Background: Graves’ hyperthyroidism commonly causes tachycardia and may result in pulmonary hypertension and high output cardiac failure. There is limited information regarding the effect of treatment on cardiac function measured using modern echocardiographic techniques. Methods: Eight individuals with Graves’ hyperthyroidism, aged 22–64 years, underwent comprehensive transthoracic echocardiography at three time points: before treatment, two weeks after commencement of carbimazole, and at six months or more when euthyroid. Exercise capacity was assessed using the 6-minute-walk-distance (6MWT), and quality of life was assessed by Medical Outcome Study 36-item Short-Form Health Status Survey. Results: All individuals were rendered euthyroid by final assessment. At presentation, there was evidence of hyperdynamic right ventricular function as measured by peak systolic velocity of the free wall of the tricuspid annulus, tricuspid annular plane systolic excursion and right ventricular ejection fraction, which normalised after resolution of thyrotoxicosis. Baseline heart rate correlated significantly with severity of the thyrotoxicosis for either free T4 (r = 0.91, p = 0.01) or free T3 (r = 0.94, p = 0.001). No individual had measurable pulmonary hypertension. Cardiac output was significantly lower in the euthyroid compared to the thyrotoxic state (p = 0.03). A higher baseline TSH-receptor antibody titre corresponded to a greater improvement in exercise capacity (r = 0.76, p < 0.05) and physical quality of life (r = 0.73, p < 0.05) on resolution of the hyperthyroidism. Conclusion: Graves’ hyperthyroidism causes increased cardiac output and a hyperdynamic right ventricle which normalise on restoration of the euthyroid state.