Reverse t3

Reverse T3 (rT3) is a biologically inactive form of T3(triiodothyronine). When T4(thyroxine) is converted to T3, "Reverse T3" is also generated. In the organism, T4 can be converted to rT3 to break down excess T4 that is functionally not needed. Increased rT3 in turn inhibits the enzymatic deiodination of T4 to T3.

fT3 and rT3 adapt competitively to the same receptors. However, when T3 binds to the receptor, it activates the receptor and thus initiates biological functions, whereas rT3 does not. High rT3 production can cause the symptoms of hypothyroidism despite normal thyroid gland values (receptor occupation by functionally inactive rT3).