The antiarrhythmic drug, amiodarone, induces thyroid dysfunction, which is potentially dangerous in cardiac patients. After discontinuation of the drug it takes several months before euthyroidism is restored. The potent antithyroid drug, potassium perchlorate (KClO4), is used successfully to treat amiodarone-induced thyrotoxicosis, but it is less well known as potential treatment in amiodarone-induced hypothyroidism. In this case report we describe the successful use of two courses of KClO4 treatment in a cardiac patient with severe amiodarone-induced hypothyroidism. The mechanisms responsible for the amiodarone-induced hypothyroidism and rationale for the use of KClO4 in this condition are discussed.