Thyroid UK
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Is FT4/FT3 ratio sometimes important? Does taking T3 enhance T4/T3 conversion? And in seek of advice?

I had been taking 200 mcg T4 (alternating 175/225) and +/- 18 mcg T3.

Due to deepening symptoms of hypothyroidism, I lowered my T4 from 200 to 150 on the 31st of January, then from 150 to 112 on February 1st, and then I stopped it completely and have not taken any T4 ever since. (I assumed I had a high rT3 due to my worsening hypo symptoms.) I went T3 only, and kept my 18-mcg dose.

My T3 dose has been the same (+/- 18 mcg) since January 15th, along with 200 mcg T4.

I was getting more and more hypo by the day (literally by the day) on 200 T4 and 18 T3. This is why I decided to stop T4 on January 30th and I lowered it to 150, 112 and then stopped it in 3 days.

The first 3 days (Jan 31st, Feb 1st, Feb 2nd) were very rough, however I seemed to notice a gradual improvement in my hypo symptoms, a reduction, which was weird enough, of course. Also I seem to be tolerating my T3 better and better. Before that the more hypo I was feeling, the less tolerance I had to T3. (It would give me an elevated heart rate and jittery feeling an hour after an 11-mcg dose.)

And today - Feb 4th - I am feeling very little hypo (I can so easily judge by the number of my skipped beats at least - my main symptom of low thyroid) and I took only 15 mcg T3 in the morning which resulted in absolutely no elevated heart rate, no shakiness...

I have mentioned before on this forum I had noticed I processed T4 very quickly.

Is it possible that my FT4 has fallen down to a more optimal for me level?

And it comes a time where the two levels (FT4 and FT3 - imagine they are lines) intersect, and the point of intersection is where I feel good at?

I am considering giving blood on Monday to see what my FT4 is. I might miss that level if FT4 falls further down. Maybe that is a FT4 level which divided by my FT3 level will result in a number less than 3. (Browsing through my labs I have noticed that I am not hypothyroid when my FT4 level divided by my FT3 level equals little less than 3 - 2.80 - usually.)

And maybe that is why I am tolerating synthetic T3 more now?

Is it possible that now with a decreased FT4 level and some exogenous T3 I convert T4 to T3 better?

I am really scared that this might be the answer and it may need a looot of fine-tuning of T4 and T3 dosages and I might skip the moment of the two levels (FT4/FT3) intersecting.

In this case, if I turn out to be right in my thinking, I need to reintroduce T4 again, at least 50 mcg of it, to keep my FT4 level "optimal" for me.

I was considering going T3 only for at least 6 weeks to clear excessive rT3 but now...

I am really surprised and at a loss.

34 Replies

You are on to something but to prove the theory you would need a home blood test lab and then test every day. Unfortunately we can't do that.

I was in a similar situation over 3 yrs ago on 150 t4 and 50 t3 and struggling. So I decided to experiment with t3 only. The most I could use was 55mcg. I went higher and was really ill. So the t3 was acting as t4 does and shutting down the system. Whether it is via the rt3 mechanism I can't say for sure. It could also be due to excess generation of cortisol blocking at the cell entry.

So I dropped back to 55 t3 and was ok but I wondered if I could add a basic t4 dose, so added 25 t4. I was very stable for a year on that dose but I thought could I use more t4 and less t3. So I slowly upped the t4 in 25 amounts and dropped the t3 to suit. I ended up at 125 t4 and 20 t3. Then a month or so in to this new dose (this is after 1 yr moving my t4 up and t3 down), I became ill again and was confused.

After a few weeks I just missed my morning t3 and was great at lunchtime. So my body was telling me I didn't need as much t3. I quickly stopped the t3 and was fine on 125 t4 for the last few yrs.

currently I have tried upping the t4 and have been having some problems. I have been on 150 t4 and it initially worked better for me but after 3 months I have started to feel more tired and my heart has some pain in that area. So I am going to be dropping back to 125 t4.

I have started vitamin d last autumn every day and I think this vitamin has boosted thyroid performance and it looks like a might need less t4.

