I'm on 50 mcg of Synthroid. Symptoms were fatigue, uncontrollable weight gain, mental fog, etc... Of course, it is worth noting that, while in range, my Tetosterone was obviously low, for me.
Inital tests were - TSH = 3.98 and T4 = .78...they didn't do T3. I went on the name brand and stopped gaining weight shortly after. Switched to the generic and started gaining again. Went back to the name brand and stopped gaining again. I got re-tested, before TRT, and the numbers were - TSH = 2.44, T4 - 1.06, T3 = 4.37 (high).
Since I have been on Androgel, I had TSH tested again. 1st time (a few weeks after starting Androgel) TSH = 1.96. Last time (6 1/2 weeks after starting Androgel) I was at TSH = 1.56. I have another set of labs, at my new Androgel level, on 5/4.
The most obvious effect on me was the cessation of weight gain. Prior to, I had not been able to stop, even with dieting and a lot of cardio. Mental fog lifted just slightly, as did the fatigue. Both of those got better when I got on the Androgel.
BUT, since being on both, for over a month, I have found that I sweat more, my resting pulse is faster and I have more heart palpitations. My gut feeling is that I have moved from hypothyroid into the edge of hyperthyroid. Hopefully the tests will sort it out.
I never really "felt" a sweet spot. Things got moderately better. Of course, I had/have more than one thing going on.