Since I have always felt that taking exogenous T3 does not alleviate any of my symptoms and it cannot be compared with the way FT3 coming from FT4 conversion acts upon me, I have googled around for answers and found this piece of information at tiredthyroid.com
Local conversion (at the cellular level)
D2, another deiodinase enzyme, converts T4 to T3 within the cell. The level of available T4 affects the conversion rate. When T4 levels are high, there is less T4 to T3 conversion at the cellular level. This is why continuing to raise the T4 dose doesn’t seem to be effective after a while, and why adding some T3 instead may be more beneficial. T3 decreases D2 gene transcription, while it increases D1 transcription. However, T3 from D2 has two to three times the effect on gene transcription than T3 from D1. This means T3 converted from T4 at the cellular level has more impact than circulating T3. Someone taking only T3 only adds to circulating T3. There would be little T3 from cellular conversion, because there is negligible T4 for the D2 enzyme to convert.
Then another question popped up: does thyroidal T3 stay in blood only? By thyroidal T3 I mean the tiny amount of T3 produced by the thyroid ITSELF, and not coming from FT4 conversion; a healthy thyroid produces 85-100 mcg T4 and 6 mcg T3. I am talking about healthy people here, actually it does not matter.
That would mean that about 25% of the 85-100% T4 turn into FT3 inside the body, so at a cellular level a healthy person would have like +/- 25 mcg T3.
The rest 6 mcg T3 come directly from the thyroid.
Do these 6 mcg stay only in blood? For any other purposes? Maybe some organs which cannot convert FT4 into FT3 into their tissues will use that T3 from blood?
I haven't been able to find anything on this question of mine?