You'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.
Inactive ingredients in Levoxyl are: Calcium sulfate dihydrate, croscarmellose sodium, magnesium stearate, microcrystalline cellulose, and sodium bicarbonate. The dyes vary according to dosage.
I didn't do well on Synthroid, but have taken Levoxyl for several years and do very well on it. The same has been true of many others on the forum.