I've had Hashimoto's thyroiditis for about thirty years. I have now turned 60.
During the first seven years of my illness, I was on L-thyroxine (T4, Synthroid) only and my symptoms hardly changed at all, i.e. I still felt chronically hypothyroid.
I saw about five or six endocrinologists during those early years and received the same response: I was told my TSH, FT4 and FT3 were all normal and that my symptoms could not be due to the thyroid treatment - which was apparently fine.