Many people here find it useful to look at the British National Formulary to see information about medicines.
Recently they have told us that they have overhauled the mechanism they use to determine the costs of medicines. This has resulted in the following about T3/liothyronine which seems to accord much better with our understanding of costs:
Tablets, scored, liothyronine sodium 20 micrograms, net price 28-tab pack = £52.46
Just below that, they have added this somewhat oddly worded but interesting note:
Liothyronine tablets manufactured by Amdipharm Mercury may be difficult to obtain. Patients switched to a different brand should be monitored (particularly if pregnant or if heart disease present) as brands without a UK licence may not be bioequivalent and dose adjustment may be necessary; pregnant women or those with heart disease should undergo an early review of thyroid status, and other patients should have thyroid function assessed if experiencing a significant change in symptoms. If liothyronine is continued long-term, thyroid function tests should be repeated 1–2 months after any change in brand.
(That link will not work unless you are already logged into BNF - and maybe not even then.)