit 6 months prior, he was prescribed tirzepatide, 2.5 mg weekly, for obesity, and the physician suggested increasing the dose every 4 weeks as tolerated and following up in a month. At that visit, his body mass index was 44.4, weight was 132 kg, thyrotropin level was 1.9 mIU/L, and he received 200-渭g levothyroxine daily. He missed the follow-up visit because he lives seasonally in different states; however, the tirzepatide dose was increased as suggested every 4 weeks, up to 10 mg. He also continued tirzepatide, 10 mg weekly, while taking 200-渭g levothyroxine daily. His weight was reduced by more than 36 kg in 6 months. After further investigation, the origin of atrial fibrillation was determined to be thyrotoxicosis in the context of rapid weight loss from tirzepatide.