I'll try and take a stab at this. Lets see... T3 is without a doubt a rambuntious hormone. If one takes it....and it feels good, it doesn't necessarily mean it will stay that way....or that more is better.
Here's why I think that is...and I could be wrong. I think it does build in the body's tissues....and may cause your t4 to build more than it would alone. So....once the stockpile hits the critical mass.....a few weeks..months later...TOXIC overload occurs. That is why, one should increase slowly, and wait...a few weeks before adding more. Then test...wait...add. You see, I think one has to just be more careful with it than t4.
Its like electricity, that harnessed properly will heat your house, cook your food....while inside a lightening bolt, burn your house down. Its all in the handling. Now...once you hit OVERLOAD....of course you can't keep putting some in...you are already saturated, and your body will revolt. You have to wait till some of the stockpile gets used before you can add again. Doesn't mean it was bad...its just something that requires proper respect.
Although...I am a Armour loyalist here, I beleive the same thing applies to Armour for the same reason. If you ask me the reason why Docs like the "T4 only" protocal is that they don't have to apply their thinking differently to the t3...which so clearly playes its own perfectly particular game in your system.
Hope that helps