It is complicated to get a balance and I think dropping thyroid meds are just as important as raising. The journey to get well not only involves thyroid tweaks but other aspects of metabolism, eg. Iron, b vitamin tweaks, adrenal tweaks, this involves vitamins, minerals and other hormones.

Many people fall into the trap of raising thyroid meds thinking more is better but this is not always the answer. More will make you more tired and hypo very often. If you don't have the metabolic performance to support more thyroid you just crash your system. A bit like using rocket fuel in a 50 year old mini car. The fuel has lots of power but the car cannot utilise it.


Your blood results will help guide you, but how you feel is more important. But I do like to see certain things in blood tests. I think ft4 in the mid to high teens is fine. And ft3 over 5. These figures correspond to the uk ranges used. Tsh 0.5 to 1 is great. It shows your thyroid is asking for more and conversion will still be happening steadily.

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I thought TSH is completely suppressed while on T3, except when the thyroid has atrophied and there is no T4 produced. Mine still produces T4, so I expect my TSH to be suppressed on T4+T3 or on T3-only.

I have also noticed, like you did, that FT4 around high teens or even 19 (12-24) is what I feel best at. Of course my FT3 has to be above 6-6.5 (3.5-7.8).


TSH will not be suppressed if using a small amount of T3. Read Dr Blanchards thyroid functional book on amazon. Very good read and simple. He shares his knowledge as he is close to retiring. His approach is a bit different to conventional forum treatment.

He says we need a TINY amount of T3 to be fully well, not these big doses. His ratio is about 98:2. So for every 100 T4 you take 2 T3.

He has found this works best for his patients. Lots of people will question this and i have posted about this on here before. I got an angry response about this from one poster.

I think if you are converting well and T4 is not quite doing it fully a small amount of T3 may make a difference. The big difference we have in the UK is that we cannot get slow release T3. He is using this product as he has it especially made for his practise. So slow release T3 in small doses might be a missing link for some people. We could try and replicate this effect with normal T3, taking very small chunks 2 times per day for example. 1/8th of a 25mcg tablet is about 3mcg. 1/16th = 1.5mcg approx.

It a new way of seeing things and certainly worth trying.

A small amount of T3 will not suppress the TSH, you will be complimenting your own production. I think this is the aim of thyroid replacement.

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It's a very interesting theory, and I'm not saying it's wrong because I have never taken such tiny amounts of T3, but if you use T4 + NDT, this would mean nobody should need more than 1/4-1/2 grain of NDT, along with T4...?

Anyway, it seems a health thyroid gland produces about 2% T3 directly, and the rest of the T3 you need comes from conversion...provided it works optimally.

But: is a suppressed TSH really a problem once you are on thyroid hormone replacement, if the only function of the TSH is to stimulate the thyroid gland to produce hormones? If the thyroid gland is unable to do that, then, does the TSH really matter...? I know the TSH is used by most doctors to determine whether you are taking enough, too much, or not enough thyroid hormones, but I have never understood WHY the TSH is a good indicator if it is no longer able to do what it's supposed to do...that is, make sure your thyroid gland produces enough hormones...?


It stimulates conversion of T4 to T3. It doesn't just ask for more T4 to be made.

So you can have a fully failing thyroid pushing out single digit FT4 numbers, the TSH is in the 50's and the patient feels totally normal.

This is because they have normal T3 levels in the tissues because the body is super converting the small amount of T4 being made.

This cannot go on forever, but people do go on for a good while and sometimes it only gets picked up via blood tests of issues not related to the thyroid. My mate gave blood and this is how he discovered he had a problem as they test your blood.

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I see...very interesting, indeed.

I found this information on the STTM:

"If you have low FT4 and a mid-range or slightly higher FT3, it usually means the T4 is converting like mad to give you the T3 you do have, which means hypo".

This is exactly how my latest labs looked on Thyroid-S...I have had a suppressed (<0.01) TSH for years. Before going on NDT, I had a GP who wanted a suppressed TSH in patients with Hashimoto's to keep antibody activity down, so my TSH has been suppressed for a dozen or so years.

