Hyperthyroidism, or overproduction of thyroid hormone, occurs fairly often in felines but relatively rarely in canines. Dogs usually suffer from the opposite problem -- hypothyroidism, or insufficient thyroid hormone production. Dogs diagnosed with thyroid cancer are the most likely to experience elevated thyroid production. The prognosis, therefore, is not good: The Merck Veterinary Manual refers to it as "poor to grave." Along with other therapies, such dogs might receive a drug called Tapazole.
Your dog's two thyroid glands are located near his windpipe, on the base of his neck. They produce hormones necessary for proper metabolism, and they affect many of the body's organs. Symptoms of hyperthyroidism range widely. They include weight loss even though the dog appears ravenous, increased drinking and urination, vomiting and diarrhea, breathing difficulties and an overall unkempt look. If a dog has thyroid cancer, he's likely to develop a swelling in the neck at the site of the thyroid glands, along with a cough if the cancer has spread to his lungs.
Methimazole for Hyperthyroidism
Methimazole, marketed under the brand name Tapazole, is the drug of choice for treating hyperthyroidism in cats, although it's used less frequently in canines because the condition so seldom occurs in dogs. While it's not approved for veterinary use by the U.S. Food and Drug Administration, veterinarians are allowed to prescribe it as an "extra-label" medication. It works by preventing thyroid hormone production.
Your vet can prescribe daily Tapazole pills for your dog to control his hyperthyroidism. However, daily medication doesn't address any underlying cancer, so the tumor will likely continue to grow and metastasize throughout his body. Since hypothyroidism is more common in dogs, medication to increase thyroid levels can end up resulting in hyperthyroidism. In that case, your vet might prescribe Tapazole temporarily so that your dog's thyroid levels get back in sync.
Other Thyroid Cancer Treatments
If the veterinarian doesn't want to go the Tapazole route with your dog, other thyroid cancer treatments are available. While more invasive and expensive than Tapazole, they might extend your dog's life if the tumor is in an early stage. These options include surgery, radiation and chemotherapy. Unfortunately, by the time most thyroid cancers are diagnosed in dogs, the possibility of a cure is remote.
Side Effects and Contraindications
Some dogs might experience lethargy, appetite loss and vomiting while on Tapazole therapy. Dogs with liver or blood diseases, or who are otherwise immuno-compromised, should not receive Tapazole.