I hope this isn't going to take too long to explain. And this is really a frustrated rant about the obsession with TSH.
I went to see a private endo, as my hypo symptoms had not resolved on a daily dose of 150mcg T4 and a split dose of 40mcg T3, given to me by an NHS endo who was prepared to accept mono therapy on thyroxine was not working for me. I asked the private endo for a return to NDT, which had worked very well for me for 7-8 years, and he agreed, but on a much lower dose than my previous Armour dose and lower than my combined synthetic dose. His reasoning was that my TSH was too suppressed.
He told me to test levels 6 weeks later and to note any change in symptoms. Here is a (slightly shortened) excerpt from my email to him and his reply:
"Blood tests after 6 weeks on your prescribed Armour dose - 2.5 grains equivalent to T4 95 mcg and T3 22.5 mcg.
Results, June 2023:
FT4 9.9 (range 12-22), FT3 3.29 (range 3.1-6.8), TSH 0.007 (range 0.27-4.2)
For comparison, here are my previous blood test results when on my previous dose of synthetic hormones T4 levothyroxine 150 mcg and 40 mcg T3 liothyronine.
March 2023:
FT4 14.3, FT3 4.84, TSH 0.01.
I monitored my blood pressure for 4 weeks prior to the change of medication and then in weeks 2-6 after changing over to NDT.
Blood pressure on synthetic hormones averaged 129/85, pulse 75; blood pressure on Armour averaged 135/89, pulse 75.
As you would expect from the blood test results, symptoms of hypothyroidism have become significant, in particular extreme tiredness and brain fog, I've had to stop my physiotherapy exercises for quite some time and become very sleepy by 2.30 in the afternoon so take my second dose then. The reduction in dose has had a considerable impact on my life. Given the dose has been dropped by a third for the T4 element of treatment and nearly a half for the T3 element, this is not really surprising."
I asked for advice re: a dosage increase as I was feeling so poorly.
Here is his response:
"Regarding the results, the ratio of T3 to T4 in NDT therapies such as Armour often gives a low or just below normal free T4 and more normal free T3, with the low TSH suggesting your body still feels that the dose need to go down rather than up. With a suppressed TSH such as this being the pituitary glands way of trying to reduce thyroid hormone production.
In some cases the low free T4 can be improved with you taking a separate Thyroxine tablet with the NDT, and that may give more longer lasting thyroid hormone levels over the day as this lasts longer than the T3 which your body will make locally in tissue as you need it from the thyroxine circulating, and that may help with the brain fog you describe."
I am very sorry, but I was on the floor with those levels of FT4 and FT3. What is particularly notable is that both my FT4 and FT3 had dropped substantially simultaneously with the TSH, yet he seemed completely disinterested in taking that into account. Even more surprising given my hypo symptoms were so much worse.
During this time, my recurrent UTIs went sky high, both in pain and frequency, and I found myself on almost constant antibiotics. I also had a severe increase in histamine intolerance symptoms of my throat narrowing, my face, mouth, tongue and throat going numb, severe headaches and throbbing over my right eye.
Fearing an allergy to Armour, I came off it and returned to my previous dose of T4/T3 combination. I also put myself on a low histamine diet. Now I haven't had a UTI flare-up in 6 days, my histamine flare-ups around my head have subsided and my hypo symptoms have started to resolve.
Of course, due to taking actions on two fronts - removal of Armour and replacement with synthetic hormones, plus significant dietary changes - I can't separate cause and effect very easily. Was it that the dose was so low it kicked off my UTI/histamine responses, or was it a reaction to Armour that built up over 6 weeks?
When I was on NDT for 7 years, much of that time on Armour, I didn't experience this response, but I was on a higher dose and my FT3/FT4 levels were at the top of the range, not at/below bottom. Can two years away from Armour mean I am now allergic or was it just the low dose?
I've messaged the endo to say I won't be continuing with Armour and thanking him for his feedback.
My frustation is really around the fact the endo reduced my dose substantially when moving me from synthetic to Armour due to thinking my TSH was too low, but my TSH crashed even further on this much lower dose along with my FT3, FT4 AND symptoms. Surely, when this happens, they should be able to see that severe hypothyroidism can exist outside of a suppressed TSH?
I doscovered that I had functional b12 deficiency - tablets raised levels but did nothing for symptoms. 
Lab Results - High T3!
Very Low TSH With Low T4
