Something is wrong with the calculation. If your FT4 is 10 with a range of 12-22, it can't possibly be 20% through the range, it is below range so it will be in minus figures, it would be less than 0%.
Only you will know what balance of T4/T3 will suit you, it's a lot of trial and error. I take a combination of Levo and T3 (nothing to do with GP though), and I found, after many months of tweaking, that I am best when both my FT4 and FT3 are around 75% through range. When I lowered my Levo to a point where my FT4 was 12.8 (12-22) I couldn't function, and it took me quite a long time to increase back to where I felt reasonably well.
I had been tweaking Levo and T3 doses for months until 100 Levo/25 T3 seemed about right, but my FT4 was 19.5 (12-22) and I thought maybe too high so I reduced my Levo from 100mcg to 75mcg which resulted in those devastating effects. I increased back to 100mcg, keeping my T3 the same dose. It has taken a year but now I seem to need 112.5 Levo with 25 T3 and I feel my thyroid is stable on those doses, last test FT4 was 72% through range and FT3 76%. But those figures wouldn't suit everyone, it is all so individual.
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