Thyroid hormone (T3 and T4) has many beneficial effects including enhancing 
cardiac function, promoting weight loss and reducing serum cholesterol. Excess 
thyroid hormone is, however, associated with unwanted effects on the heart, bone 
and skeletal muscle. We therefore need analogs that harness the beneficial 
effects of thyroid hormone without the untoward effects. Such work is largely 
based on understanding the cellular mechanisms of thyroid hormone action, 
specifically the crystal structure of the nuclear receptor proteins. In clinical 
studies, use of naturally occurring thyroid hormone analogs can suppress TSH 
levels in patients with thyroid cancer without producing tachycardia. Many 
thyromimetic compounds have been tested in animal models and shown to increase 
total body oxygen consumption, and to lower weight and serum cholesterol and 
triglyceride levels while having minor effects on heart rate. Alternatively, 
analogs that specifically enhance both systolic and diastolic function are 
potentially useful in the treatment of chronic congestive heart failure. In 
addition to analogs that are thyroid hormone receptor agonists, several 
compounds that are thyroid hormone receptor antagonists have been identified and 
tested. This Review discusses the potential application of thyroid hormone 
analogs (both agonists and antagonists) in a variety of human disease states.