Thyroid Cancer (Adenocarcinoma) in Dogs

Thyroid cancer is most often discovered when a mass on the neck is noticed by a pet owner. Sometimes a pet can be asymptomatic for problems but has a cancerous tumor growing on the thyroid which is seen once it reaches a fairly large size. Thyroid tumors can be either benign (adenomas) or malignant (carcinomas). Adenomas are typically nodular shaped and filled with fluid while carcinomas are the tumors we see that bulge from the throat area and have a necrotizing effect on the tissues within them. Most thyroid gland tumors do not affect thyroid hormone levels. Documentation shows that large breed dogs of middle-age to an older age are the most often seen patients in the veterinary field.

An adenocarcinoma is a carcinoma (neoplasia beginning in the epithelial cells) that started in glandular tissue. A thyroid gland tumor can be either benign or malignant with the adenocarcinoma being of a malignant form. Metastasis is common and the tumor is often a very large palpable mass present on the throat area.

You may notice that your dog has a mass on the neck. There have been cases of thyroid cancer growing on the neck in an area away from the thyroid, and in the chest or  under the tongue. The mass can be either movable or fixed and can sometimes cause the face to swell.

• Your pet may cough
• Your dog could have difficulty swallowing
• He may gag, especially when eating
• He might have trouble breathing if the tumor is obstructing the throat
• Often there can be a change in bark sound
• Your pet could lose his appetite and experience weight loss

It is rare for a tumor to affect the hormonal function of the thyroid. However, it can happen and in those cases you will see additional symptoms. With hypothyroidism, your pet could be lethargic, have some hair loss, and suffer from exercise intolerance. In the case of hyperthyroidism, dogs can have signs including heart trouble (rapid rate and abnormal rhythm), increased hunger and thirst, and muscle tremors.

Types

Thyroid tumors are classified as non-functional (not affecting the thyroid function) and functional (meaning they produce excess hormones that could lead to hypothyroidism or hyperthyroidism).

As with human cancers, the cause is not always easily discernable. In the case of thyroid cancer age can be a factor (most dogs are middle-aged to older), and there is an over representation of the Siberian Husky, Golden Retriever, Beagle, and Boxer breeds in studies of this adenocarcinoma.

During the physical examination of your pet, the veterinarian will assess the mass (fixed or moveable) and will check the lymph nodes in the region for swelling. While the physical is taking place, it is an opportune time to discuss your pet’s symptoms and the concerns that you have that led you to bring your pet to the clinic. The eating habits, behavioral changes, and signs of discomfort are important things to note and discuss with the veterinary team.

Testing may begin with a complete blood count, serum biochemistry, coagulation parameter, and urinalysis, as well as a thyroid panel to determine the function. Additional testing will include the following.

• Thoracic (chest) x-ray to verify if metastasis has taken place
• Ultrasound and/or computed tomography (CT) scan are the best tools to use to see how deep the tumor is and how large it has grown, and to assess the organs of the body
• A biopsy may be done, but often it is done at time of surgery if this choice is viable

There are many factors that will determine the method of therapy. The invasiveness of the mass, the size of the tumor, whether it has metastasized, and the presence of hyperthyroidism or hypothyroidism all become important points for the veterinarian to consider. Surgery to completely remove the tumor, and additional therapy to assure all signs of the cancer are removed are the goals for treatment. Because many tumors are very large upon diagnosis and have quite possibly metastasized, surgery could be accompanied by measures that may cause discomfort for your pet. Additionally, not all tumors can be removed, depending on their location and how deep they have grown. Due to these factors, surgery may be one step of the treatment with chemotherapy, radiation, and possibly radioactive iodine or cobalt irradiation necessary as additional steps to eradicating the cancer. Your veterinarian is the best person to give you guidance and advice regarding the adenocarcinoma; she can lead you toward the best decision for your pet while keeping his quality of life in the forefront.

Depending on the course of treatment, your dog could quite possibly have many follow-up visits to the clinic for additional sessions of chemotherapy or radiation after surgery. If your pet did have surgery, he will need quiet care at home once he is released from the hospital. He will have an incision that will have to be monitored by you to ensure that there is no redness, oozing or infection. Provide a soft bed for his recuperation and make sure that your dog has plenty of water and fresh food. He may have pain medication so be sure to administer this according to your veterinarian’s instructions. Call the clinic immediately if you have concerns or questions about your canine companion’s recovery.