Last year, I had a positive thyroid test , it was a fasting test: done alongside my other tests TSH was well over that needed to diagnose hypothyroid ie above 10 mU/L and T4 below the range.
My doctor wanted to test antibodies before putting me on thyroxine so I had another test plus one for antibodies. This was just 2 days after the first one, the only difference was this was a bit later in the day and not fasting. The second TSH test was (from memory , that's why haven't put T4 results ) only about 6.5mU/L. The antibodies were high though.
So within three days , I had one TSH result showing clear clinical hypothyroidism, the second a level that might not get treated. This demonstrated to me that the results can vary a lot from day to day.(and time of day perhaps)
To be honest I didn't feel 'ill' other than a lack of motivation when I was tested in the first place and was very surprised by the results.
However, my understanding is that it is (like my T1) something that will not get better and I've had a scan of my thyroid which shows it is atrophied.
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It may be that for some reason you don't need as high a strength as you are taking. It seems to take time for it to 'build' up in the body so I assume it also takes time for the effects to decrease. Dosing also seems to be very individual, that's why they make thyroxine tabs in so many different strengths. I know a friend who has no thyroid gland at all but takes less than many people who still have one.
When I went onto thyroxine, my doctor stressed that if I felt a racing heart or jittery or any odd symptoms,(long list on the packet insert) I needed to phone her .(too much is as bad as too little)
I think that you should contact your doctor and discuss your medication with him/her
