Pregnancy and T3 only - Possible Warning

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:

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????

34 Replies

Big babies often suggest a Hypo Mum :-) Mine were 9 and 9.7 - but did not know I was Hypo at that time - in fact not for another 40 years :-)

Interesting post - thank you.

My baby was 10.7 i did not know I was hypo till about 20 years later.

Gosh, I wouldn't have wanted her any bigger lol. My Aunty had gestational diabeties and had a 12 pound baby - naturally!! They didn't realise until she was half way out and very very stuck and too late to turn back lol. That was quite a number of decades ago though so I'm sure they'd be smarter about it now :-/

She could have been Hypo. Pancreas and thyroid - both Endocrine 😊

Yes, I believe she is to be honest although nothing diagnosed at mo apart from full diabeties twenty years later, her other daughter is also hashis as is my mum (her sister) and me. I became hypo in my teens but strangely the rest of them have all just gone hypo over the last two years at the same time lol??

This is reallly interesting, thanks for sharing x

Hi everyone,

I didn't know that big babies meant hypo mums. My three babies were big with the 1st being 8lb 9, the 2nd 9lb 5 and the 3rd being 11lb 8. I suspected I had hormone problems following the birth of my 3rd and final child but the medics couldn't really find anything wrong except my progesterone levels were not normal. I never got any treatment for my hormones until 4 years ago when I was diagnosed with thyroid cancer and now I have an underactive thyroid since my partial thyroidhectomy to remove the cancer.

As regards thyroid hormones in pregnancy, I've not done any research into this but I'm surprised by Saggyuk's post above. T4 is a storage hormone that the body makes and stores approximately 1 months amount in the body. It circulates throughout the body and converts to T3 in the thyroid and more so in the liver. T3 is the active hormone but as it circulates around the body it loses an iodine atom and becomes T2 (needed for metablism) and T1 (needed for the brain). As it is circulating in the blood it will pass over to the baby via the placenta, and as I understand it, it will support the baby until the baby's own thyroid is formed and starts functioning independently of the mother. So, I'm not sure how adding T4 during pregnancy would help. If you take T3 your body stops making T4 as it does not feel the need to make it because there is no need to convert it, especially so if you're on a high enough dose. I think Saggyuk for future pregnancies you should consult an endocrinologist with experience of treating pregnant mums should you decide to have another baby.

Have a good weekend everyone.

TT x

I'm sure I read on here once that T4 is converted to T3 by the placenta. But that still doesn't explain why a baby can't make use of T3. If T4 can pass through the placenta then surely T3 can too - it's a smaller molecule!

But I'm just hypothesizing, I have no actual evidence to offer.

Anyone on T3-only who wants to get pregnant could consider NDT if they are worried and need some T3 in their meds in order to function. There have been a few reports on here of NDT pregnancies that were successful when the same person on T4 only either couldn't get pregnant or couldn't stay pregnant.

Apparently in rats, it can cross the placenta, just not the fetal brain barrier? I am also not saying it would apply to humans, I just think I wouldn't risk it? NDT has both doesn't it?

I certainly am not advising anyone to stop their T3, I continued on my normal dose at the time throughout which was 50mcg, they may just want to add some T4 like I did. The T4 didn't change my TSH nor my T3 levels at all (which is why I'm on T3 only in the first place) so it was going somewhere else it appears lol?

Yes, NDT has both T4 and T3.

I do find the idea that T3 can't cross the blood-brain barrier rather strange, and I'm not sure I believe it to be honest. I've seen the same thing suggested in adult humans as well. But there are people on here who have no thyroid hormone production of their own and yet they do perfectly well on T3 only. So how does that work?

And if it is the placenta doing conversion of T4 to T3 in the case of a pregnant human, the fetus obviously does something with T3.

I'm not trying to persuade anyone to do anything by the way. I'm just musing. :)

Haha, I like musing lol

Umm, can't quite remember as was yesterday but I think it said only T4 would pass through the blood brain barrier and then be converted on a local level???? I think T3 could be used elsewhere but not the brain? - in rats at least - apparently lol! I'm assuming they had no issue with experimenting on rats in a way that would be shocking on a human - poor little babies :-(

I can only go by own experience, which has not been good when it comes to the brain since producing zero T4. I have looked at and tried to rectify everything else possible and in the last three years removed nearly all my lifelong health issues - apart from these new ones and have returned to square one which is the only thing that makes any logical sense? Time wise, symptom wise, blood test results wise. I suppose we'll soon see if my tsh is finally lowered on my next blood test and see what happens with me brain lol!


