Thyroid Tumors in Dogs

Thyroid tumors in dogs develops as a result of abnormal replication or growth of the cells that make up the thyroid gland; specifically, the glandular tissues.

In dogs, most thyroid tumors (about 90%) are malignant but nonfunctional, meaning they do not cause excessive thyroid hormone production.

At the time of diagnosis, 35 to 40% of the dogs will have visible evidence of spread of the tumor to other parts of the body (especially the lungs). As high as 60% of dogs will have the cancer affect both thyroid glands, which causes the glands to malfunction in 50% of these cases. In about 10% of the cases, the tumor will produce excessive amounts of thyroid hormone, which can have systemic side effects.

Thyroid tumors are commonly seen in middle-aged to older large-breed dogs such as boxers, beagles, golden retrievers, and Siberian Huskies. They account for only 1.2–3.8% of all canine tumors.

Causes of Thyroid Tumors in Dogs

The reason why a particular pet may develop this, or any tumor or cancer, is not always straightforward. Very few tumors and cancers have a single known cause. Most seem to be caused by a complex mix of risk factors. With thyroid tumors, although there are no known dietary or environmental causes, there appears to be a genetic component, as certain breeds are more commonly affected, including Golden Retrievers, Beagles, Boxer Dogs, and Siberian Huskies. No breed predisposition has been identified in cats.

Symptoms of Thyroid Tumors in Dogs

Dogs with thyroid tumors may have no symptoms or may develop a lump in the neck region. If the mass compresses the windpipe (trachea), these dogs may present with difficulties breathing or coughing. If the mass is pressing on the esophagus your dog may gag or have difficulty swallowing. Some dogs may have a change in their bark, weight loss, or loss of appetite.

Thyroid tumors can occasionally be found in a location in the neck away from the normal thyroid glands or even under the tongue or in the chest.

Diagnosis

Diagnosis can be made by a fine needle aspirate or a biopsy. These tumors can have extensive blood supply, which can preclude identification of cancer cells in some samples and makes biopsy of them carry an increased risk of hemorrhage. Ultrasound and/or advanced imaging (CT/MRI) is often helpful in evaluating the degree of invasiveness and potential for surgical removal. A CT scan can also be helpful for evaluating lymph nodes which can be involved and for radiation treatment planning.

Treatment of Thyroid Tumors in Dogs

Surgery is recommended for dogs with mobile thyroid tumors, and those with limited invasion. Dogs with small, freely moveable tumors treated with complete surgical excision have median survival times of 3+years reported.

Radiation therapy is used to treat thyroid cancer that is invasive into underlying tissues. Typically one daily radiation treatment is administered from Monday to Friday with the weekends off for a total of 18 to 21 treatments. By administering small multiple doses of radiation, the side effects of radiation are lessened.

There are additional treatments that may be used in conjunction with surgery, radiation therapy, and chemotherapy. Non-steroidal anti-inflammatory drugs (NSAIDs) have been shown to have anticancer effects against some types of carcinomas