What is thyroid carcinoma in a dog?

Q: “My dog has a large mass in front of his throat. My veterinarian suspects it is bilateral thyroid cancer and has scheduled some tests. I am looking for any information regarding thyroid cancer in dogs. Can you tell me anything about it?”

A: It is a cancer of the thyroid gland that may spread.

Typically the type of cancer that affects the thyroid gland in dogs is called a carcinoma. These tumors are usually large, solid, obvious masses that cause few clinical signs that can be recognized by owners, aside from regional swelling. Most affected dogs have normal amounts of thyroid hormone circulating in the blood. However, some may have deficient amounts and will show signs of hypothyroidism (low thyroid). Uncommonly, some dogs with thyroid cancer will develop hyperthyroidism. Affected dogs may lose weight even though they are also eating and drinking excessively. These dogs may also urinate excessively.

Thyroid carcinomas may extend (or invade) into the esophagus, trachea, larynx, muscles of the neck, nerves, and surrounding blood vessels of the thyroid gland. This type of cancer tends to spread (metastasize) rapidly to the lungs, lymph nodes, and surrounding organs. During the physical examination, the veterinarian will determine if the suspected thyroid carcinoma is movable or fixed. Movable tumors tend not to have spread as of the time of physical examination. They have a better prognosis than fixed tumors. Fixed tumors are very likely to have invaded surrounding tissues and spread to the lungs and other organs. Unfortunately, bilateral thyroid cancer is most likely to be fixed, and thus carries a poor prognosis.

Diagnosis of thyroid carcinoma is based on clinical signs, physical examination, bloodwork, and the results of a fine-needle aspirate cytoanalysis (tumor cell study) or a biopsy. Following diagnosis, it is necessary to take x-rays of the chest to look for evidence of metastasis to the lungs. Abdominal ultrasound can be used to look for evidence of metastasis to the liver, kidneys and other organs.

Treatment may involve surgery, chemotherapy, radioactive iodine therapy, and radiation therapy. In general, complete surgical removal (in movable or well-encapsulated tumors), or debulking of as much of the tumor as possible (in fixed or nonencapsulated tumors) is generally and ideally the first step. Additional forms of therapy may be indicated according to findings at surgery and on x-ray and ultrasound studies. Fixed or nonencapsulated tumors especially may require radiation and/or chemotherapy, since complete surgical excision may not be possible and spreading tumor cells to other parts of the body may not be surgically treatable. If the thyroid cancer causes hypothyroidism, the dog patient will need synthetic thyroid supplementation. Dogs that have had the entire thyroid tumor and glands removed will need thyroid supplementation as well.

Surgery is often performed by a board-certified veterinary surgical specialist, or an experienced general veterinarian. Your veterinarian can talk to you about the surgery as well as additional treatment with chemotherapeutic agents and radiation therapy. Your veterinarian may find it necessary to refer your dog to an oncologist or radiation oncologist for a consultation; then decisions concerning the best treatment can be made. The prognosis at that time can also be discussed based on additional tests.

You may also find helpful information on the VetCentric Web site at www.vetcentic.com. Enter the terms "hypothyroidism," "hyperthyroidism," and "cancer" into any search engine and select "search entire site."

Good Luck.

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