All medicines which are approved in the UK have their documentation posted on the MHRA's products database.
That includes
(Where the product is available due to approval within the EU, some documents will not be available here.)
It has a search facility but that is very crude and it is difficult to get just want you want. You can include active ingredients, excipients (inactive ingredients), PL numbers, etc. But you can't be clever and search for "levothyroxine that doesn't contain lactose".
It will also list all products which say there are any interactions or warnings about taking while already taking levothyroxine.
Therefore, you would very often want to choose only the Patient Information Leaflet (PIL) documents, and the PL number or active ingredient. Then have to work your way through the documents it finds.
And, if you search for "levothyroxine", you'll probably find all the documents for liothyronine as well - because they include words like "stronger than levothyroxine".
While it is the only complete source of documents, it is tedious to use.
If you find anything incorrect, misleading, typos, links
that don’t work, etc., please let me know. Go to my profile and use the contact details there: