Thyroid UK
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What's the difference with T4 and T3 medicine


I see lots of advice/info that T3 is X amount stronger than T4.

So, what's in the these tablets that makes a the difference?

What is actually in T3 tablets that makes it more powerful?

Is it a more concentrated form of T4?

I ask coz I'm about to start my T3 journey when it arrives. I've also cut back to 50mcg of T4 to prepare.


8 Replies

Mark, they're distinctly different hormone replacements. T3 is the active hormone we need and is 3 x stronger than T4. T4 consists of 4 molecules which are absorbed in the gut and converted to T3 and reverse T3 in the liver, kidneys, heart and brain. T3 has only 3 molecules which cuts out the conversion needed for T4 although it is still absorbed via the gut.


T3 tablets contain T3. T4 tablets contain T4.

In the body, T4 is converted to T3 - but, at the same time, some gets converted to reverse T3, and some is excreted. Rule of thumb - one third is converted to T3, one third to rT3 and one third disappears.

As it is the T3 that is used within cells, this conversion is a vital step. From the above you can possibly see that taking, say, 100 micrograms of T4, you might end up with 33 micrograms of T3. It is that which gives rise to the "three times as strong" idea.

Like absolutely everything to do with the thyroid, it really isn't that straightforward. Some people can only convert a little T4 into T3. If I suggest that for 100 mcg of T4, they produce only 10 mcg of T3 that makes it all look very different. In one way of looking, that would make it appear ten times as "strong".

Looked at another way, T3 has a function but T4 doesn't itself do anything except act as a relatively safe way of storing and distributing a source of T3 round the body.

I am seriously concerned at what you appear to be doing. There would usually be no reason to cut back on T4 before starting T3. Why have you done that? I appreciate that most people go to T3 because of some problem or other. But why would you want to make yourself distinctly low in thyroid hormone first?

Most people seem to try a combination of their T4 with a small amount of T3. Even if they end up taking T3 only, the idea is that a gentle, slow changeover allows you to stick at any point if you feel it is right, or go into reverse if you are feeling worse. Dropping T4 significantly before and then, I assume, trying to make up for that with quite a bit of T3 could be a rocky road. And if you have to reverse you have no clear place to go back to.

Take care.




Thank you for your reply.

For 10 years I was on 50mcg and soon realised this wasn't making a difference. My TSH was around 1.2 and FT4 11-13. I increased to 75, then 100 and then as high as 125. None of these levels made a difference . In fact, on 125 I felt toxic within and then dropped right back. My logic ( if any here...) was to start back on my original dose and introduce T3 at very low levels. I must add on the higher dose of T4 my TSH became quite suppressed.

I'm struggling here, quality of line could be a lot better. Endos and my doctor are not interested at all in what I tell them. I've been left dismayed by the treatment I've had.

I'm not sure it's a good idea or bad, but I have to try something.



This is a link to an archived website (Dr Lowe was an adviser to but unfortunately had an accident and died two years ago) and there is a wealth of information and these two in particular may be helpful. Particularly the questions about too low a dose of thyroid medication.


I started T3 in April and I have never been told to reduce my thyroxine. I took 100mcg thyroxine prior to starting T3 and I still take this dose. I take 60mcg T3 also. I don't understand why you have reduced.


It might be reasonable to reduce the T4 a bit when you are actually taking T3. In your case, your effective dose (using fairly common conversion factors) went from 33mcg T3 to 93mcg.

(I am perfectly happy for you or anyone else to re-work that with any published conversion factor. It will still end up being a substantial increase in effective dose.)

Not arguing against you being on that dose - it might very well be what you need - but it is an increase. Looks like you were considerably under-dosed on 100mcg of T4. Had you not been under-dosed, then adding T3 without at some pint reducing the T4 a bit probably would have ended up with you being over-dosed.



I know it was a massive jump. I had only every been on 50mcg of T4 when on it and I had always felt under dosed. Bloods were always in the range. I was increased to 100mcg T4 in February. Then in April I started the T3. The T3 was started gradually and I expected to be told reduce the thryoxine and I never have been. My FT3 and FT4 were all tested and I had very low FT3. Everything is at the top end of the range now, and I feel normal. I am regularly monitored.

This chap sounds like he is dosing alone. My concerns for him would if you don't see an improvement, or if you end up a few months down the road heart palpitations and going overactive, how would you know what product to increase or decrease?


The difference is that T4 is a4 alone a storage hormone and T3 is the 'working' hormone. Your body is supposed to convert T4 to T3 but most times can't. Because most people are given levothyroxine which is T4 only they do not get the hormones they need so some clued up doctors will prescribe T3.


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