Thyroid UK

Does our body needs T4 or not?

This is my first post, I don't know will I manage to explain properly what I would like to ask. I am taking T4/T3 therapy. I spent 3 months on only T3 cleaning my RT3. Now I am back on T4/T3 but I think I was feeling better when I was on just T3. I am confused about this subject. Does organism needs T4 at all or not? I have problems with converting it to T3, but does it have any purpose on its own, like just T4, when it is not converted? There are so many different opinions, so I am lost in them all. I know there are people doing well on just T3, but some are saying that they need some T4, can anyone please explain why there are so many opinions because I can't make up my mind what to take. Thanks.

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28 Replies

Hi 123-go

These are two excerpts which might explain better:-

"Thyroid hormone is comprised of two major parts – Thyroxine (T4) and Triiodothyronine (T3). T4 is the most abundant form of thyroid hormone in the body. It makes up nearly 80% of the thyroid hormone that is released. T4 is converted in the brain, thyroid, bloodstream, and other body tissues to the more active form of thyroid hormone – T3.

T3 is the real thyroid hormone that we are after to increase. People with thyroid disorders usually have one of two problems. They either don’t produce enough T4, or they lack the efficiency of converting T4 into the more effective T3. Whether you are prescribed T4 or T3 depends on which of these two problems you have.

Thyroid hormone is comprised of two major parts – Thyroxine (T4) and Triiodothyronine (T3).

T4 is the most abundant form of thyroid hormone in the body. It makes up nearly 80% of the thyroid hormone that is released. T4 is converted in the brain, thyroid, bloodstream, and other body tissues to the more active form of thyroid hormone – T3.

T3 directly boosts metabolism in those little cell powerhouses called mitochondria. It regulates fat, protein, and carbohydrate metabolism. The mitochondria are where fatty acids go to be metabolized.

So, our body produces thyroxine and converts to T3.

Levothyroxine is a synthetic thyroid hormone and is only useful if we are able to convert it into sufficient T3 (liothyronine) which is needed in every cell for us to function.

If our body is unable to convert it to sufficient T3 (liothyronine) we can replace the T3 directly.

So the answer is we can live without T4 as its pupose is to convert to T3. T3 drives our whole metabolism.


T4 is virtually inactive biologically. Its purpose in the body is to act as a control so that we only produce just enough T3 from it in the appropriate tissues and neither too much or too little. If T3 only was produced by the active thyroid and T4 didn't exist, then the body couldn't control the different needs of the different organs as all would have to put up with the same amount.


Brilliant response! Thank you. :)

123_go, there are differing opinions because people utilise and respond to T4, T3 or T4+T3 differently. I was well on T3 only, very unwell on T4 only, and I'm good on T4+T3. Overall I prefer T4+T3 because I think it is slightly smoother than T3 only, less peaks and troughs, and I like the security of having T4 in my system should I be unable to take T3 for a couple of days. If you felt better on T3 only perhaps the T3 dose in your T4+T3 combination isn't high enough and you need to tweak the doses, or perhaps T4 doesn't suit you.


I am not a medical professional and this information is not intended to be a substitute for medical guidance from your own doctor. Please check with your personal physician before applying any of these suggestions.


Thank you all very much for replying. My conversion is obviously not as it should be. But what I wanted to ask is does T4 has any purpose as just T4, not when it is converted to T3? This is the thing I am confused about. Why some people say that we need some T4? I mean if some like me have problems in conversion, do we still need T4 as just T4 for some reason. That kind of writings on the internet confuse me, they say that but they don't explain why.

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If you felt well on T3 alone you culd drop the T4 and see how you get on. Personally I wouldn't take levo if you paid me huge amounts of cash but there you go, each to their own! T4 lasts up to 7 weeks in the system so is nothing to worry about.

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Thank you. So T4 doesn't have any usefulness as it is? I mean when it is not converted to T3?

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T4 is a storage hormone and in my view its the one that causes most damage!! Personally I think this is what encourages the body to store fat!! I could be wrong but it seems logical to me.

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Glynisrose, read 'The Obesity Code@ by Jason Fung. It is insulin that causes us to store fat. Totally necessary but if you eat too much sugar and/or starch its stored as fat. Of course those of us who are hypo need less, but feel constantly cold and sluggish so tend to stoke up on sugar and starch in an attempt (vain, of course) to get going. Its all unconscious most of the time, of course. I'm not sure if there is also a direct effect on insulin production when we are hypo, I don't think the research has been done.

