Thyroid hormones are essential in maintaining and regulating the body's metabolism. Reverse T3 (rT3) is an inactive form of triiodothyronine (T3) that is formed particularly during periods of stress in the body. T3 is the most potent of the thyroid hormones. Approximately 85% of circulating T3 is produced by monodeiodination of thyroxine (T4) in tissues such as the liver, muscles and kidneys. Selenium and zinc are required for this monodeiodination. Under normal conditions T4 converts to both T3 and rT3 continually and the body eliminates rT3 quickly. However, under certain conditions, more rT3 is produced and the desirable conversion of T4 to T3 decreases. This occurs during periods of fasting, starvation, illness (e.g. liver disease) and particularly during times of increased stress. This becomes a vicious cycle as rT3 competes with T3 as a substrate for the 5'-deiodinase enzyme, inhibiting the conversion of T4 to T3. Therefore an increased amount of T4 is converted to rT3. An increased production of rT3 is often seen in patients with metabolic disorders. Measurement of rT3 is therefore valuable in identifying thyroid disorders. SUMMARY OF CAUSES Test Results Cause Increased rT3 Increased Cortisol and other stressors Increased Adrenaline Wilsons Thyroid Syndrome Euthyroid "Sick" Syndrome