The thyroid works differently than the HPTA so terms like "shutting down" aren't really applicable. Any amount of T3 make the thyroid work less efficiently once you come off. (Which is why all the weight will return) If you use so much that the thyroid can no longer function on its own, there's a good chance you've caused irresperable damage. T3 is a big risk for a small payoff.
T3 increases nutrient uptake? Not really. It helps uultilize calories more effectiveley, obviously. The reason you go the bathroom alot is because the drug has the side effect of causing diarhea.
I'm not trying to scare anyone here. But that's the deal with T3. Don't kid yourselves otherwise. If you must use a thyroid med, I'd suggest synthyroid and in the wy that serge recommended. -- after bulking.