Recently there was a flurry of "can't get any Mercury Pharma Liothyronine" that we see from time to time. As I often do, I emailed MHRA and got this response:
We have contacted the Department of Health's Medicines Supply Team regarding this issue. They have looked into this and have confirmed with Amco that supplies of Liothyronine Tablets are currently available and they have not reported any recent supply issues. We would advise you to contact your local pharmacy as pharmacies should be able to order supplies via their usual routes, if you do have an issue then please come back to us with any further detail you can provide.
Hopefully, whatever was actually happening to cause an apparent shortage was local and transient.