Thyroid dysfunction can markedly increase cardiac morbidity and mortality. Identifying and treating thyroid dysfunction promptly is critical and may be particularly challenging in patients with underlying cardiac disease. Thyrotoxicosis often presents with cardiac manifestations. It is essential to establish etiology and to recognize if treatment can be deferred or if the condition is life-threatening. Instances in which urgent management is required include uncontrolled tachyarrhythmia, unstable angina, and thyroid storm. Similarly, hypothyroidism may require prompt diagnosis and management, especially in the setting of profound hyponatremia, respiratory failure, or myxedema coma.