Reverse T3 (rT3)
Thyroid hormones are essential in maintaining and regulating the body’s metabolism. Triiodothyronine (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.
Do you know what Reverse T3 is?
Reverse T3 (rT3) is an inactive form of T3 that is produced in the body particularly during periods of stress. When the body is under stress, such as during a serious illness, it tries to prevent many tissues that depend on T3 from being metabolically active by producing more reverse T3 than T3. This is believed to be a way of conserving energy until the stress is relieved and it causes a syndrome called non-thyroidal illness (NTI). RT3 may also be elevated in hyperthyroidism. Use of the RT3 test remains controversial and it is not widely requested. Reverse T3 differs from T3 in that the missing deiodinated iodine is from the inner ring of the thyroxine molecule compared with outer ring on T3. reverse T3 is measured by a blood test.
Under normal conditions, T4 will convert to both T3 and reverse T3 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 reverse T3 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 reverse T3 is often seen in patients with disorders such as fibromyalgia, chronic fatigue syndrome (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.
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