Yes, your natural thyroid production will decrease through negative feedback when you tack exogenous thyroid meds including T3. Whether your thyroid will come back to normal will depend on your particular situation- genetics etc. Most people will probably bounce back just fine. In terms of how long it will take to get back to normal, I'm not sure on this one. I would assume 8 weeks would be long enough to be sure your levels were coming back to normal but I think this will vary quite alot just like it does for testosterone levels in different people.