As far as T3 vs T4 goes in terms of treatment is dependent upon how sensitive your body is. T3 has about two to three times the "strength" of T4, making it far more efficient but much harder to regulate. T3 also starts out as T4--from my limited understanding of the chemistry, the molecule loses an iodine (not sure where it goes lol) and becomes T3 anyways.  So by taking T4 as the standard treatment you're really giving your body the chance to use a process that is closer to normal--the thyroid puts out about a 4:1 of T4 to T3. However there are instances when T3 is prescribed, but I'm pretty sure it's when something has gone "wrong" (well, more wrong than having a non-working thyroid lol) and they just need to stabilized the hormone levels.  That's pretty much what I've got--my extremely limited understanding of the whole thing is from my grandmother and a friend from high school who had thyroid cancer her freshman year of college, so probably not the most technical or in-depth lol