Several versions of generic Levoxyl exist. Until recently, pharmacists were uncertain about how different levothyroxine products compared to each other, because the FDA had not approved or tested levothyroxine products. With testing, the FDA found that several generic versions of levothyroxine are the same as Levoxyl. You may still find information warning that generic levothyroxine is not the same as Levoxyl; equivalent may be a better term to describe generic versions of levothyroxine. Hence, most doctors will prefer that you consistently use the same brand of levothyroxine once you and your doctor determine which version and dosage is best for you. Doctors prefer this consistency because levothyroxine has a "narrow therapeutic index," which means that very small differences in dosage can cause very large differences in its effects on the body. If you decide you must change brands, you can ask your doctor to run blood tests to check that you are receiving the correct dosage.