Introduction: Dilated cardiomyopathy (dcm) can result from a prolonged thyroid hormone deficiency. If pericardial effusion is the most frequent cardiac involvement; found in 30% of cases; myxedematous dcm is rare and therefore its frequency is difficult to determine. Clinical Case: We report the case of a 53-year-old patient admitted to the emergency room for management of an attack of global heart failure related to a dilated cardiomyopathy with severe left ventricular dysfunction secondary to a hypothyroidism discovered in the aftermath radiation therapy for breast cancer. The evolution was favorable after the administration of hormone replacement therapy with total disappearance of clinical signs, normalization of the thyroid balance and recovery of left ventricular function. Discussion: The clinical signs of post-radiation hypothyroidism are frequently blunt and unspecific. They can be confused with clinical signs and treatment of the disease as well as with its nutritional, physical and psychological consequences. The average time to onset of post-radiation hypothyroidism is estimated by the majority of authors between 2 and 5 years. The prolonged deficit in thyroid hormones leads to cardiovascular disturbances. This suggests that this deficiency changes the geometry, relaxation and contraction of the heart muscle. Dcm is therefore considered a rare presentation of hypothyroidism. Conclusion: DCM is a progressive dilation of one or both ventricular cavities with impaired contraction of the heart muscle. When it is idiopathic, it is usually an irreversible condition. If the cause is reversible like the case of our patient, the cardiac functions can be restored after treatment of the etiology. This is how we recommend the adequate and diligent treatment of any form of hypothyroidism in order to avoid the passage to the myxoedematous heart stage.