But now, I am beginning to wonder if the info on the STTM means that, with a suppressed TSH, you may not be converting enough T4 to T3...? And that this in turn could mean that you need more T3, simply because you are not making enough due to your suppressed TSH...? I never made that connection, to be honest, I just thought the TSH's "job" was to make sure the thyroid gland produces T4 (and tiny amounts of T3).


I found this article very interesting; the occasional necessity of a suppressed TSH is discussed under the heading "Optimal treatment":

I also noticed the mentioning of thyroid hormone resistance in some patients, requiring them to take very high doses of NDT...


Hashimoto's is a tougher nut to crack compared to failing thyroid. I think the suppression is a good idea and this will lower antibody action.

I am not sure whether the quote from STTM needs to be interpreted as a bad sign. This is probably a normal state of affairs in many peoples bodies on the planet. They just start to get symptoms when old age comes along. But they have been going for years quite well with low FT4 numbers and decent FT3 numbers.

If you have this blood set up and issues, then time to take action.

In basic under active issues, if your TSH is suppressed on T4 only then i believe this tells us the issue is with the adrenal side of things. I had this issue for years on T4 only. It wasn't until i addressed my adrenals that my TSH popped up to 0.4. I was very happy to see this happen. This basically showed the thyroid hormones were now being absorbed much better into the cells and so clearance was higher. FT4 came down and TSH went up. The metabolic cycle had been repaired !

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Would you mind telling me more about how you addressed your adrenal issues? I have been diagnosed with adrenal fatigue and find it hard to wean off prescription cortisone.


Progesterone was the key for me. I use 6mcg per day, but i am male and so we don't need much. If you google the steroid hormone cascade diagram you will see how our hormones are made. They all start from cholesterol and it is the T3 which stimulates the cholesterol to break down into the steroid hormones.

HypoT patients have high cholesterol because they don't have enough thyroid pushing out the cholesterol into steroid hormones.

The first hormones we make are pregnenalone and progesterone. These then break down into the other main hormones. Many people find pregnenalone supplementation to be useful.

Again the key here is to use small doses. Some say pregnenalone in 100mcg per day is good, but i find 10mcg doses to be more effective.

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Thanks a lot:-)


I know that progesterone and FT3 work synergetically. low FT3 can lower progesterone and cause estrogen dominance in women.

I can feel this, my luteal phase's length is proportional to my FT3 level. The more hypothyroid I am, the shorter my lutheal phase is. Progesterone governs lutheal phase.

And low progesterone and estrogen dominance can lower FT3.

There was a doctor (I have to look up his name) who suggests that progesterone improves thyroid function a bit.


yes progesterone is pro thyroid action. It is breaking down into cortisol and so this aids T3 absorption. Someone has mentioned it balances high/low cortisol. So if you have high cortisol it helps lower it.

The main reason why women outnumber men with thyroid issues (10:1) is down to oestrogen. This is the most antithyroid hormone of them all. It interferes with conversion and loads more stuff.

Read the articles produced by Dr. Ray Peat in the USA. This guy is ahead of his time and has been saying what is pro metabolism for years. The main stream is only catching up. A lot of his stuff concerns diet, but he talks hormones as well. This link takes you to this article on these hormones, but you can read loads more about oestrogen, thyroid, cancer etc:

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I am beginning to wonder more and more if putting me on Estrogel five years ago (when I was 42), from day 5-25 of cycle, along with progesterone day 15-25, was the right thing to do...? I was told my estrogen and progesterone levels were lowish in blood and 24 h urine, but I have also read that you cannot just look at each hormone individually, but need to compare them to each other...and, quite honestly, although my doctor is a so called hormone specialist trained by the great T. Hertoghe, I have often been wondering since if I would have been better off without Estrogel...I am thinking about skipping Estrogel this cycle, and only take Utrogestan (200 mg) from day 15 to 25. I was told, five years ago, the everyone with adrenal fatigue needs both E and P, but I have begun to question that statement lately, for various seems I have a conversion problem, and I am trying to figure out what the reasons for this could be.


oestrogen interfere with conversion i think.

Here is another ray peat article:

I am not too up on the female relationship between prog and estro so hard for me to comment.

