Thyroid UK
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Question about GPs and amount of T3 that they will permit to prescribe

I have been allowed by my GP to try out T3 instead of the T4 I have been on for 17 years (100mcg).

I was previously referred to an endo who gave me T3/T4 but I didn't improve at all and for a short time felt worse - I was on only 20mcg T3 and 50mcg T4 but only for 6 weeks.

Then I read Paul Robinson's book and decided to ask if I could do a T3 trial along with the CT3M as I knew my cortisol was low. The endo decided to discharge me into the care of my GP which seems a bit odd as she knows less than him, but I was happy to take charge of this experiment.

I am 4 weeks in nearly and so far up to 27.5 mcg T3. I am phasing this is very slowly as per Paul's suggestion so I don't get hyper symptoms.

My question is this.......... I don't know how much my body will need and I have noticed on forums that some people are taking huge amounts of T3. And yet the endo has noted that T3 is equivalent to 4x the strength of T4. I think I have read though that an average dose is 40-60mcg of T3 ....... but even this is 160-240mcg equivalent of T4. So what is going on here, and how does one tackle ones GP if they argue that this is too much?

I am just worried that I may have this conversation soon and won't know what to say. So far there have been no wonderful breakthroughs either - slightly higher temps and BP, but I am clearly still hypo so cannot claim that it's making a huge difference yet to me.

How much do people on here take of T3 and did you have any issues over getting even 40-60 mcg amounts agreed?

15 Replies

The British national formulary says dosage of Liothyronine is up to 60 mcg a day. You might have a bit of a problem persuading the doc to go over, but you have scope to at least double what you take..... Anyone can log on to the bnf.... Worth signing up and printing the info.


What is the "bnf" please Rosee45?


British national formulary


Thank you for that info Rosee - just looked that up, and well worth knowing this. I will print it off and have it with me if she queries my dose.


The standard statements about how much T4 is equivalent to how much T3 are, at best, a very rough estimate.

In a "normal" human, approximately one third of T4 is converted to T3, one third to rT3 and the final third into other forms (sulphation, glucuronidation , etc.).

If you are taking levothyroxine, then some of what you take does not get absorbed. (We have very little decent information about absorption of T3. So it is difficult to be sure of how much is absorbed.)

So in total, you might need something like four times as much T4 to have a similar effect to T3.

I would rather keep away from the word "strength" - because in many ways T4 itself does not have any strength, it needs to be converted to T3 to be effective. So it is really "how much of the T4 is converted" that matters.

If anything affects the proportion of T4 that is converted to T3, then the equivalence ratio changes. We know all too little about the precise mechanisms which control conversion. Therefore it can be difficult to make an accurate assessment.



Thank you Rod, that is a helpful way of understanding it.

In fact from the sound of it, if I have been on T4 and perhaps not enough has been reaching the cells then in fact in theory one would need LESS of T3 rather than more compared with T4?

My concerns are that this is not going to work for me for other reasons - for example I see in Paul's book that there can be a host of reasons why T3 might not be fully utilised by the cells. I have had a mitochondrial function test done some years ago via Dr Myhill and I didn't come out well on that, so that worries me a bit. I don't know if T3 can put things right in that respect.

However, I didn't have a rT3 test first so maybe if there was a problem there, going on T3 will put that right.

Sorry this is going off on a tangent from my original question....


I think you are getting the real meaning!

And I suspect this is behind some of the apparent issues of excess rT3 and high T4 levels while still not being well. (But I am not one of the "blame rT3 for everything" brigade.)

I think there are many reasons for thyroid hormone not being used well/properly. Including genes.

If you click on the blue Reply to this then I will now if you respond to any of my posts. :-)


My experiences with T4 haven't been as dire as some people I think. However it has left me with some problems that have got worse with time I feel.

I am hoping that some time on T3 will sort them out (including possibly RT3 which I may have had in the past possibly, but find it hard to believe I have now).

If not, then I guess I haven't lost anything much though I may still not know why I have the problems! I may just have to learn to live with it perhaps. :)


Please keep in mind that research shows that FT3 does not cross the blood/brain barrier and FT4 is required for deiodination to FT3 by D2 enzymes in the brain

The latest guidelines from the European Thyroid Association THE USE OF L-T4 + L-T3 IN THE TREATMENT OF HYPOTHYROIDISM.which appear to have been adopted by the UK can be found at: -

The paper is interesting for what it says and the ridiculous conclusions it draws.

The FT4:FT3 ratio in people treated with levothyroxine monotherapy (L-T4) drops from 3:1 to greater than 4:1, which explains why people feel so bad on L-T4 monotherapy, yet it concludes this is irrelevant and people should not receive L-T3.

If they still feel unwell it is that they have a mental health issue.


A Better Link To The ETA Guidelines


Thanks for that MM. I doubt my GP will have read the guidelines, but my ex endo (decided to discharge me into care of GP who knows nothing!) might have done.

I am still struggling with the science part of thyroid issues, but understand more than a few months ago I think.........'Please keep in mind that research shows that FT3 does not cross the blood/brain barrier and FT4 is required for deiodination to FT3 by D2 enzymes in the brain' - in layman's terms does this mean that FT3 goes straight to the cells, and doesn't need the conversion that FT4 does?

I think I am getting some subtle improvements today, but I am crossing my fingers still as it's early days!!


Not convinced what MM is saying.

FT3 and FT4 are "free" as in not bound to proteins in the blood. But when you are talking about deiodination, that happens within cells. So the Free versus Bound T4 or T3 is not really a meaningful distinction.

The point is that some people think T3 cannot cross the blood-brain barrier (or the blood-cerebrospinal fluid barrier), so the only way to get thyroid hormone to the brain is for T4 to be transferred across these barriers.

Once the T4 is within the brain it can be taken into cells and deiodinated.

I believe that T3 CAN get into the brain. I find it impossible to believe that people with no thyroid, and taking T3 only, are surviving without any thyroid hormone getting into their brains. My suspicion is that the mechanism is simply not fully understood. Possibly it only happens in some circumstances? Or at quite a low level?

And yes, if T3 can get into the brain, then into cells, it of course does not need conversion because it already is T3! :-)



Well I am a complete newbie to thyroid research but a quick google brought up this research which seems to indicate that T3 can cross the blood/brain barrier. Since I am new to all this, I had no idea that it didn't :)

Also I suppose we don't individually know if our thyroid gland is still working - not sure how they test this? If I have been medicated for 17 yrs since I was borderline hypothyroid might it not still be capable of working a little and therefore some T4 to T3 conversion still happening too?

Apologies if my science is completely wonky here, but I find it really hard to understand this stuff!


T4 to T3 conversion takes place in many places - a bit in the thyroid, more in the liver, kidneys, heart, skeletal muscle, brain - and some in end-use cells like hair follicles.

Yes - this conversion process has to continue if we are surviving on T4-only.

And what a very good link! :-)



I am on T3 only and have been for over 2 years now. I feel well and I am on 60mcg per day. I was mostly well (95%) on 40mcg and the dose has crept up to 60mcg over the course of that 2 years. I take it in 3 doses per day. 20mcg just as I go to bed (as I did not get on well with the CT3M and this works for me). 20mcg as I get up and 20mcg at 11.30am. I also take supplements as I was deficient in iron and Vit D and other supplements to support my adrenals and general well health. My GP is happy I am on 60mcg as my good endo said I could go up to 60mcg. If I need to go higher then I would probably have to go back to him for approval.

I hope you start to feel some benefit soon, perhaps when you increase the next couple of times.


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