In the UK we have BNF and EMC (MHRA) to get "official" information about medicines:
(Requires free registration)
EMC contains a massive collection of Patient Information Leaflets (PIL) and the more comprehensive Summary of Product Characteristics (SC) documents. Not, unfortunately all, but a sizeable proportion are listed and available for download.
But sometimes, for various reasons including Internet discussions and simply wanting to know, we wish to access information from other countries.
Medicines in the USA are controlled by the Food and Drug Administration which has equivalent information in the somewhat oddly named Daily Med site:
The amount of information there is staggering (and this is only one arm of the FDA) but it does appear to be slow to get updated. So be careful.
The European Medicines Agency is a decentralised agency of the European Union, located in London. The Agency is responsible for the scientific evaluation of medicines developed by pharmaceutical companies for use in the European Union.
This too has a considerable amount of medicine information but not, currently, about Levothyroxine or Liothyronine.