http://www.fda.gov/AnimalVeterinary/.../ucm482928.htm

"January 29, 2016

The U.S. Food and Drug Administration has issued warning letters to six manufacturers of unapproved levothyroxine sodium drugs for hypothyroidism in dogs. These unapproved products have not been reviewed by the FDA for safety and effectiveness.

Thyro-Tabs Canine (levothyroxine sodium tablets), (NADA 141-448) is the only FDA-approved drug for replacement therapy for diminished thyroid function (commonly known as hypothyroidism) in dogs.

FDA is concerned about unapproved animal drugs. Animal drugs that have not been reviewed and approved by FDA may not meet FDA’s strict standards for safety and effectiveness. They also may not be properly manufactured or labeled.

FDA issued warning letters to manufacturers of the following unapproved levothyroxine products:

Thyrosyn
Soloxine
Levocrine
Thyromed
Thyroid Chewable Tablets
Thyrokare
Thyroxine L
Thyrovet
Leventa (letter to be posted soon)
.... "