There are really two separate processes going on here, as you mention. Normal thyroids (anyone remember those?!) produce all the T4 that we need plus a small amount of T3. The rest is converted, as you say, by the liver mostly, but also at many other sites throughout the body. Since T4 is the "storage" form of the thyroid hormones, it circulates in your body until your cells need T3 and then is converted to T3. T3 is quite quickly neutralized if it is not used promptly. So, taking T3 only has a couple of drawbacks. T3 is very fast-acting and eliminated quickly if not used, so it's very difficult to avoid "peaks" and valleys". T3 levels are regulated by the conversion process, so keeping conversion in place by taking some T4 maintains that "natural" process and helps to keep T3 levels more responsive to body demands.
You might see what you find in the archives concerning ERFA. Many of our members have struggled over the past year with the whole old Armour/new Armour/old NT/new NT mess. It's been a long year. Best of luck.
I was very weepy, despressed, almost suicidal, joints ached (left knee in particular), cold, hair falling out, massive brain fog, lots of mistakes at work. I had been on armour since my PT in 2006 and felt good sometimes and great sometimes and so-so sometimes. (Hashimotos' so the thyroid was dying) Armour changed and I felt worse. Changed to Naturethroid but didn't know I was supposed to take more to equal old Armour so still felt the same as on reformulated Armour. Took Synthroid to please my husband and mother who only know about it and convinced me I might feel better and couldn't fee worse on Synthroid). I did feel worse. Not sure if because Endo had me cold turkey NT and start Synthroid (so no T3 at all). I had a NT pills left and took 130 one morning when I felt like I was going crazy and within 2 hrs felt normal again. I even tried adding Cytomel but was too jittery. I was on 12-15-09 thru 2-12-10. Went back on NT and worked up to 5 grains which was a tad too much. I was on 4 grains 2-13-10 until 6-27-10. the 27th I started the reformulated NT and am headed down hill again. I'm giving it until the 24 when I have my next labs and will switch to ERFA (if I can get it) or compounded. I had 1/3 of my thyroid left after my PT but at the ultrasound on 1-14-10 it couldn't be found very much. Perhaps it atrophied because I was taking the Armour and it didn't need to work or because it died from the Hashimoto's? Either way I think I'll need T3 from now on and I know I feel much more human without it.
I can't answer this question, but can you tell me exactly how you felt on synthroid as far as the horrible symptoms, because i'm working on getting back to normal after my TT in Dec and praying that these hypo symptoms "leave"....