Levothyroxine v thyroxine

[Tiggey22]

Levothyroxine and Thyroxine, are they exactly the same drug ? If not what is the difference ? Thank you

[Heloise]

I always felt they were the same but I just googled it and there is a slight difference. They are both T4 formulas and act much the same way in the body. They should convert to T3 during processing. quora.com/How-do-thyroxine-...

[radd]

Tiggy,

Levothyroxine is a brand name for a synthetic version of thyroxine (T4) that is made and released by the thyroid gland. Both contain the same number of iodine molecules and are converted (hopefully) into T3 by one being released.

To complicate things further some call T4 "thyroxine" whether referring to either a synthetic hormone or the one made within our thyroid glands.

If you are referring to Levothyroxine v Thyroxine Sodium they are the same thing, although different makes may contain slightly different fillers.

[helvella Administrator]

Until something like ten years ago, you would not often see "levothyroxine" on a packet of medicine. It was usually just "thyroxine".

In the world of the chemist, it had always been known, from very early days, around 1928, that there were two forms of thyroxine.

If you think of shells on a beach, for any species, almost every shell twists the same way. The other day I collected some common turret shells - every single one had the same twist. Yet, if you read up, you find out that just occasionally you can find one that twists the other way.

Similarly, many compounds occur in two forms. One which twists clockwise, the other twists anticlockwise. This is the only difference - they have exactly the same atoms in exactly the same shape - other than twist. Chemists have long called these laevo (levo or just l-) and dextro (d-) forms. If mixed together in similar amounts they would be called dl-. Laevo = left. Dextro = right.

Again, many years ago, dextrothyroxine was tried as a medicine. Trouble was, it was less effective and caused heart problems. (Some people think any positive effect was due to levothyroxine or other impurities!) The very fact that the word thyroxine was not sufficient to identify which form (D or L) meant that the naming was unsatisfactory even if it mostly worked. Imagine if for some reason D-thyroxine was found to be of some use? The confusion would be serious.

So it was decided to remove the ambiguity and called the substance levothyroxine on the packets.

Therefore when you read "thyroxine" it will always mean "levothyroxine". And vice versa.

The links below are to three-dimensional images of the two forms. These links are unlikely to work on a phone or tablet but do work (for me) on a PC running Windows 10. You might get a warning as it starts to display - just click to ignore that.

D-thyroxine

chem.nlm.nih.gov/chemidplus...

L-thyroxine

chem.nlm.nih.gov/chemidplus...

[Treepie]

A small but important point .Levothyroxine is a hormone not a drug.

[diogenes Remembering]

There are two forms of thyroxine. One is used by the body the other not so well. The shape of the thyroxine molecule allows it to exist in two mirror images. That is, if you looked at one form in a mirror, what you would see is the other form. Just like looking in a mirror where the parting in your hair is opposite to the one on the real you. One is called left handed the other right handed. Left handed thyroxine is levothyroxine, the active hormone. Right handed is not so easily used and isn't present in pills.