The only difference is that westhroid actually has less fillers and is the most hypoallergetic thyroid med out there....:) :)
Westhroid does look to be the same thing as Armour. I haven't used it but it's natural dessicated thyroid like Armour.
http://thyroid.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=thyroid&cdn=health&tm=12&f=00&tt=12&bt=0&bts=0&zu=http%3A//www.wes-throid.com/