As I understand, both T4 and T3 can be carried across the blood-brain barrier.

However, this is an active process. It doesn't just flow like heat from hot to cold. Each molecule of thyroid hormone, whether T4 or T3, is individually carried through the barrier. There are things like MCT8 (Monocarboxylate transporter 8) and OATPC1 (Organic anion-transporting polypeptide C1) which do this function.

Questions exist as to how effective these are in a foetus. Are they fully effective from the very first cells that form the blood-brain barrier? Or do they become more abundant and effective as the foetus matures? What controls these processes? Do they transport T4 and T3 equally or do they "prefer" one over the other?

There is information about this, to some extent, from those who suffer Allan–Herndon–Dudley syndrome. In this genetic disorder, the MCT8 transporter does not work properly, and the results are severe. Ironically, given this post, it is the inability of the MCT8 to transport T3 which seems to be affected - though perhaps it also affects T4?

My point? It isn't at all clear what the best approach is for anyone taking T3-only who is pregnant.

Oh thanks, I've never heard of that syndrome before, I might take a look. That is interesting.

Yes, I am not at all suggesting this to be definite - as mentioned, this is in rats but it would make me not want to take the risk. Having looked at it back then, there was no research at all to help and this has been the first one I've found on the subject with any related research.

Thanks :-)

Just jumping in with something my endo told me recently: don't keep taking T3 if you become pregnant because they don't know the effects of it on the fetus.

I didn't ask him why or how but he works in an endocrinology clinic and was very insistent.

Yes, I am aware of this but obviously my endo is aware I literally cannot convert and it seemed nonsensical to take the normal route and put me on T4 as it would likely mean my body would not be able to carry a baby full term and even a large change like that might mess up my system even if just temporarily and cause miscarriage. We agreed to just kind of try and guestimate how much T4 a baby might require based on some certain facts, went a little on the higher side, and went with it and closely monitor for any tiny changes that might indicate something not right. I had bloods every month and many more scans than usual and we observed no changes to any of my results - nothing more than 0.1 anyway and nothing in the scans. This was all we could do and thankfully it was all okay :-)

Did your doctor give a reason for the warning of a T3 only pregnancy?

No, simply a lack of research and fear of the unknown. We looked into it but could only find research concerning T4. He wasn't as concerned as others and didn't suggest it as a warning and was more annoyed that his team had tried to force me onto T4 while he was on holiday as he knows it is a problem for me - it was a very unplanned thing meaning we had to make a decision fast lol! Suffice to say, I didn't listen to his team and was very glad when he returned :-)

Don't know the effects on humans! I wouldn't worry too much, they barely know that T3 exists and they are basically ignorant of most thyroid issues, so tend to err on the side of extreme caution.

"Don't know" is most definitely is not the same as "is bad".

As Saggyuk says, the change onto T4-only is quite possibly a far more dangerous step.

As many have said, T4-only is categorically not what we are set up to live on.

Also, "don't keep taking T3" sounds as if it means that even combination therapy is being decried.

Umm , yeah helvella, you can tell I didn't read the whole thing through hey!. Just quickly glanced at the rest of it and see where you got that from. I just can't read stuff online and take it in, especially complicated long stuff lol. Plan to print off and read properly at some point soon ;-)

I agree with you.

What we did during my pregnancy was decided upon by both myself and my endo because of lack of research/evidence at the time. From my understanding, most endos will force pregnant mothers off T3 in most cases because of lack of research but that would have made me ill. He is a professor as well so is up on his research most of the time lol.

I suppose this is more a possible warning for people who no longer make T4 or very little of it like myself at the time. Even when my tsh is raised to 30-50, I still do not make a tiny iota of T4 and they've ran my thyroid low with a tsh of 11/12 for six months now and still nothing so appears my thyroid is long gone now. Also before this, they have been unable to lower my tsh and hyper symptoms regardless of my T3 levels which have been as high as 9 and way over range - it appears my body still wants something and it's not T3. The endo and me having tried everything else over the past year or so, have concluded that the only possibility is that you do need a little T4 somewhere to some degree which I have just started.

