Hi Abby, yes, your FT4 is on the low side and this is what your own gland is producing so why is it slowing and YET your FT3 going up even a little seems a sign the other factors are all good. You obviously are converting what T4 you have into a fair amount of T3. Sometimes T3 will pool in the blood but since you even feel all right, that probably isn't happening. It's difficult to pinpoint.
I've been showing videos by a functional medicine neurologist and he explains the 22 reasons for low thyroid function and I think he gets to the root cause for some people. There are several about antibodies which might possibly reduce output. They are only 5 min. videos. I'll link to ones I think may be your problem if you would like to check them out.
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