Congestive heart failure is a recognised complication of uncontrolled thyrotoxicosis but isolated right heart failure is rarely seen in association with thyrotoxicosis. Two cases of right heart failure associated with thyrotoxicosis are presented. In a 45-year-old man with right heart failure, investigations for all common secondary causes of right heart failure were negative. The only concurrent disease identified was thyrotoxicosis. The right heart failure subsided after treatment of the thyrotoxicosis. In a 36-year-old woman, the right heart failure had two underlying causes, thyrotoxicosis and an atrial septal defect. Treatment of thyrotoxicosis alone resulted in improvement of pulmonary hypertension and right heart failure. Thyrotoxicosis should be considered as a possible cause of pulmonary hypertension or isolated right heart failure.