Canine thyroid cancer occurs more frequently in middle-aged to older larger breed dogs. Thyroid tumors in dogs are not very common. When they occur, they can either be benign or malignant.
The thyroid glands are paired, lobed-like structures that wrap partly around the windpipe (trachea), about halfway down the neck of dogs.
The thyroid glands' function is to produce and release thyroid hormones (thyroxine and triiodothyronine). These thyroid hormones are vital for normal body function; specifically they regulate the metabolism of the dog's body.
Thyroid tumors in dogs are not as common as other forms of canine cancer such as bone cancer. When they do arise, they can be either benign (adenomas) or malignant (carcinomas).
Benign thyroid tumors tend to increase in size over time and cause the over-production of thyroid hormones.
Malignant thyroid tumors, on the other hand, seldom cause the problem of hormone over-production. Instead, they tend to spread both locally and to other parts of the body, especially the lungs and the regional lymph nodes, and they can also grow into other structures nearby such as the windpipe, esophagus, and blood vessels.
Unfortunately for dogs, most thyroid tumors that arise (about 90%) are malignant and deadly (as you can see from this post.)
Canine thyroid cancer is more commonly seen in middle aged to older, larger breed dogs, such as boxers, beagles, and golden retrievers.
One typical sign of thyroid cancer in dogs is a mass or swelling in the neck. The presence of the mass (tumor) will also cause other signs depending on how the tumor is affecting the neck area.
Please also read other visitors' experience:
For example, if the tumor compresses the windpipe, the dog patient may cough and have difficulties breathing.
If the tumor presses against the esophagus, the dog patient may experience difficulty swallowing.
If the nerves of the voice box is affected by the tumor, a change in the tone of the dog's bark may be noted.
As mentioned above, most canine thyroid tumors are malignant, and they rarely produce excessive quantities of thyroid hormones. Therefore, dogs with thyroid cancer rarely show signs that are associated with hyperthyroidism (e.g. restlessness, hair coat abnormalities or drinking and urinating more than usual).
Like other types of cancer, the exact cause of thyroid cancer is not clear.
One possible cause of thyroid cancer is believed to be stimulation of the thyroid glands over a prolonged period of time.
For example, a hypothyroid dog who has been receiving excessive dosages of thyroid hormones, or a dog who is on a diet that is excessively rich in iodine.
This over-stimulation may progressively cause the thyroid gland cells to change. First, a part of the gland may experience abnormal cell overgrowth (a condition known as hyperplasia). This cell overgrowth may eventually form a benign tumor (adenoma) which may then develop into a malignant cancerous tumor.
Another possible cause of canine thyroid cancer is the insecticide fipronil (as contained in some flea preventives such as Frontline®).
In fact, the U.S. Environmental Protection Agency classifies fipronil as a carcinogen (cancer causing substance) as they have found that exposing animals to fipronil in a lab caused benign and malignant thyroid tumors in such animals.
Also, according to the Journal of Pesticide Reform, fipronil can cause, amongst other things, "drastic alterations in thyroid function" in lab animals.
Some visitors to this site also commented that their dogs who had been using Frontline® developed thyroid cancer later on.
If you notice a lump or swelling on the neck of your dog, you should get your dog to the vet immediately for a complete examination.
Tests to properly diagnose dog thyroid cancer include:
For thyroid tumors that are freely movable, i.e. if they are not attached to the neck tissues, surgical removal is the treatment of choice.
If the tumor has not metastasized and if complete removal of the tumor is possible, a dog with thyroid cancer may live for up to 3 years after operation.
However, if the tumor is invasive and is attached to the underlying tissues, life expectancy is much shorter (about 6 months to one year).
Radiation or chemotherapy is often recommended for tumors that cannot be completely removed, or are too large for surgical removal.
Radioactive iodine treatment is also sometimes used to treat canine thyroid cancer.
A study by Massey University Veterinary Teaching Hospitals shows that radioiodine treatment, either used on its own or as an adjunct to surgery, is effective at extending survival time of dogs suffering from thyroid cancer.
The study recommends radioiodine treatment for cases where surgery alone is difficult to achieve good results because of metastasis or local invasion, or for cases where complete surgical removal has not been achieved.
However, not too many veterinary hospitals and facilities offer this treatment option since it involves the handling of high doses of radioactive iodine.