Thyroid hormones play an important role on the physiological chemistry of heart and vascular systems in healthy subjects. Any thyroid disorders accompanied with alteration of effective concentration of thyroid hormones cause heart dysfunctions. Thyrotoxicosis is a term given for the clinical manifestation of hyperthyroidism which can invoke heart and vascular abnormalities through the mechanism at heart muscle cells nuclear level. Thyrotoxicosis can play positive roles for heart disorders including atrial fibrillation, left ventricular hypertrophy and right ventricular systolic dysfunction, which are considered as major risk factors for heart abnormalities. Miscalculation of heart dysfunctions related thyrotoxicosis in cardiovascular patients might be avoided through careful laboratory measurements of T4 and T3 to exclude any possible thyroid hormone-related heart diseases.