I do think self experimentation is the best way we can help treat ourselves however. This involves the simple step of removing or adding a hormone and vitamin and testing the outcomes for a few weeks. No doctor has the time do do this with us in the UK, and it would be expensive to do this with a private doc.

If you get a rough plan of how you want to test things out you then add/ remove/ lower / increase a hormone or vitamin.

Sounds like you have a plan, so give it a go. Monitor blood pressure, HR, and temperature as these indicators will help you understand if things are positive or negative.

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A very important point to remember is to make one change at a time and see it out for weeks, maybe a few months. Even if you add a basic vitamin to a new hormone you still don't know what is doing what fully.

I think this has happened to me very recently. The introduction of Vitamin D in the autumn has had a major effect on my thyroid hormones. I think i understand what has happened now and am taking steps to correct it. The vit D has improved thyroid performance and so i now need LESS T4. But instead of decreasing, i increased before christmas to 150 T4. I was good while 2 weeks and then still not right.

I have been experiencing a tighter chest and it has really come out in the last few weeks. Also i was consistently seeing my BP in the 130/70 area when i take it at bedtime. I just couldn't understand why the pressure had gone up.

Well now i know because too much thyroid meds will push up pressure (i also understand low thyroid will give higher BP as well). So it can all be a bit confusing.


Going slowly, small changes at a time is something I find very hard to do.

It is my unbearable skipped beats (20 000 a day when I am not medicated) that drive me crazy and drain me off. I sleep propped up since my heart skips every third beat when I am severely hypo. I hardly get any sleep. (There is nothing wrong with my heart! It's just my FT3 level!)

Another thing I've noticed is that I process T4 pretty quickly!

To feel the full effects of a particular T4 dose (either an increase - a decrease, or commencement - stopping T4) I need only 3-4 days. And I can feel that I have altered my T4 dose the very next day.

It is as if I am taking T3 instead of T4.

I have observed my symptoms on different T4 doses over a year now and know this for sure. Also I tolerate great amounts of T4 with no tissue over-stimulation at all.

In this case I don't know whether it is appropriate to wait for 6-8 weeks for a dose to settle. I have done that in the past and it made me really ill.


You sound the same as me. T4 acts like T3 almost. I can sense a change very quickly if i up or drop a dose. I think this is a good sign. I know i convert well so you are probably doing the same.

I also never get tissue over stimulation from T4 or T3. The opposite happens. I close down and go hypothyroid. My HR is always in the 40 area. 39 at nighttime. Does this mean i have a very good protective system in place ?

I always say to give a few weeks, maybe months between changes, but if you are in touch with your body you can choose your own time length.

However, it has taken 3/4 months for the vitamin D to really show up as having some positive effect on my system, so vitamins will be slower to show up.

Hormones are the main drivers to getting well and if you get settled on the right meds you can then tweak with additions or removal of vitamins. This process could take many years, but the key thing to watch is improvement over time. Once you get to a certain happy level life is ok. It's been that way for me for over 3 years and i am still making tweaks and changes.

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I am the same with the exception that I don't tolerate T3. I only tolerate it with T4 in my system but have never gone above 18 mcg T3 while taking T4+T3. If my T4 dose is too high and I am hypo I don't tolerate even an 11-mcg increment T3.

Same with a low T4 dose rendering me hypo - cannot tolerate 11 mcg T3.

The more T4 I take the more it shuts me down. So - same as you.

I don't think I am a good converter though. :(

However now that I stopped my 200 mcg T4, TSH must have gone up and maybe that is the reason I have felt an improvement - TSH enhancing T4/T3 conversion.

I am beginning to feel worse now, so - only 4 days with no T4 and I can already sense it...

My initial improvement lasted 1.5 day.

I am also wondering how come I process T4 so quickly when my whole metabolism must be slow due to hypothyroidism..

Are you really sure that 12-15 mcg T3 daily won't completely suppress TSH?

Also I am taking T4 +/-2 hours before waking. I always wake up at night. I guess it has to do with low FT3 levels or low cortisol, or just my mind knowing it has to wake me up to take my pill. lol.

Taking T4 that way has just been more comfortable. I had been taking it this way during pregnancy so that I didn't have to wait for half an hour or more before having breakfast. I keep taking T4 in that way and I take my T3 upon waking now - so +/-2 hours apart.

