You stopped the NDT too soon. As NDT contains T3, you should just leave a gap of 12 hours, so now you have a false low FT3. The actual level would be more like 5.346. So, just over mid-range. If you feel fine, it's good. But, plenty of room for an increase of 1/4 grain.
TSH is irrelevant.
FT4 is low because you're taking NDT.
Antibodies are negative, but one negative result does not prove you don't have Hashi's.
I take it that B12 test is an active test? If so, it's OK, could be higher, though.
Your rT3 is possibly creeping up due to your low ferritin. It's certainly not due to your FT4 level. Yes, increasing ferritin could make all the difference, as you say.
You should increase your NDT by 1/4 grain, not half a grain. Increases should be done in small amounts. But, only if you feel the need. If you feel good, leave your dose as it is.
How much vit D3 are you taking? And are you taking vit K2 - MK7 and magnesium with it?
How much B12 are you taking? And are you taking a B complex with it?
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