Amiodarone is a widely used antiarrhythmic drug. It is related structurally to thyroid hormone and is stored in adipose tissue. The drug has a high iodine content as well. Taking amiodarone on an ongoing basis can lead to hypothyroidism by inhibiting deiodinase activity and by acting as a direct antagonist to T4 . In some cases, amiodarone-induced hypothyroidism resolves within a few months; however, in many, especially when accompanying anti-TPO antibodies are present, treatment with levothyroxine is needed. This can be easily monitored and adjusted. While answer a is a consideration, the patient likely needs the amiodarone given her tendency toward paroxysms of atrial fibrillation and the attendant risk of stroke. While starting a beta-blocker may prevent episodes of rapid ventricular response, this decision would be made with the help of her cardiologist, which is not mentioned. Anti-TPO antibodies increase the risk of hypothyroidism but are not necessary to guide therapy in this patient. Although prednisone may be used for treatment of amiodarone-induced hyperthyroidism, this patient has hypothyroidism, and prednisone is not indicated.