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Introduction: It is well established that hyperthyroidism is a strong independent risk factor for atrial fibrillation (AF). However, the relationship between hypothyroidism and AF is complex and not fully understood. The abnormal thyroid hormone levels can disrupt the electrical signals that regulate heart rhythm, potentially leading to AF. Some symptoms of hypothyroidism, such as fatigue, weakness, and shortness of breath, can mimic the symptoms of AF. This can make it challenging to diagnose AF in individuals with underlying thyroid issues. Meanwhile the treatment of hypothyroidism in A patient with AF can significantly improve the symptoms. Clinical Case: A 54 years old female was referred to the Therapy department with fatigue and palpitations. She was diagnosed one year ago with paroxysmal tachysystolic atrial fibrillation. CHADSVASK-1, EHRA-3. She experienced paroxysmal episodes four to five times a month for which she took propafenone. At presentation echocardiography (ECHO) showed LVEDD-50, LVEDS-35, IVS-12, LVPW-11, LA-42, EF-57%. The blood sample revealed TSH-8.41 (reference range 0.3-4 mIU/l), free T4-1.15 (reference range 10-25 pmol/l), free T3-3.76 (reference range 2.5-5.8 pmol/l), AbTPO-1040 (reference range 2.5-30 IU/ml). On thyroid ultrasound examination dimensions of right lobe were 18 *23 * 39 mm, left lobe were 22*20*38 mm, isthmus 3.8 mm, and no nodules were present. Total thyroid volume was 19.5 mL. The gland had heterogeneous echotexture and normal vascularity. Treatment with levothyroxine 50 mcg was started. After 6 weeks of treatment with levothyroxine TSH improved to 3.8 mIU/l and patient's fatigue and palpitation symptoms significantly improved, and no new paroxysms were detected. Conclusion: Managing hypothyroidism through thyroid hormone replacement therapy can often help control and sometimes even reverse AF if the arrhythmia was related to thyroid dysfunction. However, some individuals, as in this case, may continue to have AF even after their thyroid levels are normalized. In this case we emphasize the importance of understanding the effect of hypothyroidism on AF. [ABSTRACT FROM AUTHOR]