Armour thyroid is not a licensed medicine in the UK. Licensed thyroxine preparations are available and widely prescribed.
There is no rationale behind the use of a combination of levothyroxine and liothyronine, or of dried thyroid hormone extracts. A combination of the two, in both non- and physiological proportions, has not consistently been shown to be more beneficial than levothyroxine alone with respect to cognitive function, social functioning and wellbeing in a limited number of patients.
The Royal College of Physicians do not recommend the use of Armour Thyroid
UKMI Q&A 56.6 Dec 2013