Thyroid UK
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Liothyronine (T3) Potency

A few days ago there was some uncertainty about the potency requirement for liothyronine in the UK. Because I did not know, I asked the MHRA who have responded:

•British Pharmacopoeia Volume III

•Formulated Preparations: Specific Monographs

Liothyronine Tablets

Action and use

Thyroid hormone replacement.


Liothyronine Tablets contain Liothyronine Sodium.

The tablets comply with the requirements stated under Tablets and with the following requirements.

Content of liothyronine sodium, C15H11I3NNaO4

90.0 to 110.0% of the stated amount.

Have to say, I think that is a big range. Though it was only a few years ago that the standard for levothyroxine was tightened from this same range to 90 to 105%. (95 to 105% in the USA.)

Also bear in mind that Mercury Pharma seem to have repeated difficulty in manufacturing to the existing standards - what would they do if they were tightened up? (Mind, we don't actually know that the problem is with potency, it could be other issues.)


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Thanks Rod. That is quite a big variance, particularly for someone on T3 only. Rather impressed MP responded to you so quickly.


MHRA, Clutter! It was a nice simple one. :-) But yes, so was I.


I wrote to Mercury Pharma before and after I returned their tablets. Their heading both times was Efficacy of Liothyronine. I wrote it is not the efficacy of liothyronine I am complaining about but the filler/binders used which are most probably the cause of symptoms could they use hypoallergenic ones.

Why I said that is if I am taking T3 and begin to have symptoms, if I increase/decrease slightly they'd disappear but not if fillers/binders are the problem.

Now that you have posted the range of liothyronine which can vary between 90% to 110% why have the MHRA allowed an even wider potency of the sodium?

With only one liothyronine being prescribed in the UK, should it not be perfect. MP seem not to worry that their product isn't perfect as so many have complained or think symptoms are still to do with hypo or that it is the liothyronine that's doesn't agree with them, whereas I who have been able to have absolutely no symptoms with another product and initially with MP T3, developed symptoms I never had previously with MP T3 which went when I switched to another T3, plus the fact that they leave patients high and dry when it runs out of the product which has happened a few times now.

As I quote from Dr Lowe "Thyroxine is what we call an "orthomolecular" substance. This means that thyroxine is natural to the human body and necessary for health. Allergic reactions to orthomolecular substances are incompatible with health and extremely rare."

(so my assumption is that liothyronine is also orthomolecular).

I assume that a Brand name has to remain identical in everything but not if it's generic like Mercury Pharma's T3?


When they changed from Tertroxin to generic liothyronine there was no change whatsoever in the product. I believe that the same product is sold in South Africa as Tertroxin. Looks to me as if the change was purely to allow them to increase the price.

All changes of any significance should go through various approval steps with the MHRA - indeed, failure to do so was stated to be one of the factors in the great Teva levothyroxine scandal. In that case, it was what appeared to be a modest change to one of the ingredients - dextrin.


Just remembered, even to this day the SPC Leaflet has the Tertroxin name on it - as well as the generic.

1. Name of the medicinal product

Tertroxin Tablets 20mcg

Liothyronine Sodium BP 20micrograms Tablets.


Have you any suggestion why some of us complained about the product? The only way I could think of was to stop it to see if it there was a connection and symptoms resolved. I had been fine on it for approx 9 months.


I could speculate until the cows come home. And get milked, chew the cud, have a good sleep, and go out in the fields for their breakfast. :-)

There are so many possibilities - all of them seemingly unlikely but something must be making that difference. And many of those reasons might apply in some people but not others.

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Thanks. At least I cannot complain at present as I o.k


Just a small comment.

Paddock Labs Liothyronine is available in the UK - 5mcg and 20mcg tablets.

I admit that it may be very difficult to get it prescribed, but Boots have no problem fulfilling my NHS prescriptions, although it has to be ordered.


But as it is not licensed here, it can only be prescribed by a doctor who is willing to take the responsibility both for the medicine and for prescribing a very expensive substance.

There is a small number of specialised importers of medicines which are not licensed. If the demand is sufficient, they can and do keep modest stock levels here.

