The Pharmaceutical Services Negotiating Committee (in the UK) has a page on writing prescriptions for specific brands of levothyroxine. I have attached an image which shows an example of how it should be done.
If the prescriber just adds a note, the pharmacy might not get the appropriate payment.
Their full advice page is here:
psnc.org.uk/our-news/levoth...
Adding:
Of course, the same advice would apply to other medicines such as Liothyronine.