I have been on T3 only for well over 10 years and produced very little T4 of my own. Well under range anyway and apparently my thyroid died and stopped producing any at all either during pregnancy or shortly after. This was in 2010 and me and the endo scoured the research to see if anything mentioned the fetus being able to take T3 as I cannot convert T4 so didn't want to make myself ill during pregnancy which may risk it anyway. We could only find evidence of T4 being passed through and there was a lack of research on t3 only in general at the time anyway. I just didn't want to take the risk and I decided that I would add 25 or 50 mcg thyroxine just for the baby to be on the safe side. I can't quite remember how much but can find out exactly if you think it would be helpful. I didn't decrease my T3 dose and the added thyroxine didn't appear to do anything to my levels and remained stable with tsh close to 1 without any further changes through the rest of the pregnancy so must have been the right dose anyway.
Having had a few problems with my brain since stopping this added T4 dose and therefore having no T4 in my system, I've been reading up on the need for T4. I have just read a research paper in 2015 that states that at least in rats, the fetal brain is totally impermeable to T3 (cannot pass through) and all fetal brain T3 comes directly from T4. I know these are rats and it may be different for humans but I think it's evidence enough to really not take the risk. I wouldn't normally give warnings like this but it turned my stomach thinking about what the outcome might have been or what brain damage might have occured if I hadn't have made this decision and our babies/pregnancies are so important to us, I wanted to ensure it was known about just in case on the off chance people weren't aware of this possible issue.
I'm sooo glad I went with my gut!
Edit - apologies - link to research paper: ncbi.nlm.nih.gov/books/NBK2...
Edit - I've just noticed a few other posts regarding peoples concerns with T3 only and pregnancy with not many people being able to answer so I would also like to add that my little one came out 100% healthy (in fact a lot stronger than most new borns), was 8 1/2 pounds and has passed all milestones earlier than expected so certainly no damage. For anyone in the future, I am happy to provide all details of my pregnancy if you pm me and can find out what my exact levels were throughout if you would find it useful (did monthly blood tests)
I also find it interesting that precisely since I have had no T4 in my system, I have had 0% sex drive - not a flicker and I always had a high one even after birth so it might be possible that my body is aware that a pregnancy is not viable????