Thyroid UK

Thyroxine

Thyroxine

The picture attached to the post shows the two forms of thyroxine:

Levothyroxine (on the left) and Dextrothyroxine (on the right).

All tablets called thyroxine (or, of course, levothyroxine) are as shown on the left.

The structures are identical but they are twisted differently. For all us ordinary folk, dextrothyroxine might as well not exist.

[ This was posted as we have had several questions about the difference between levothyroxine and thyroxine, if any. ]

30 Replies
oldestnewest

So what is Dextrothyroxine's function? Is it just synthetic or does it occur in the gland?

Reply

So far as I know, it is just synthetic. If anyone knows better, please add to this thread!

1 like
Reply

helvella What a GREAT EDUCATIONAL MIND OPENING POST . Thank you for sharing ....

Reply

helvella,

It is interesting to see two chemically identical substances that exert such differing effects . ie D-T4 has been used for lowering high cholesterol.

As both convert to T3, it leads onto the question “How different is D-T3 to T3 or rDT3 to RT3 ” ? ? ….

3 likes
Reply

Yes, quite, we know nothing of D-T3.

Doses of dextrothyroxine have been much higher than those of levothyroxine. That is why impurities, possibly including levothyroxine, are very important. There is little clarity!

Reply

Meant to make it quite clear I was questioning the evidence on D-T4 - if it was good at reducing cholesterol, surprising that a pharmaceutical company didn't pick it up and run towards the bank with their profits. :-)

4 likes
Reply

Supposedly too many side effects. Obviously not as bodily compatible (& so profitable) as ol' Levo.

1 like
Reply

Bizarrely:

D-Thyroxine therapy increased serum concentrations of T4 from 90.5 ± 25.5 to 147.5 ± 40.8 ng/ml (P < 0.001) and of T3 from 1.8 ± 0.4 to 4.1 ± 1.2 ng/ml (P < 0.001) and decreased TSH serum concentrations from 1.1 ± 0.8 ml-IL to <0.01 ml-J/L (P < 0.001). All other variables, including C-reactive protein, albumin, total protein, and fibrinogen concentration, remained unaffected in control and D-thyroxine-treated patients throughout the study period.

jasn.asnjournals.org/conten...

So how did they come to the conclusion that it was the D-T4 that affected cholesterol? Rather than the effect it had on increasing T4 and T3 levels.

1 like
Reply

Yes, agree as optimal thyroid hormone rectifies many health conditions but I guess it depends what you read ... must be other studies if you were to go searching late on this lazy sunday eve ? ! ...

1 like
Reply

Yes, I was thinking the same. Surely Levothyroxine also reduces cholestoerol.

1 like
Reply

It apparently has negative cardiac effects (according to wiki) which is why it was pulled.

1 like
Reply

Thanks - another interesting post.

Reply

Thyroxine, natural or synthetic, refers to levothyroxine and dextrothryoxine. Although levothyroxine tablets are often informally referred to as 'thyroxine' they are levothyroxine with no dextrothyroxine content. This is why I always refer to L-T4 rather than T4 (except in cases where I really mean T4). Levo has far more potent activity than dextro, something like 15x as much (a rough guess from memory). I think the thyroid produces much more levo than dextro - I'm not certain. The role of dexto is not understood.

Similarly there are levo and dextro versions of triiodothyronine with L-T3 being far more potent than D-T3. D-T3 is sometimes called dextrothyronine and also has another similar sounding name that I can't remember!

2 likes
Reply

jimh111 I thought these were synthetic so not made by the thyroid glad. Is there a natural version of the dextro types that the gland makes?

Reply

Levothyroxine sodium is a synthetic. Once the sodium atom is stripped off the levothyroxine from the tablet is the same molecule as the levothyroxine that comes from the thyroid. The thyroid also makes a little dextrothyroxine which is not in the levothyroxine tablets. So the synthetic molecules are exactly the same as the ones produced by the body (they have to be) but the thyroid produces some substances that are not in the tablets. These other substances are presumed to be of little or no importance (which seems to be true at least for patients who do well on levothyroxine but perhaps not true for all patients).

1 like
Reply

Would NDT have small amounts of dextrothyroxine, too?

Reply

SilverAvocado

Surely it must have it as well. Why would it just be humans that make it? Of course I have no idea 😂😂😂 so could be completely wrong in my supposition 😉

Reply

That would be my guess. Another example of the little subtle differences between NDT and synthetics.

