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Canine Thyroid Cancer

Issue Description
Thyroid tumors include cystic structures called goitres, multinodular overgrowth (hyperplasia), benign (non-spreading) cancers (adenomas) and malignant (spreading) cancers (carcinomas). Hyperplasias and adenomas grade into each other and most produce excessive quantities of thyroid hormones. These induce complex clinical syndromes. Malignant thyroid tumors rarely produce hormones but they may spread both locally and to the lungs. Up to a third of thyroid carcinomas in dogs may originate from specific cells in the thyroid glands (C-cells) that act in combination with the hormones of the parathyroid glands to regulate blood calcium
Other Names
Thyroid Cancer, Thyroid Neoplasm




Causes
The reason why a particular pet may develop this, or any cancer, is not straightforward. Cancer is often seemingly the culmination of a series of circumstances that come together for the unfortunate individual.

One non-cancerous tumor of the thyroid called 'colloid goitre' is due to inactivity of the gland. This is often caused by tumors in the pituitary gland preventing production of the pituitary hormone that controls the thyroid. There are numerous drugs and illnesses that can produce similar non-cancerous goitre but such goitres are rarely large enough to see clinically.

Prolonged stimulation of the thyroid gland often causes cancerous changes with a continuous spectrum from small areas of overgrowth (hyperplasia) to benign thyroid adenoma and then malignant cancer (thyroid adenocarcinoma). This multi-step process is called tumor progression. Some cancers never progress past the first stages so remain benign.

Only about 10% of canine thyroid tumors are functional, meaning that they secrete excessive levels of thyroid hormone. This can result in hyperthyroidism, a condition in which the body's metabolism is revved up, resulting in symptoms such as rapid heart rate, elevated blood pressure, hyperactivity, and weight loss despite a voracious appetite. These thyroid tumors are usually located within the main thyroid glands, which sit on either side of the trachea (windpipe), just below the larynx (voice box). However, carcinoma can also arise anywhere there are thyroid tissue remnants, extending from the base of the tongue, down the neck, through the thoracic inlet (where the windpipe and esophagus enter into the chest) and into the mediastinum (the area within the chest cavity in front of the heart and between the lung lobes).


Symptoms
Many dogs with thyroid cancer don't show many symptoms. Some symptoms of thyroid cancer in dog may include urinating more than usual, weight loss, increased appetite, and nervousness. The thyroid gland, located at the base of the neck, may appear enlarged. Some breeds of dogs are more susceptible to thyroid tumors than others, including beagles, boxers, and golden retrievers.

Diagnosis
About one third of thyroid cancers have already metastasized (spread) to the draining lymph nodes and lungs at the time of initial diagnosis. The best way to evaluate the location and extent of functional thyroid tumors is by using a nuclear medicine scan. This test uses a radioactive tracer, such as technetium or iodine, which binds to any residual thyroid tissue. The areas of radiation uptake are then measured with a gamma camera. If this test is not available, an alternative is to perform a contrast CT (computerized tomography) scan of the neck and chest, which uses intravenous dye and digital X-ray pictures to outline any cancerous tissues. If a CT scan is not available, radiographs (X-rays) of the neck and chest combined with an ultrasound of the neck are a reasonable alternative.

Treatment
There are many options for treating thyroid cancer in dogs. The best option for your dog will depend on the size of the tumor, how it is attached to the surrounding tissue, and whether or not it has spread to other parts of the body.

If it is possible to separate the tumor from other tissues without damaging any major blood vessels or other vital parts, surgery is the best treatment. This is the case whether or not the cancer has spread to other parts of the body. If the cancer has spread to other parts of the body, additional treatment will be needed, however.

If the tumor involves the surrounding tissue and therefore cannot be removed surgically, radiation therapy is often used. Radiation is directed at the tumor, causing it to shrink. Sometimes after it shrinks sufficiently, the remaining tumor can then be surgically removed.

Finally, there is chemotherapy. Chemotherapy is often prescribed along with surgical or radiation therapy. It is used when cancer has spread to other organs of the body to slow its growth and is also used to prevent the recurrence of cancer. There are a number of drugs available for these purposes.


Prognosis
Early diagnosis and treatment is key to survival. There is a high rate of success in treatment that is begun in the early stages of thyroid cancer in dogs.