Levothyroxine, also called L-thyroxine, is a synthetic form of thyroxine (thyroid hormone). In the United States, brand names include Synthroid, Levoxyl, Levothroid, Eltroxin, Tirosint and Unithroid; Generic forms are Levothyroxine sodium USP and L-thyroxine. There is still debate over whether generic thyroid supplements are as effective as thyroid brand name supplements.
The active ingredient in levothyroxine and levothroid is the same; however, each manufacturer uses different fillers/binders to make the pills. Some people can have reaction to those.
I agree that there is debate over the use of generic vs brand name. I actually did better on generic levo than I did synthroid, but everyone is different.