Levothyroxine sodium – Anhydrous vs. Pentahydrate - Thyroid UK

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Levothyroxine sodium – Anhydrous vs. Pentahydrate

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helvella – Levothyroxine sodium – Anhydrous vs. Pentahydrate

If you read about levothyroxine, especially as a medicine, you will see mention of two forms used as the Active Pharmaceutical Ingredient (API) in levothyroxine medicines: levothyroxine sodium (anhydrous) and levothyroxine sodium. Although the second form doesn’t make it clear, it could refer either to the anhydrous form or to a hydrated form. We have to try to interpret from context and any other information present.

There is a genuine difference between levothyroxine sodium (anhydrous) and the most common hydrated form, levothyroxine sodium pentahydrate.

Five water molecules are attached to the levothyroxine sodium molecule in the pentahydrate: none in the anhydrous

The rest of this document includes formatting and images which I cannot include here. The document has just been updated - 6th March 2021 - to include some information I have recently found. Here is a link to a PDF:


4 Replies
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Thank you for your hard work on this Helvella.

Thank you so much for posting this info! Amazing and very interesting indeed. It never ceases to amaze me how more there is to know about this subject. Best wishes x

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helvellaAdministrator in reply to Morning_gl0ry

The question over why some makes put the word "anhydrous" in their Patient Information Leaflets when some don't has been asked several times. I tried to answer that and couldn't easily, in a few sentences, on the forum.

However much we think we know, there is much, much more to be known. :-) The history alone is fairly incredible.

Thank you for another very informative post helvella .

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