I've heard a lot of people with thyroid deficiencies tend to feel better on a t4/t3 blend rather than either hormone alone.
I've also heard a reason people feel sluggish/tired while taking t3 is because t4 is not only important in the conversion to t3, but that also other tissues in the body require t4 to work properly..... or something
As you can see, I'm not very knowledgable when it comes to t4... or even t3 for that matter. Im glad someone started this thread.