Thyroid Cancer in Dogs and Cats

By Kendra Hearon | VMD, DACVS-SA, ACVS Fellow, Surgical Oncology

What is the thyroid gland?

The right and left thyroid glands are located near the trachea. There are several important structures near the thyroid glands, like the jugular veins, carotid arteries, the esophagus, lymph nodes, and important nerves. There are also four parathyroid glands. The thyroid glands produce thyroid hormone, which affects the body’s metabolism. The parathyroid glands regulate calcium and phosphorus levels in the body.

Thyroid tumors in dogs

Thyroid tumors in dogs are not very common and only account for one to four percent of all canine tumors. There may be a breed predisposition for Boxers, Beagles, and Golden Retrievers, although they can be seen in any breed. The average age of dogs diagnosed with thyroid tumors is over nine years old. Most thyroid tumors in dogs are malignant (which means that they spread to other parts of the body and invade normal tissue). However, most thyroid tumors in dogs are not functional, meaning they do not secrete extra thyroid hormone (which would cause hyperthyroidism), or prevent extra thyroid hormone from being excreted (which would cause hypothyroidism). Approximately one-third of dogs with thyroid tumors will have metastasis (spread of the tumor into other areas of the body) at the time of diagnosis and most will go on to metastasize over the course of one to two years. The tumor tends to spread to the nearby lymph node first, and then the lungs.

How do I know if my dog has a thyroid tumor?

Dogs with thyroid tumors usually present with a firm mass in their necks. If you notice that your dog has a mass in their neck, your veterinarian will perform an examination of the area. Masses that feel encapsulated are more able to be surgically removed than masses that are diffuse (invading into surrounding structures). Also, masses that feel fixed to the neck are more difficult to remove than masses that are able to be moved around in the neck. A biopsy of the mass is not recommended because these tumors can have a lot of blood vessels within them. An aspiration of the mass with a needle can be attempted, especially with the help of ultrasound guidance to avoid major blood vessels. An ultrasound of the neck can also help to determine whether the mass is arising from the thyroid gland, if it is encapsulated, and if any of the surrounding lymph nodes appear enlarged. If the mass feels fixed to the neck or invading into surrounding structures, a CT scan is recommended. In addition, routine bloodwork with a thyroid level should be performed as well as either chest X-rays or CT scan to look for evidence of metastasis.

Surgical removal is the ideal treatment if the mass is not invading major structures. If the entire mass needs to be removed, sections of the jugular veins and carotid artery can be tied off and removed, as well. If the recurrent laryngeal nerve is damaged (a nerve that controls the voice box), it can lead to a condition known as laryngeal paralysis, where the muscles of the larynx become paralyzed. Laryngeal paralysis is not usually clinically significant if it is only on one side, but owners should be warned of the risks as well as symptoms and treatment. If the tumor is present on both sides, then the risks of laryngeal paralysis are greater, and patients may need a procedure known as a tie-back procedure which will hold the voice box open so that air can flow across it normally. In general, surgery is very well tolerated and most patients can go home the following day.

Any enlarged local lymph nodes should also be removed at the time of surgery and submitted for biopsy. If the mass is invasive and not amenable to surgery, then radiation with or without chemotherapy would be considered. Chemotherapy may be indicated post operatively based upon tumor size, stage (evidence of spread), and/or findings on histopathology such as invasion of the blood vessels or lymphatic system vascular or other indicators of aggressive behavior.

The long-term prognosis for thyroid tumors is dependent upon many factors including whether it is surgically removable, size of the tumor, whether the tumor is bilateral or unilateral, whether there is any evidence of metastasis, blood vessel or lymphatic invasion.For dogs with small tumors that can be surgically removed without evidence of spread or blood vessel and lymphatic invasion, survival time with surgery alone is greater than three years.

Anecdotally, many surgeons feel that thyroid tumors can have a very slow progression of metastatic disease and can still do well for upwards of a year with removal of the tumor, removal of any affected lymph nodes, and follow up chemotherapy. Medical therapy such as Palladia is emerging as a treatment option for several types of carcinomas, including thyroid carcinomas, when there is evidence of progressive disease.

Thyroid tumors in cats

Thyroid tumors in dogs and cats tend to have very different behaviors. Thyroid tumorsin cats are common, with a majority of them being on both sides. They are almost always benign and do not metastasize (called adenomas or hyperplasia). However, they are functional, meaning they lead to an increase in thyroid hormone release, causing hyperthyroidism.

How do I know if my cat has a thyroid tumor?

Cats with thyroid tumors are usually found on routine bloodwork or when a cat presents with any clinical sign ranging from weight loss despite a ravenous appetite, hyperactivity, fast heart rate, heart murmur, and a possible mass on the neck. Your veterinarian will perform an examination of the neck area on routine physical exam and may or may not find a small mass in the neck. Cats can also have ectopic thyroid tissue, where there is thyroid tissue present in other areas of the body other than the neck and thus not have a mass within the neck.

Cats with hyperthyroidism are often treated with medical management (methimazole) or radioactive iodine therapy (I-131), which is offered here at MVA. Some cases may be controlled on a prescription diet food with limited iodine, controlled phosphorus, and low sodium to decrease thyroid hormone production. However, they can also be surgically removed.