I am wondering what T4 and T3 dosages effects on TSH will be depending on the time of day they are taken and whether they are taken together or apart...

If I consider going partial thyroid replacement only, I must take that into account as well since I will need a good TSH level.


Exactly, and that is one of the reasons women on the pill may need higher doses of thyroid replacement hormones.

Excessive estrogen also raises TBG (Thyroxine-binding globulin) thus causing less free Ts in blood and cells.

I have never come across anyone with a high TBG due to excessive estrogen though.

Moreover, constantly excessive estrogen by itself is a very bad thing and, thank God, a rare occurrence I think.

One of the reason women outnumber men when it comes to thyroid issues is the upset balance between progesterone and estrogen.

Progesterone opposes estrogen and helps "tame" it.

When this balance is disrupted due to some reason, a woman becomes estrogen dominant.

That is why perimenopausal women are usually estrogen dominant, not estrogen deficient - because they start experiencing more anovulatory cycles/periods with age which:

1. lowers their progesterone - when there is no egg to be fertilized, there is no need for much progesterone to be produced since no possible pregnancy can occur;

2. disrupts the balance between estrogen and progesterone by lowering the later;

3. the follicular cyst that has formed due to the failed ovulation raises estrogen.

All these affect thyroid. That is why many perimenopausal women become hypothyroid at that time. These two hormones balance is greatly disrupted then and only aids hypothyroidism.


There is one question that has been bugging me for a while.

As TSH stimulates some part of T4/T3 conversion, does it mean that most of T4 from NDT goes unused and people on NDT feel better only due to the T3 in it - as TSH is suppressed on NDT?


No i think T4 is still being converted, just not at the higher rate you would get naturally if your TSH was 1 or 2.

NTH was no different for me from T4 only or T4 and T3.

This info is useful to read and talks about the enzyme activity which changes T4 into T3:


Thank you. Yes, it sounds logical, otherwise they would extract T4 from NDT.

Is it true that added T3 sometimes enhances conversion?


Not sure about added T3 improving conversion. It may be the extra T3 helps push along the T3 into the cells and so you get a better flow of hormones.

Or can the system get knocked off track and the new T3 helps reestablish natural patterns ?


Thат is a rather interesting theory. And I believe it is applicable to my case.

This theory could be said to unravel the mystery why I felt superbly good (not hypo at all) only 7 months ago during my pregnancy on ONLY +/-42 mcg T4 (alternating 50, 50, 25) with a TSH of about 1.6-1.9 and a low-normal FT4 level and a lowish FT3 (4.1).

However, we all know that results look different during pregnancy.

Otherwise (when not pregnant), I need an FT3 level of at least 6-6.5 and a mid-range FT4, and an FT4/FT3 ratio - little short of 3.

Whenever I would increase the T4 dose a little (up to 50 mcg), my FT4 would go up, my FT4/FT3 ratio would get skewed towards FT4, my TSH would go down and I would immediately start experiencing skipped beats - my main hypothyroid symptom. That was during pregnancy.

Currently, only 7 months later, when I take 42 mcg T4, my FT4 level goes high normal and my TSH goes down to 0.5-0.7, with my FT3 level at about 4.9. And I feel veery ill.

When I raise Т4 I feel better (little hypo) on a higher than 150 mcg dose but my well-being is only short-lived. I guess I reach something like a "sweet spot" at which FT4 is still converted to almost enough FT3 and beyond which FT4 starts building up and (maybe turning into rT3) and making me more hypothyroid.

If I lower my T4 dose to 37.5 from 42, I feel hypothyroid, don't get well.

I assume that during pregnancy the demand on my thyroid was greater. The foetus was draining some of mine FT4, also my whole body needed more of it.

That at least explains why my FT4 would go higher and TSH lower on 42 mcg T4 post pregnancy. There is nothing to "leash" my FT4 now.

So in order to replicate my pregnancy state now by taking 42 mcg T4, I would have to take some T3 along which would act as a "leash/brake" on FT4 and hold it mid-range or little over mid-range (the FT4 level I need when not pregnant); The added T3 would also boost my FT3 level little. However the T3 dose should be so low as to not fully suppress my TSH and hinder conversion. lol.