And do we know the potency required by the USP?


Does your GP have to prescribe Paddock on a 'named person' basis?


Does that variance refer to batches or individual tablets in a pot? If the latter, no wonder I struggle to feel the same from day to day. I also cut into quarters because my GP refused to prescribe 5mcg tablets, so there is also the chance of each little crumb being different.


I think that they have to take a few tablets in order to perform an assay. So technically, probably the variance found in any sample of <so many> tablets randomly selected from a pot or a batch. But I too would like to be sure of the procedures.


See Rod? I wasn't full of bullshit in the past and truth is the T4 is not titrated to a narrow range either as per propaganda.

You have no idea what you are taking with these meds. It could be 90 mcg, it could be 110 mcg. That's a big difference.


I must say that range surprises me. With the short half-life of t3 such variances could be very problematic, especially when you take t3 only and in a large dose.

For what I understand these are requirements and not actual measured variances, actual measurements would imo be very interesting to know. Of course this measurement should come from an independent party as I don’t think a manufacturer would admit 20% potency differences between pills.

I’m also curious about similar requirements for other brands.

Personally I take tiromel and have tried uni-pharma, I was actually kind of shocked by the difference in effect that I perceived and am pretty sure that difference is not only related to differences in the amount of active ingredient.

I looked into this a little and read in some older posts that dr John Lowe once tested and compared some generic t3 and grossman cynomel and found a difference in chirality between the t3 molecules or something and concluded that grossman t3 was more pure. Now I can’t find any source for this, only some old posts but I would be very interested if someone knew more about this. (I could also open a separate topic for this if it’s too much off-topic)


Absolutely - these are the requirements. And the MHRA might require that at the point of distribution the range is tighter - I shall be responding to them on that point. This requirement applies to the full life of the product from manufacture to expiry date.

Without knowing what they actually achieve, it is rather less useful.

Don't remember seeing anything about chirality as you suggest. The one document that I found which compares quite a number of makes is so old, the information has got to be suspect and it was all subjective.

I think it is on-topic but do whichever you wish - start anew or add. :-)



I took Paddock liothyronine for a couple of years after switching from branded Cytomel because of the cost. I found the potency could vary significantly between batches, which was exemplified when the American pharmacy sent me 5mcg tabs in error, instead of the 50mcg. While I waited for the correct tabs to arrive they said just to make the 5mcg up to my normal dosing regime. I take several timed doses of T3 through the day, but that time I suddenly developed hyper symptoms with the 5mcg potency. I ended up having to drop the total daily dosage by 15 mcg!!

I've switched to Mexican Cynomel by Grossman. It's a cheaper and more consistent product.


Can you please OM with details of where you get your T3? I'm on Mercury Liothyronine 10mg day plus 100 Levo. Been on that 7 weeks. Initially felt better but anxiety/panic attacks starting to return with internal shaking. I'm due to Endo this Wednesday. Sorry, mean PM me! Thanks x


I also can’t find much objective information that goes more into detail about (perceived) differences between makes. I did see some studies comparing other types of generic and brand name drugs. In this article for example the author notes differences between bioequivalence studies and actual efficacy of generic drugs vs brand name drugs and advocates that generic drugs be tested more extensively on actual patients:

I think that if you would professionally compare the efficacy of let’s say uni-pharma t3 with a bench mark product (guess this would be king’s pharmaceutical cytomel) in actual patients that there is a big chance that it’s just less effective independent of the dose. Of course then there are also people that report doing great on the brand.

Anyway I think that this stuff may be more complex than some people think and patients would imo benefit from more thorough analyses and comparison of different replacement products.

@Flowerpower, if you are certain that the returning symptoms are from too little thyroid effect you could also try some more t3. Some people experience an initial improvement in symptoms when just starting a new dose that later reverses and improves again with raising. Apart from this I think it is a good idea anyway to try a couple of different brands and see how you respond, just ordered some cynomel myself.


Ask yourself why they are so liberal with the overage/underage?

Almost every other drug is 95%-105%


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