Thyroid UK

T4 equal to what t3

H this may be a question for Rod.

My normal dose to have a correct TSH is 100 Mcr of NHS T4. What very roughly would that be in T3 terms. At present I am taking 22.7 of t3 twice a day but have just cut down to 15 Mcr of sustained release t3 :twice a day.

My maths tells me that t3 is 5 time more potent that t4. 15 x 5 = 75 MCR of T4? does that mean if I am taking that twice a day I am taking 150 of the equal amount of T4. Or does it mean I am only taking the equal amount of 75Mcr T4 and because T3 only has a very short half life the second dose is just filling my body with 75 MCR of equal T4. This is vey rough maths.


john C

14 Replies


I think the 3:1 ratio T3:T4 is more accurate than 4:1 or 5:1. On that ratio 33.3mcg T3 = 100mcg T4. It doesn't matter whether the dose is single or split and time release will even out the peaks and troughs of bog standard T3.


I am a great disbeliever in calculated equivalents. These calculations have their place - in providing a start-point guess and to start questioning if the amounts are too far out, but that is about it.

I tend to think of it in this way: If you take T4, one third converts to T3, one third to rT3 and one third gets excreted. Then allow these thirds not to be strict mathematical thirds but simply "three portions of vaguely similar size". That would suggest something like 35 micrograms of T3.

But sustained release is a very different beast. A lot of sustained release products do not fully release their dose - whether because some remains attached to the substance used to slow the release, or because it does not get absorbed in the same part of the gut. So you might need somewhat more to cover that.

My final guess (and that is ALL it is) is that 30 micrograms of sustained release T3 might be equivalent to a bit less than 100 micrograms of T4.

Worth thinking of the extreme - someone who cannot convert T4 to T3 at all. If they take 100 mcg of T4 then it does nothing and is equivalent to 0 mcg of T3. Even 1 mcg of T3 would be more potent. That is why equivalents can be so misleading.



The 3:1 ratio is based on equivalent effects on the pituitary by measuring TSH. It's possible that other tissues may convert a little less T4 to T3 and so the ratio would be slightly higher. This is a good measure because it doesn't rely on any measurement or calculation of absorption, free / total levels, receptor binding or whatever. It just measures intake and final effect. The amount you swallow against the effect in the (pituitary) cells.

Contary to almost every endocrinologist and alternative physician on the planet I don't believe sustained release T3 has much benefit and may have disbenefits such as unpredictable absorption / availability. This is because the half-life of T3 is not so important. What matters is how the body uses it. Thyroid hormone receptors require many hours of stimulation by T3 for expression. I have taken large doses of L-T3 twice daily and used a heart rate monitor (like what exercise nuts use) overnight and my heart rate was steady throughout the night, it didn't reflect the level of T3 hour by hour. As an analogy, you drink a glass of water and it's elimination half-life may be just a couple of hours or so. However, you body's response to it is evened out by all the processes that the water goes through.

Finally, rT3 is produced by type 1 deiodinase (D1) which produces one rT3 for each T3 it makes. D2 is now believed to be the major deiodinase and it only produces T3. Thus a lot more T3 is produced than rT3. The only exception is when serum thyriod hormone levels are high, then D1 expression is increased. D1 is upregulated by high levels of T3! This is not what one might expect and is probably a mechanism to protect against thyrotoxicity (by producing more rT3) and to allow recycling of iodine (the larger T4 molecule is not reabsorbed into the gut).


Thanks for this Jim. My experience supports this, taking 80mcg T3 in a single dose I have no noticeable symptoms over overmedication or thyrotoxicity, and have concluded that whatever excess T3 I have is either metabolised out or converted to rT3 as necessary. I think in effect that I am relying on the deiodinasing mechanism to properly regulate the T3 available. The research which Clutter references above would appear to support this. I should add a word of warning here to anyone considering this radical step, however - I have only done this as a last resort after being so unwell on T4, it has taken over a year to reach this apparently stable point, and I would never recommend this to anyone other than a completely thyroidless person with proven conversion problems, and that I am yet to meet an endocrinologist who supports this regimen.

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Hi Jim,

I think I understand what you are saying!! The bottom line is I have no thyroid and since this was removed in 1999, I have had a pretty poor life quality. The NHS want to treat me with Anti depressants , taking these you still have the same problems only you are a bit happier about it.

I was once told my Rt3 was too high and that was the cause of my problems. I have now been on sT3 since february. I have to say I feel a lot better but I know I cannot stay of this or afford to stay on this for the rest of my life.

One thing I have noticed that my body temperature on NHS t4 was not rise above 97F now its steady around the right mark. I have also noticed that where as before if I took t3 with the NHS T4 I came out in a rash and that has subsided. I agree this may not be the best way forward but I have tried every dr, Quack and suchlike and this is a clutch at straws maybe also used straws. I am hoping I can use Westroid and take some comfort and normality with this med.

Yours and thanks

John c


Are you sure 100mcg of T4 (or it's equivalent in T3, whatever that may turn out to be..) is enough, John? (see my response to Jim above for what I do now ) . I found that it took months for the adverse effects of years of T4 to get cleared out, and for a while the peaks and troughs of T3 were quite alarming, until I finally figured out I wasn't taking enough. I can now take T3 like I took T4, without worrying if I miss a dose, as the short half-life is irrelevant once you've got enough in the system. Best wishes Richard.


