There are a number of manufacturers of Levothyroxine and each one
uses different ingredients, except for the levo, itself. For instance, Synthroid
uses acacia and corn starch, either of which bothers many people... I didn't do well on Synthroid, but I do well on Levoxyl. Others do best on Tirosint, which is a gelcap and contains only the levothryoxine, gelatin, water and glycerin.