T4 is a relatively inactive hormone. T4 only becomes useful to the body after one of the D1 or D2 deiodinase enzymes converts it into the biologically active T3 hormone. Another deiodinase enzyme (D3), is able to convert some of the T4 into reverse T3 (often shortened to rT3). T3 cannot be converted into rT3.
The deiodinase enzymes can be upregulated or downregulated depending on the need to increase conversion from T4 to T3 or reduce it, and increase or decrease rT3.