Reverse T3

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Abstract
Reverse triiodothyronine (rT3) is an iodothyronine produced mainly by enzymatic inner ring [5-]monodeiodination of thyroxine (T4). The 5-monodeiodinase (5-MD) enzyme is most abundant in skin, cerebral cortex, and placenta. In contrast to triiodothyronine (T3), which is the most active thyroid hormone, rT3 has little or no calorigenic or thyroid stimulating horomone suppressive activity. However, rT3 may exert other important functions, e.g., regulation of T3 production (from T4), by its ability to interact with and inhibit both type I and type II 5'-monodeiodinases. Some suggest that rT3 may influence brain development in the fetus. On the other hand, elevated rT3 levels may interfere with recovery from hemorrhagic shock. Serum rT3 levels are increased in systemic non-thyroidal illnesses except chronic renal failure. This increase is related to decreased type I 5'-MD activity and decreased metabolic clearance rate of rT3 in non-thyroidal illnesses. Serum rT3 levels is also increased in patients taking drugs such as amiodarone, propylthiouracil, dexamethasone, propranolol, and ipodate. This too is related to decreased activity of type I 5'-MD caused by the drug. High serum rT3, low T3, and low T4 are correlated with bad prognosis in systemic illness. (C) Lippincott-Raven Publishers.

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