Deiodinase enzymes D1 and D2 convert T4 into active T3, while D3 converts T4 into reverse T3.

“the three most relevant are D1 D2 and D3 so let's talk for a moment about these three diogenases D1 and D2 are quite similar in that they both convert T4 into T3 more about that in a moment it's just where they do it that's slightly different D1 is extracellular it's on the cell membrane facing outward whereas D2 is on the membrane of the endoplasmic reticulum and it's facing internal to the cytosol but put that aside for a moment and just keep in mind that D1 and D2 both convert T4 into the active hormone T3 now D3 is different in that D3 takes T4 and makes something called reverse T3”