Synthroid is the brand name and levothyroxine sodium is the generic name. They contain the same active ingredients, but they do differ in their inactive ingredients. The Food and Drug Administration (FDA) considers most levothyroxine sodium preparations to be equivalent to each other. A pharmacist is allowed to substitute a generic levothyroxine sodium product for a brand name product, unless your prescription specifies. Many patients and doctors still report noticing such differences in symptoms when switching from one manufacturer brand to another. In general, it is a good idea to stay on the same brand name of levothyroxine sodium.