Sounds like a horrendously exquisite balance to be achieved.

All in all, as marsaday said - LESS may turn out to be better in my case.

I may be completely wrong of course.

Marsaday, if you want me to give you a SR T3 source, you can PM me.

If I fail to achieve this balance I will then need a full thyroid replacement dose of T4+T3.


It is useful that you have so much blood data. We just can't get this in the UK, unless we pay high costs. The fact that you have all this data will give you a bigger insight into what is happening in the body. So many people could work out these issues if we had cheaper access to blood tests.

I would read Dr Blanchards book because it would give you some more insights into what is happening.

So yes i would look at using a lowish amount of T4 and then supplement a tiny dose of T3. Start with an 1/8th of a tablet which is not too bad to cut up using a stanley knife blade.

See how one dose works for you on that morning and monitor signs and symptoms. Do you get some improvements ? The next day you could try adding another 1/8th in the afternoon. Always start with the lowest dose possible and work up or hold if things look positive.

I do think the balance between Ft4/3 and TSH is often over looked and have realised myself that nurturing this balance is the way to improve metabolism. I don't have hashimotos though and so i do think hash's sufferers have to do things differently than basic underachieve patients.

It will be interesting to hear how it goes. I think many people are over medicated on thyroid meds. If increasing and increasing what ever thyroid med you take is not working you need to reset everything and start again focusing on the basic measurement relationships.


I am form Bulgaria and I do all the blood work privately. I pay out of pocket. Insurance covers only an annual TSH test. lol.

I have Hashimoto's.


Ok so you have hashi's. It will be interesting to see if the approach of T4/T3 ratios and in lower quantities can help here.


Me to. Hope it helps, since I am tired of being so ill.


Thank you, Marsaday, I guess I have to apply trial-and-error approach, like you did. There is nothing else I can do...


Thank you for sharing this.

Your body uses T3. It has to work to convert from T4 to T3, and needs selenium and iodine to do so. There's a lot that can go wrong and sound like it has in your case. Also, T3 may not be evenly distributed or be evenly converted across your body. Kent Holtorf has a good write-up on thyroid conversion.

You may just do best on T3 only. I took it alone for 4 years and did just fine, taking in divided doses, 25-50mcg in both morning and early afternoon. Lots of people in the US are on T3 only, the UK NHS has made T3 difficult to test and to get because they stupidly contracted with one supplier who jacked up the price. It's quite cheap in the US and mist other countries.

You haven't mentioned whether your thyroid antibodies have changed or are a factor. If so, a gluten free diet is important.

And how are your adrenals? If they're a mess, hard to fix thyroid til they're tended to.

If it were me, I'd go by your fT3 level and your symptoms and dose T3 to where you feel good. Take a good multivitamin with iodine and selenium.

Once you're stable you can try switching to part T4, but from the sound of it, you aren't converting properly and T3 may be your answer.

NDT might help, too, but I'm allergic to the fillers in all the products, so can't advise.

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Learner, my antibodies have always been within range - Only twice did Anti-Tg go out of range and then came down by itself. I am on a gluten-free diet. It does not help at all. Makes no difference to how I feel.

I have tried going T3 only and it was a disaster! I could not tolerate T3 and felt extremely hypothyroid. The moment I brought T4 back into my system, there was a relief of symptoms and I started tolerating T3. Except I should not have raised T4 dosage so much I guess. It seems that being low on FT4 and being high cause the same symptoms, make me hypothyroid.

I do have a conversion problem. It's Hashimoto's causing it.

In Bulgaria no doctor checks out adrenals. My current symptoms are ascribed to depression. lol.

Оh, I forgot: I supplement many things.

And also making my body ready for iodine intake by taking selenium for the time being. I will take the chance to try and take little iodine.


It is very positive that you react well to T4's reintroduction. This is a clue as to how your body works. You need the T4. It is a case of finding the correct replacement amount and the mix with some T3. Set a plan out and record results. This is the only way to get well for some people. This is what i went through.

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