(Although generic Westhroid is no longer available, some people may find the following section informative as it describes the regulatory status of Westhroid and generic versions. This information may become applicable in the future, if generic versions return to the marketplace.)
Westhroid, as well as all other thyroid USP medications, has never been approved by the U.S. Food and Drug Administration (FDA), despite what you may read on the Internet. Many people are unaware that many prescription medications (usually old medications) have never been approved by the FDA. Almost always, these medications were already around before many of the laws and regulations were established. Because Westhroid and other thyroid USP medications were not approved by the FDA, there is no way to know if any of these products are equivalent, since the FDA makes this decision.
Some states have laws that permit the substitution of a generic medication for the brand-name version by a pharmacist only if the FDA determines that the generic version is equivalent to the brand-name drug. In these states, generic thyroid USP products cannot be automatically dispensed in place of Westhroid, unless a new prescription is obtained. Other states leave this up to the discretion of your pharmacist. Either way, many people are more comfortable staying with brand-name Westhroid, especially since it is already a fairly inexpensive medication.
It is also for this reason (the lack of FDA approval) that Westhroid, Armour® Thyroid (a similar brand-name medication), and Nature-Throid™ (a similar brand-name product made by the same manufacturer as Westhroid) cannot be considered to be equivalent.
If you are considering a switch from one natural thyroid product to another, be sure to check the strength in milligrams, not in grains. The strength of natural thyroid products is usually given in grains, an outdated measurement. Since there is no consistent agreement as to exactly how many milligrams are in each grain, a 1-grain tablet of one natural thyroid product may not be the same as a 1-grain tablet of a different product. For instance, a Westhroid 1 grain tablet contains 65 mg, while an Armour® Thyroid 1 grain tablet contains 60 mg. Therefore, it is the milligram strength, not the strength in grains, that is important.