Until recently, there were a variety of levothyroxine medications on the market for dogs with hypothyroidism. The U.S. Food and Drug Administration issued warning letters to 6 manufacturers of unapproved levothyroxine products, which had not been reviewed for safety and efficacy by the FDA. The manufacturers were warned to stop production and marketing of the products. Because of this, Thyro-Tabs Canine is the only FDA approved drug for hypothyroidism in dogs and is currently the only medication available. Letters were issued to the manufacturers of Thyrosyn, Soloxine, Levocrine, Thyromed, Thyroid Chewable Tablets, Thyrokare, Thyroxine L, Thyrovet, Leventa. More information about the FDA and the letters can be viewed on their website.
Thyro-Tabs Canine is manufactured by Lloyd, and is a levothyroxine medication.

Levothyroxine is a replacement for the hormone T-4, which is naturally produced in the body by the thyroid gland to regulate metabolism and energy. When the body is unable to produce enough thyroid hormone on its own because of an imbalance, an enlarged thyroid gland, or other medical conditions and treatments, it is called hypothyroidism. Hypothyroidism in dogs can be found with a blood test or by signs and symptoms involving metabolism, energy, nerve and muscle function, skin and haircoat. Veterinarians often prescribe levothyroxine for dogs with hypothyroidism for hormone replacement. You can find more information about Thyro-Tabs by the FDA and can find it available on the VetRxDirect website.