Contrary to what is implied by many Web sites, Armour Thyroid (as well as all other thyroid USP products) has never been approved by the U.S. Food and Drug Administration (FDA). However, this is not necessarily as bad as it sounds. Many prescription medications have never received FDA approval. Almost always, these are old medications that were in use long before the FDA existed or before certain laws and regulations were in place. For instance, phenobarbital (Luminal®), a prescription seizure medication, has never received FDA approval.
Synthetic thyroid replacement medications, sold under the name levothyroxine (Levoxyl®, Levothroid®, Synthroid®, and others) were also unapproved until recently. The FDA required each of these medications to perform the necessary studies and seek approval. It is possible that the FDA may require natural thyroid products to do the same in the future. Until then, the lack of FDA approval for Armour Thyroid may be a cause for concern. For instance, the prescribing information for the drug does not meet the usual FDA standards, since it does not list the possible side effects, except for a few listed with the warnings and precautions.
Armour Thyroid contains thyroid USP, which means that the manufacturer has prepared the medication according to the formulation or "recipe" of the United States Pharmacopeia (a non-governmental organization) and that Armour Thyroid should meet certain consistency and potency standards.
The thyroid gland makes two different thyroid hormones: triiodothyronine (T3) and thyroid USP (T4). Usually, the thyroid gland produces much more T4 than T3; however, T3 is much more active than T4. The body can convert the T4 hormone into T3 as necessary. If your thyroid does not make enough thyroid hormones, there are a few different ways to increase your levels.
Most people with an underactive thyroid are treated with synthetic T4, also known as levothyroxine (Levothroid®, Levoxyl®, Synthroid®, Tirosint®, Unithroid®). However, some people prefer to be treated with natural thyroid products, such as Armour Thyroid. Armour Thyroid consists of ground-up pig thyroid glands, so it contains both T3 and T4.