Question: I have hypothyroidism and have been on thyroid replacement for almost 40 years. I currently take 0.175 micrograms of Synthroid, but a generic is available that is much less expensive. Should I switch?

Dr. Hibberd's Answer:

Thyroid replacement drugs are given in such small amounts (microgram — one thousandth of a milligram) that a very small difference in dosage can have a very significant effect on you, the patient. Purity guidelines for generic drugs are regulated by the FDA, but are sometimes less stringent than those of the original manufacturer. For this reason, it is generally advisable for you to stay with the same manufacturer for your medications, as similar dosing from different manufacturers is not bio-identical and not interchangeable.

Many drugs fall into this category, especially seizure medication (such as Dilantin), select cardiac medications, and some hormone preparations (such as Synthroid). Medications such as antibiotics generally are supplied in larger milligram increments, and a small dose variation may not be identifiable clinically. While generic substitutions are supposed to be identical, they often are similar but not always identical.

The choice on generic substitution should be discussed with your doctor before making changes, because there are cases where generics shouldn’t be substituted. If you do decide to use a generic thyroid product, be sure your pharmacist buys from the same manufacturer and not from multiple manufacturers. Always ask, since you won’t always be told.
Never buy off the Internet unless you are buying directly from the manufacturer or from another well-known reliable source. Some sources sell pirated and impure drugs that are often untraceable. Recently reported cases found excessive impurities (including cyanide and digitalis, a heart medication) only after adverse events, including deaths, were reported.