1 like
Reply

I showed the two molecules of natural thyroxine and levothyroxine to a pal whose dad was a prof of chemistry - sadly the prof had developed severe Alzheimer's so all his brilliance and knowledge was lost, but his son said he'd take the natural version rather than the synthetic for sure because they were not identical and this could be significant. Interesting to know a mirror version (or is it just a different twist) of Levo does something quite different.

3 likes
Reply

Always seems to me that there is some confusion, somewhere.

So far as I am aware the only difference is in the sodium atom in the synthetic versions.

At least some pictures of "natural" thyroid hormones are of the hormone attached to the large thyroglobulin molecule from the thyroid from which it was derived. That is jolly impressive on screen, but unless we know that is how it transfers into the blood stream, it means little in itself. That will affect delivery in the gut - but that does not mean the actual hormone molecules differ between "natural" and synthetic.

Also, all forms of thyroid hormone will be hydrated (have lots of water molecules more or less attached round them). This is not shown in most diagrams because it adds complexity and confuses.

1 like
Reply

helvella that is interesting. I have read that some angles in the synthetic version differ and this might have significance but I claim no knowledge of molecular chemistry so no idea if this claim is sound.

Reply

I can't see why any bond angles would be different.

Such things are affects by levels of hydration - so it could very well depend on precise circumstances. For example, the anhydrous levothyroxine that could be used as an ingredient versus how that same molecule is when it has been hydrated (e.g. from the atmosphere or in a slurry being made up into tablets).

1 like
Reply

I have seen research on the net where the synthetic T4 and the glands own T4 molecular structures were reviewed side by side - you didn't need a phd in chemistry to see they were totally different .

Reply

Would you care to post a link as to where you have seen that?

I have certainly seen some comparisons which were wholly wrong-headed.

Reply

helvella

This mentions them as being above the article but they don't show on my iPhone

tpauk.com/main/article/synt...

Reply

TSH110,

Thanks. I couldn't think where I had seen them.

One link shows that desiccated thyroid is not the same as levothyroxine.

The image shows the huge thyroglobulin and the tiny thyroid hormone molecule nestling into it.

I think it is completely accepted that in desiccated thyroid the thyroid hormone is partially bound to the thyroglobulin. But that says absolutely nothing about the thyroid hormone once it has been absorbed through the gut into the bloodstream.

The other link is inaccessible without signing up. However, I remember seeing the picture from there on Mercola's site. Here:

articles.mercola.com/sites/...

Afraid I am very unimpressed by that. The two diagrams are drawn to very different conventions. The upper diagram shows additional water on the synthetic but not the "natural". Sorry - but just where in the body could thyroxine exist without being hydrated? Choosing to ignore it in one diagram but not the other shows either ignorance or deviousness.

Yes - we also all see "levothyroxine sodium" on packets or other documentation - no-one is trying to pretend that it isn't the sodium salt of levothyroxine. And "natural" thyroxine from the thyroid will have a hydrogen atom instead of a sodium atom. There is plenty of reason to believe that this actually makes no difference once it has passed through the stomach.

In both cases it is rather like the same sitter having been painted by Rembrandt and then by Picasso, or van Eyck and Mondrian. The subject might be the same, or very similar, but you'd have a hard job convincing yourself if all your could see were the canvases. :-)

Reply

helvella Love the painting analogy - I totally agree it is bonkers to compare incompatible diagrams of molecules using different conventions - of course they will be different due to the way they were interpreted graphically but that is not the same as them being different in reality. I think I would like Rembrandts portrait of levothyroxine best 😂🤣😂

Reply

Synthetic levothyroxine is identical to natural levothyroxine. Otherwise the assay would not pick it up and the TSH would not respond in the same way and millions of patients on levothyroxine only would not get better.

1 like
Reply

jimh111 So why do millions of us not get better on it - just a conversion issue of T4 to T3? Surely that other stuff the thyroid pumps out has to be significant. Do we know people really are well on levothyroxine for sure and it had no long term health consequences? It certainly did not make me feel well again but my sister and mother like/d it very much.

Reply

Lots of patients do fine on levothyroxine, many don't. I guess there are a number of reasons they don't. It's wrong to suggest levothyroxine never works and wrong to suggest it always does. Two sides of the same coin.

My view is that very few strictly have a conversion issue, perhaps indirectly as a result of an underperforming thyrotrope but not directly. If it were a conversion problem we would all be perfectly well when given a small dose of liothyronine. This doesn't happen so clearly there are other unknown problems.

1 like
Reply

You may also like...