I've sent Louise Roberts at TUK two short articles showing why switching T4 brands isn't the easy thing that manufacturers believe. I cannot make these generally available because they are paywall shielded. They are:
Clinical Pharmacokinetics
ISSN 0312-5963
Clin Pharmacokinetics
DOI 10.1007/s40262-020-00887-x and
Impact of a forced dose-equivalent levothyroxine brand switch on plasma TSH; a cohort study:
DOI
:
10.1089/thy.2019.0414