I worked with one man whose previous coach had him on 300 mcg of T3/daily in the of-season. You guys can probably guess the coach, as he recommends gross amounts of drugs to aspiring bodybuilders and has a history of trying to swoop up young talent by telling them he won't charge them for his services! Anyway, the first thing I did was remove ALL the T3 from his program. All of it. I anticipated at least some short-term crash (i.e. fat gain) as his body attempted to re-start endogenous production, but I wanted him to recover as quickly as possible (there simply isn't any reason to gradually reduce one's T3 dose, as the feedback loop for the thyroid is much more resilient than most other feedback loops). I did not expect what happened. Not only did not not gain any fat at all, but he actually gained 20 lbs of lean tissue in 30 days and his BF went down! He did this while cutting his steroid use dose to 1/3rd of what it was, as he was on atrocious doses at that time I met him.
I have since speculated as to what exactly allowed this guy to gain so much lean tissue in such a short period of time, while simultaneously lowering his BF%. In my opinion, there was probably some "over-reaching" going on, as his body had likely upregulated many of its muscle-sparring processes in order to hang onto lean tissue in the face of an overwhelmingly catabolic metabolism. Therefore, when that catabolic metabolism was suddenly removed from the equation, those muscle-sparring processes remained in high gear for a while, enabling him to achieve great growth over a short period of time. Of course, I do NOT recommend anyone abuses T3 for the purpose of trying to achieve something similar, but it is an interesting observation, none the less.
This experience showed me, quite clearly, the ability to T3 to absolutely crush muscle gains. Now, there is definitely some evidence to show that low-dose T4 may beneficial for growth, but as far as T3 goes, I am not in favor. We want normal levels of T3 when attempting to grow, not supraphysiological levels. There may be some exceptions to this, but if so, it is certainly not the norm.