The reason I've been researching it is because of the difficulties I've been having with certain areas of my brain and a few other matters since having no T4 which seem to coincide with some of the latest evidence I can find, that T4 does have a use of it's own in these particular areas and smaller less important areas such as hair growth. Apparently Rt3 is actually made from T4, and not T3 and this is what is used to stop too higher levels and balances the level in your system etc etc. I haven't finished reading up yet as just starting but will post what I find at a later date with more credible sources if I find anything useful.

If this does apply to humans, I suppose this would make more sense to me if I think that the brain, under construction at such an important time, might be really sensitive and possibly need controlled levels and maybe why it doesn't want T3 and a more controlled T4? Or maybe it's because T4 seems to have a function other than turning into T3.


Very interesting article, also for those not contemplating pregnancy! Ever since my diagnosis, 17 years ago, I have felt I needed some T4 along with T3, although T4 alone was never enough for me, meaning T4 must play a role on its own. It's difficult to find objective information about this, though, so thanks for the link!

Yes, I didn't think I need it either until the probs I've been having since producing none (none produced even with tsh of 30-50). Even with my T3 way above range (9), my tsh has continued to be raised suggesting it wants something else. I have also had problems with hyper symptoms and have had to be kept low with tsh of 11 for six months now because I can't up my dose. The stuff I can find about the role of T4 outside of just being turned into T3 seems to correlate with my new symptoms/issues. I've just started looking through but I can message you anything else credible I find later if you're interested?


Yes, please do:-)

All we can is be as healthy as we can be and make informed choices. Every pregnancy journey is different and offering knowledge and your experience is great.

I wanted to add I've 2 babies since being diagnosed as hyper and my FreeT3 was above range the whole way through both and actually still hasn't settled. However both boys are soon to be 2 and 5 and have no issues linked to my hyperthyroidism and were both large babies.

Hi saggyuk,

Thanks for posting this. I am on T3 only and am considering getting pregnant in the not too distant future. I was wondering about switching back to NDT which I did quite well on in the past as am worried about the effect on the baby. Will have a look at that paper. Thanks again.

Hi Blue Lagoon, I stayed on my usual T3 dose throughout, I just added T4 just in case for the baby so not at all suggesting you would need to change your normal meds, just might be worth considering your T4 levels I suppose - if you do produce some still etc? I had no issues with the pregnancy.

Good luck with it all whatever you decide though :-)

Saggyuk thank you for your post. I find this research dubious. There have been reports on Recovering with T3 group of members having healthy babies although they stayed on T3-only through their pregnancies. The idea that T3 isn't being passed on from mum to her baby doesn't seem to make much sense.

Yes but many of them would have been producing their own T4 as well regardless, I did for at least the first 10 years on T3 only - I still had some function left at first. this is more possibly a problem for those like me now who no longer produce T4 or not enough of it with little or no function left and it's these people that maybe need to be more aware that this could be an issue :-)

Well, I personally find it a bit odd that people choose to stay on T3-only if they feel well on T4/T3 combo. T3-only is more difficult to manage.

There are some people who cannot tolerate T4 medication at all - are they not supposed to have kids?

I think we will agree that more research is needed in this area ....although I cannot imagine an ethics committee, which would ever agree to carry out a study like that on humans. Unless it would be a retrospective study, I guess e.g. measuring how healthy are children of women who took T3-only during their pregnancy.

I'm glad your baby is o.k. ;))

Take care


Thanks :-)

I can't convert T4 so have no choice, we're just considering that I may need T4 for other things at the mo- not for conversion so have added a little. Yes it has it's downsides but better than the alternative.

Yes, definitely needs looking at more and yes, only option would be retrospective, don't think anyone would be signing up for anything else lol!

Take care too :-)

I suppose I would add that the reason I started thinking this way more recently was that Nature is generally very efficient and always knows what it's doing where we often don't lol. If nature has put T4 into our systems, then there must be a good reason for it even if we don't know it yet.

.........apart from Autoimmune antibodies - Nature can go take a blinking hike on that one, stupid bleep bleep bleeep !!!! ;-)

Very interesting post thank-you

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