Although it would be wonderful if a dose of 0.125 mg of Levoxyl, 0.125 mg of Synthroid, and 0.125 mg of levothyroxine were interchangeable, the fact is that they aren’t. Despite the best efforts of the Food and Drug Administration, these medications are made slightly differently, and their potency is slightly different. That means if you get on one of these preparations, staying with the same preparation is important to be sure you experience no change in your thyroid function. What conditions may cause your preparation to change? If you change pharmacies, the new pharmacy will likely use a different source for its medications. The pharmacy you use, even if you don’t change, may change its source. If you change health plans, your new health plan may use a different manufacturer. If you buy your drugs from Canada or Mexico, you can bet you’re getting a different potency. If you change doctors, the same thing can happen. Will you realize you’re on a new preparation? Probably not, because the changes in your body may be too subtle for you to notice. But the different strength may cause damage to your body, depending on whether it’s too much or too little. How can you prevent this from happening? Check the name of the medication on your new bottle each time you get a refill of your thyroid medication. Ask your pharmacist if he or she is using the same source each time you refill. Try to use the same pharmacy or drug program each time you refill. Avoid discounted sources of thyroid medication. It should be a very inexpensive drug. If all else fails and you have to change the source of your thyroid medication, get thyroid function tests four to six weeks after you make the change.
Anticipating Drug Interactions So many drugs interact with thyroid hormones that you must check with your doctor whenever he or she places you on a new medication or takes you off an old medication (see Chapter 10). Your thyroid function can be affected not only when you start a new medication but also if your doctor takes you off an old medication or changes the dosage significantly. The way to avoid a problem is to perform (or have your doctor perform) a search for interactions between thyroid hormone and the drugs you’ll be taking.