Abnormal circulating reverse T3 concentration Elevated circulating reverse T3 concentration 2022-06-26T21:04:45Z Elevated rT3 Elevated circulating reverse triiodothyronine concentration Concentration of reverse T3 (3,3',5'-triiodothyronine or rT3) in the blood circulation is above the upper limit of normal. Reverse T3 is the third most abundant iodothyronine circulating in human blood and is produced by the inner ring deiodination of the pro-hormone thyroxine (T4). The number 4 in T4 means it has 4 iodine atoms. It can lose 1 of them, yielding either T3 or reverse T3, depending on which iodine atom it loses. Loss of iodine from the five-prime position on its outer ring yields T3, the most potent thyroid hormone. On the other hand, when T4 loses an iodine atom from the five position on its inner ring it yields reverse T3. Reverse T3 is inactive.