Aim: Assessment of prevalence of thyroid dysfunction in patients with acute atrial fibrillation. Materials and Methods: A total of 110 patients participated in this cross-sectional study, admitted to the coronary care unit with AF not more than 48 hours from symptom onset and investigated by TSH, T3, and T4 to show how prevalent thyroid dysfunction in patients with acute AF, a study done at Al-Sadr Medical City – Najaf Al-Ashraf city, Iraq. Results: This study showed Prevalence of thyroid dysfunction was 21.7% which represents 24 patients that include (one patient with subclinical hypothyroidism 0.9%, 4 patients with subclinical hyperthyroidism 3.6%, 5 patients with hypothyroidism 4.5%, 14 patients with hyperthyroidism 12.7% and patients with euthyroid represent (N=86, 78.3%) of patients. Conclusions: For patients who admitted to the coronary care unit with acute atrial fibrillation both hypothyroidism and hyperthyroidism were frequent with the latest exceeding the prevalence in the general population necessitating screening for hidden thyroid dysfunction for early diagnosis and management to reduce morbidity and mortality.