Sorry to jump in here but I was interested in all your comments because I had low DHEA and Adrenal issues as well as thyroid. I was on 125mcg of T4 and 40 of T3 and decided to try the CT3M approach (having read Paul Robinsons book ) and I dropped T4 and stuck with 40mgc of T3. I don't feel any different, perhaps slightly less tired maybe? but otherwise the same. The only significant difference is that hair loss which is my main concern has started to increase whereas I thought T3 helped that?

Now from reading these posts it would appear there is a school of thought that says T3 only could be a mistake because it isn't support or proven (and the fact that my hair loss is increasing worries me that T3 only may be the wrong path for me....

Or is there a counter argument that I just need to reintroduce a little T4 to get the T3 to work properly - I am very confused now on this - apologies


Hi I am not sure if I can answer your question.

I believe that 99% of the problems here on this board has nothing to do with Thyroxin. I am not saying its not thyroid related but its not the Thyroxin medication it self, either T3 or T4. I do believe that it is something else, what it is I have yet to find out. in my case I have no thyroid and if I take NHS t4 I get all the good signs of Thyroid issues cramps drepression pins and needles etc. I am hopping I can take NT i.e. westroid after I have completed a long course of T3. I also believe there is some link to adrenals. Vit d. Prenenalone, DHEA and in Ladies Oestrogen men totestorone. I went to a great dr who suggested that I take dhea with my drugs but that helps but seems to trick the body into thinking that you need more thyroxin hence you overload you body and get better that way.

One thing that works is to take 5 mg of pregnisalone and that works for me but long term use is dangerous I would not recommend this to you ..

I would try armour or Westroid NT, I have seen a video on You tube of two ladies that petition the scotch parliament and one of them was complaining about what you have mentioned.

One other thing you can try is to take you temperature with a mercury thermometer 3 hours after you get up and then three hour again and three hours again . It should be around 98.4-7. that way you can see how your body is performing and what is going on.

Too high a body temperature you body has not enough thyroxin or too high temp to much thyroxin.

There is a lot to take in here but I wish you well.

All I can say is Keep calm and smile.


john C


Thanks John, I have been on Armour on the past - it was ok but didn't resolve all the issues, particularly hair. Re temps I have been taking them in the morning and they are between 35.0 and 35.5 so sort off low really. However BP and Pulse are ok.

I agree with you that there can be other hidden issues, otherwise why is it so difficult to resolve. It's an exhausting search for wellness


Hi Liz,

there is your problem, your temperature. You must use a digital thermometer and take your temperature three hours after rising though to be sure.

I am not sure of your area but try and see jean hanging she is an expert with this .




Hi John - yes I have been using a digital thermometer and taking temp as soon as I wake and then 3 hours later and the temp is around 35 to 35.5 - rarely do I get to 36 if ever.

When you see the problem is the temperature - and I have read that basically when your temp is normal then you are pretty much ok - but just wanted to check if that is what you meant?

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Hi Liz,

my temperature for years was hovering around 36C 97F and I felt very lethargic most of the time(I have no thyiod) I was able until today to get this up to the right level but last night I took some Cq 10 and that has messed me up.

Before you go out and spend any funds. May I suggest the following housekeeping. Take your thyroxin in the morning with water a good half a glass or full glass. wait for at least an hour then have breakfast try keeping away from cereals or any milk related items. (if you can wait for two hours that is better. The if yo have any supplements to take, swap the times until just before you go to bed. Stop taking Calcium, soya , any antacids, Vit d. Then see how you go for a week and then see how you go for a month and make a note of your temperature. Ihave noticed that while on this T3 dose I am having bad hay fever and I am susceptible to any supplements as if affects my temperature.

Just a bit about your temp. years ago before the TSH test, they use to tell if you were on the right medication by your heart rate and BP and your temperature.

I hope that helps and try it. don't bother with your temperature on rising its always low.

What you may have is RT3 where your body is taking in Thyrxoin but its not being absorb .

Good luck

John C


Some people do seem to do better on a little t4 as well as t3 but I don't know if this is a counter argument as much as anecdotal evidence.

Like you I stopped taking t4 and am now on t3 exclusively and don't feel any better or in fact any different with the possible proviso that I may be less bloated (I notice less swelling in my legs and belly). This would be consistent with my initial experience when I first added t3 to my t4 and almost all my swelling seemed to go almost overnight. Over the next year it seemed to fluctuate a little more and became more of an issue again and now it seems to have got better again. Possibly coincidence.

Your hair can fall out if you're undermedicated or overmedicated, so is it possible you're on a little too much t3? My hair began to grow back when I was finally on enough t4 and continued to do so maybe with even more vigour when I added t3, but it has never really stopped falling out. It is now as thick as before dx, but with all the little regrowing hairs all of different lengths waving around my head I look a bit mad and it's hard to make it look tidy.

How are your levels of d, b12, ferritin, folate etc? And what is your gut up to (under or overactive)? Pulse?

This whole thyroid palaver is a big mess and very confusing, so rest assured your response is completely normal! :-)


Thanks for your help - I always feel a bit thick trying to understand all this, it is beyond me! I have been on Armour and then on T3 and T4 now just T3. I have no idea id I am under-medicated or overmedicated, I have no idea what to try next, I have had periods where hair loss is normal but has just never regrown. However around November last year I noticed hair line receding again (I am female in 40s) and that hair loss increase was on T3 and T4. So I decided early this year to try T3 - it hasn't stopped general increased thinning.

Completely out of ideas now...


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