Are Synthiroid And Levothyroxine The Same Medicine?

I have endometria cancer. I have taken synthiroid for years. Now I am asked to change to Levothyroxine, the generic. I take 6 other medications and am afraid to take a generic. Can you tell me the difference between the two drugs and are they compatible....and what SORT OF FILLER do they use. Is it a weight gain drug?