Toxic multinodular goitre (TMNG) is a thyroid disorder characterised by an enlarged gland containing multiple autonomously functioning nodules that produce excess thyroid hormones, leading to hyperthyroidism. It is clinically important because it often causes symptoms of thyrotoxicosis and may require specific management distinct from other causes of hyperthyroidism, such as Graves’ disease. A key complication of TMNG is the development of atrial fibrillation due to prolonged thyroid hormone excess.