re correct that Levoxyl and Synthroid are separate brands of thyroid medication.
Levothyroxine is the active ingredient for both of them; the difference is the inactive ingredients (fillers/binders and dyes) they each contain. Levothyroxine is also the generic for all brands of T4 medications.
Inactive ingredients in Synthroid are: acacia, confectioner's sugar, D&C Red No. 27 & D&C Red No. 30, FD&C Blue No. 1, lactose, magnesium stearate, povidone, and talc.