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 D3 deiodinase also blocks T3 access to the cell nuclei. The higher the D3 level the more difficult it is for T3 to work.