While you are taking Synthroid, you should avoid foods that can interfere with its absorption, thus making it less effective. These foods include walnuts, cotton seed meal, soy (including infant soy formula), and anything which is high in fiber. You should also avoid changing brands of levothyroxine or switching to a generic version if possible. This is because different preparations can work differently. If you must change, you should let your doctor know so that he can order blood tests to determine whether you are receiving the correct dose of the new drug.