Peter Attia· MD
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