It is irrelevent. I have read loads more info and this is rubbish. T4 is only worth it if you convert it to T3. Things are differentif you are hypo!

You having a bad day Glynisrose?



123_go, you aren't likely to get people taking T3 only to agree that T4 has any usefulness so have a look at this link

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Yes, this is what I saw before and I don't understand it. Because it says that human organism needs T4 but there is no explanation why if it is completely inactive and doesn't have a purpose unless it is converted to T3. I would like that someone gives the answer from the science point of view on a very simple question - does T4 have any direct function in our body?



Aside from anything else, T4 is a relatively safe transport form of thyroid hormone. For example, T4 can be carried around the body, and through the blood brain barrier, without any likelihood of it directly affecting the tissue it travels near/through. Once T4 enters the brain, it can be converted locally into T3 - as needed.

(This is an idealised description - of course the real version is messier and more complicated.)

T4 is safe for storage and transport. T3 isn't.

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Can T3 pass through blood brain barrier? Some say it can and some that it can't?

I believe it can. Have posted this in the past. And how could we have so many people seeming to do well on T3-only if it can't? (Mind, I am wary, and certainly don't think that T3-only is right for everyone.)

But even though it can, it does not mean that it is the best and safest way of distributing thyroid hormone and getting it into the brain!

123_go, T4 wouldn't exist if it had no direct function. However, it is only useful if it can be converted to the active T3. If not, it remains a storage hormone.

Dr.Lowe made the analogy that a can of beans is food yet the beans are not nutritionally useful until they are unlocked from the can.

But does anyone knows what is T4 direct function? I am on both at the moment but I would like to understand things so I can make my decision, should I start taking only T3 from now on, like I did for 3 months, in case T4 doesn't have any function of its own, I mean when it is not converted to T3 and my conversion is not going well.

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123-go, There are some scholarly links here

I found a similar post and there are probably plenty more of them. And as always there is not even one scientific proof that T4 has any purpose of its own. I saw quoted on many places but woman who keeps this blog is neither a doctor or a scientist, just the person as you and me.

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Yes, it was my post. I searched internet for months but I wasn't able to find any serous readings explaining the purpose of T4 as it is, not when it is converted to T3. So, after posting this I decided to go on T3 alone. I am doing pretty well - no need for T4 for me at all.

It has long been claimed that TSH has absolutely no function or effect other than on the thyroid itself.

There is now a growing list of claims as to its direct effect elsewhere.

I'd most certainly NOT be claiming that thyroxine is not needed by any human body, anywhere, and under any circumstances.

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I have to agree with you adrewyy. Since nobody can provide a single scientific paper that T4 has some purpose in our organism as just T4 (not converted to T3) than you are right.

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There was a discussion regarding this a while ago and a link posted to this article,

That being said plenty of folk do best on T3 only, and there isn't any research into why that might be. Most likely the functions of T4 can, if absolutely necessary, be fulfilled by other means.

But synthetic T4 is not the same thing as what we have in our bloodstream. Its bound up in a different way, and therefore likely to be transported and metabolised slightly differently. So describing it as bio-identical, as the drug companies do, is disingenuous. In my case, levothyroixine did absolutely nothing for me, other than affect my TSH (see? there must be receptors!) But I didn't convert it to T3.  

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This sounds very logical. You just explained here something many wanted to know but could not figure out themselves. I must admit I never thought of it myself. Thank you.

I take only T4. It is bioactive in me in 5 minutes, sometimes less, and it hits its peak in me in under 30 minutes. If I am crashed it brings me back to a normal (chest pain/heaviness and twitches subside). If it too much at that moment, it makes my pulse climb and feels like I just took an amphetamine. A terribly high dose for me would blow my heart out. So to all who say T4 is not active, you have me as an outlier: either my body is converting it to T3 immediately or you must conclude that my body is using it as a direct source in addition to a catalyst/source of T3.

So is T4 needed? My advice is that each person's endocrine system is not operating the same, so answer the question based only on your personal experience: the breakages in our endocrine systems are at different points on the railway track.

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I read somewhere that we have T4 receptors, so it must be active at some level. And clearly it works for you!

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