Reverse T3 (rT3)
Thyroid hormones are essential in maintaining and regulating the body’s metabolism. Triiodthyronine (T3) is the most active of the thyroid hormones. Approximately 85% of circulating T3 is produced by monodeiodination of thyroxine (T4) in tissues such as liver, muscle and kidney. Selenium and zinc are required for this process.
Reverse T3 (rT3) is an inactive form of T3 that is produced in the body particularly during periods of stress. rT3 differs from T3 in that the missing deiodinated iodine is from the inner ring of the thyroxine molecule compared with outer ring on T3. rT3 is measured by a blood test.
Under normal conditions T4 will convert to both T3 and rT3 continually and the body eliminates rT3 quickly.
Under certain conditions, more rT3 is produced and the desirable conversion of T4 to T3 decreases. This occurs during fasting, starvation, illness such as liver disease and during times of increased stress.
This becomes a vicious cycle as rT3 competes with T3 as a substrate for the 5-deiodinase enzyme. This inhibits the conversion of T4 to T3, with more T4 being converted to more rT3.
An increased production of rT3 is often seen in patients with disorders such as fibromyalgia, chronic fatigue sydrome (CFS), Wilson's Thyroid Syndrome and stress. Measurement of rT3 is also valuable in identifying sick euthyroid syndrome where active T3 is within normal range and rT3 is elevated.