I agree with Gimel as usual.
Question: When you site the dosages for the test date. Was that the dosage that you were on at the time of the test, or was that the recommendation at that date and the next test would indicate the blood levels from that dosage change?
It is almost imperative that you only make ONE dosage change at a time. That is change ONLY T4 dose, OR T3 dose, not both at the same time. As it is harder to see what is happening if you change both at the same time.
I would highly recommend that you start with only changing (increasing) the T4 dosage. I would further recommend that a consistent T4 dosage for simplicity for every day (7 days a week) makes sense if possible. Increasing the dose so that you are at least 50% which would be a FT4 level of at least 1.3. Gimels estimate for the total of about 650 mcg per week (100 for 6 days plus 50 Mcg one day ) makes sense. it may take a bit more but you have to work with the dose increments available. After you get your FT4 level to be mid range or a tad higher. Then see where the FT3 level settles at and more importantly what your symptoms are like.
Please note: Any T4 dose takes about 6 weeks to stabilize. as the half life is measured in WEEKS. While T3 dose is nearly immediate with a half life measured in HOURS. This explains why with each T3 increase in dose you feel well almost immediately, but the delay in the reduction of the T4 dose lags by a few weeks and over time you feel bad again. This is why I recommend stabilizing and focus only on the T4 dose. increasing your FT4 level should also help your Ft3 levels rise slightly due to conversion of T4 into T3. There is still some room for your FT3 levels to rise most likely before you MAY start to feel Hyper. If you do start to feel hyper, you can reduce your T3 dose.
I expect the reason your FT3 dropped is the continual reduction in your T4 med. In your situation, your T3 is dependent on your T4 level, since the T3 has to be converted from T4, plus any T3 med. When trying to eliminate hypothyroid symptoms, a good target for both FT4 and FT3 is mid-range. Both have to be taken into account because they both affect symptoms. FT3 directly. FT4 because it is converted to FT3. As your levels are adjusted toward those targets, you can determine the levels at which you feel normal.
Looking at your test history, it seems your best level to try and get your FT4 to mid-range would be 100 mcg for 6 days and 50 for one day. Along with that, I think it would be best to continue with the 20 mcg of T3, and see where both FT4 and FT3 are at next round of tests. I expect that you will need further increases in your T3 med dosage, but you can best determine that by how you feel. Be sure to give any new dosage some time to affect symptoms.
Also important for you are Vitamin D, B12 and ferritin. Your D is good. I suggest testing for B12 and then supplementing as needed to get it into the upper third of its range. Your ferritin is inadequate. It should be at least 100. Ferritin affects conversion of T4, and also when too low it seems to cause unwanted effects when raising FT3 levels. So I suggest supplementing with iron. A good supplement would be Iron + C, available inexpensively from CVS. One tablet, which contains 65 mg of iron, should be adequate. If it affects your regularity, take as much additional Vitamin C as needed to alleviate.
You can confirm what I have said by reading a link to a paper I co-authored. Here is a link.
Also, when you say you feel hypo, what symptoms do you have?
First thing I would like to know is whether you take your thyroid med the morning before the blood draw. Also were the blood draws all done about the same time of day? the reason I ask is that thyroid med should be delayed until after blood draw, in order to prevent fals high results. This is even acknowledged in the AACE/ATA Guidelines for Hypothyroidism.