Dogs have two thyroid glands located on each side of the windpipe in the neck. These glands produce thyroxin, a hormone that regulates the body’s metabolism.
Thyroid tumors in dogs are relatively uncommon, but golden retrievers, beagles and boxers between the ages of 9 and 11 are most common to develop this tumor. These tumors are almost always malignant, meaning that they tend to spread to the lungs and lymph nodes and can grow into nearby structures such as the windpipe, blood vessels and esophagus.
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.
The most common clinical sign of this tumor is a mass in the neck. Other signs usually are attributable to the tumor affecting the nerves of the voice box and may include a change in the tone of the bark, harsh breathing sounds, difficulty breathing and rapid breathing.
If the nerves that control swallowing have been affected or the esophagus is compressed by the tumor, difficulty swallowing may be noted. Other less common findings include bruising under the skin and swelling of the head.
If the tumor produces excessive amounts of thyroid hormone, weight loss, marked increased appetite, muscle wasting, increased thirst and increased urination may be seen.
A complete blood count, chemistry profile and urine testing are completed in the initial stages of the evaluation of the patient that has a suspected thyroid tumor. Blood tests can be run to see if the thyroid glands are functioning normally or are hyperactive. A fine needle biopsy of the tumor provides a diagnosis about 50% of the time.
Ultrasound of the neck frequently can show the extent of the tumor and may demonstrate spread of the tumor to lymph nodes in the neck. Chest X-rays are important to identify visible spread of tumor to the lungs. It is important to note that no imaging test can identify microscopic spread of cancer to the lungs and other internal organs.
It has been estimated that only 25 to 50% of patients presenting with a thyroid tumor are candidates for surgery. Only tumors that are not attached to the underlying tissues in the neck should be removed.
Surgery involves making an incision on the underside of the neck, separating the muscles that cover the thyroid gland and then removing the thyroid tumor. Both thyroid glands are inspected as some patients have tumors in both glands.
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.
Thyroid tumors tend to be moderately responsive to chemotherapy; therefore, there is value in this treatment approach.
Unlike humans, most dogs receiving chemotherapy do not lose their hair and usually have only mild side effects, which may include transient loss of appetite and vomiting.
Treatment with radioactive iodine can be successfully used to treat some thyroid tumors. Since this treatment requires a high dose of radioactive iodine, most hospitals do not offer this treatment due to staff safety concerns.
Negative prognostic factors for patient survival include large tumor size (tumors larger than 100 cm3 have a 100% spread rate), presence of measurable metastasis, bilateral tumors, immoveable tumors, and tumors that cause paralysis of the voice box.
One study that included 20 patients that received surgery alone for treatment of freely removable thyroid tumors with no spread of tumor to the lungs or lymph nodes found median and mean survival times of 20.5 months. Of these patients, eight died of non-tumor-related diseases. With removal of these patients from the study, the median survival time was more than 36 months. Surgery therefore is the treatment of choice in patients that have moveable tumors.
Another study included 13 patients that were treated with cisplatin chemotherapy. Most of these had tumors that were invasive and not freely moveable. One dog had complete remission, six dogs had partial remissions; three dogs had stable disease; and three did not respond at all. The mean survival time was 191.8 days.
One study included a group of dogs that had infiltrative thyroid carcinomas that were treated with radiation therapy alone. The tumors shrunk to undetectable disease in all dogs. Metastasis to the lungs was seen in three dogs and to the bone in another dog. Overall median survival time was 24.5 months.
Another study that included 13 dogs with invasive thyroid carcinoma showed a median survival time of 96 weeks. Four dogs died of metastatic disease and four from progression of disease. It is concluded that radiation therapy is a better treatment modality than chemotherapy alone. Although the studies have not been published, chemotherapy combined with radiation therapy may be more beneficial than a single treatment modality.