Amiodarone is an extremely useful and fairly well tolerated anti-arrhythmic agent that prolongs repolarization and refractoriness in both atrial and ventricular tissue.  However, Amiodarone has potential adverse effects including corneal microdeposits, photosensitivity, pulmonary toxicity, QT prolongation, and thyroid dysfunction, among others.  Amiodarone commonly affects the thyroid in up to 33% of patients and can cause either hypothyroidism or hyperthyroidism.  Hypothyroidism is more common.  Amiodarone’s effect on the thyroid is complex and interpretation of TFT’s can be challenging.  Amiodarone has a high iodine content and can be directly toxic to thyroid tissue.  It can also decrease production of T3 (leading toward hypothyroidism, as T3 is the active hormone and T4 more a circulating storage reservoir) and decrease clearance of T4 (leading toward hyperthyroidism).  The average time to onset of thyroid dysfunction is 3 months.  Amiodarone is extremely lipophilic and has a very long half-life, up to 180 days.  Thus, it can cause toxicity, including thyroid dysfunction, weeks or months after discontinuation of the medicine.  Stopping Amiodarone, after carefully weighing both pro’s and con’s, may not be the ultimate solution in the near-term as it may not reverse thyroid abnormalities any time soon given the long half-life.  Baseline TFT’s should be checked prior to initiation of Amiodarone, including TSH, Free T4, and anti- TPO ab’s (anti TPO ab’s can sometimes help differentiate Amiodarone-induced thyrotoxicosis type 1 (present) from type 2 (absent)).  Repeat TFT’s 4 weeks into treatment, and then every 3 months, in general.  Symptoms, if any, and severity of thyroid dysfunction, on interpretation of thyroid function tests, among other factors, should guide treatment.  Treatment needs to be individualized. Many choose to continue Amiodarone given the arrhythmia benefits and deal with the thyroid dysfunction with additional medication, if needed.  Bottom line:  Beware of the multiple toxicities of Amiodarone, including thyroid dysfunction.