Heart failure is one of the most known complications of hyperthyroidism, more commonly high-output heart failure, but some patients may develop dilated cardiomyopathy with low ejection fraction. We report a 35-year-old man, with hyperthyroidism, atrial fibrillation, and severe heart failure with 43% of ejection fraction. After the definitive treatment of the hyperthyroidism with radioiodine, heart failure was reverted, with symptomatic improvement and echocardiographic normalization including a normal ejection fraction (69%). There are several cases of reversion of heart failure due to hyperthyroidism treatment, but most of them with a high-output heart failure. Mechanisms by which hyperthyroidism can lead to heart failure and its treatment are discussed. We conclude that treatment of hyperthyroidism may reverse this thyroid related heart failure, even in severe cases with systolic dysfunction.