You know I wondered the same thing and still do actually. My endo says all you need is t4 and your body will convert what it needs into t3. Ive read ALOT of comments from people that have had their thyroid removed and was prescribed t4 only and never felt good at all and when they switch to a brand that has both t3, t4 they feel much better.
I asked my endo and he is set on synthroid only...I may go to a different endo and get a second opinion