A mass in your dog's neck should alert you to a possible thyroid tumor

Thyroid tumors are uncommon in dogs and usually are malignant

Treatment includes surgical removal of thyroid tumors that are freely moveable

Tumors that are attached to underlying tissues of the neck are best treated with radiation therapy

Chemotherapy is recommended for all patients that have a thyroid tumors, however, very small thyroid tumors are usually self contained and may be cured with surgery alone

What is a thyroid tumor?

• Dogs have two thyroid glands that are located along side of the neck and produce a hormone called thyroxine that regulates the body's metabolism.
• A mass located on the bottom side of the neck, should alert you to the possiblity that your dog may have a thyroid tumor.
• These tumors frequently are malignant, meaning that they have a high risk for spread to regional lymph nodes and the lungs.
• About 35 to 40% of dogs that are presented for evaluation of a thyroid tumor will have evidence of spread to other areas of the body.
• About 60% of the dogs have these tumors in both thyroid glands and half of these patients have malfunction of the thyroid glands.
• About 10% of thyroid tumors produce excessive amounts of thyroxine which can cause serious side effects.

Clinical Signs

• Average age: 9 to 11 years
• Most common breeds: Boxers, Labradors, Beagles
• Most common sign: a mass in the neck
• Signs caused by the mass damaging the nerves to the voice box
  • change in the tone of the bark
  • harsh breathing sounds
  • breathing difficulty
  • rapid breathing
• Damage to the nerves that control swallowing or compression of the esophagus by the tumor may cause swallowing difficulties
• Less common signs include bruising of the neck and swelling of the face
• Signs associated with excessive thyroid hormone production
  • weight loss
  • increased appetite
  • muscle wasting
  • increased thirst
  • increased urination

Diagnosis

• Initial tests that may be performed include a complete blood count, chemistry profile, thyroid hormone levels, and urine testing
• Fine needle biopsy may provide a diagnosis, but has been reported to be an accurate test in only half of the patients. This is due to the fact that thyroid tumors have a vast blood supply, thus the biopsy usually only retrieves blood and not tumor cells. As a result, if the aspiration of the mass reveals a lot of blood, there is a very good chance that it is a thyroid tumor even if tumor cells are not seen on the cytology.
• Chest x-rays are needed to rule out spread of tumor to the lungs

Treatment

• Surgery is indicated for thyroid tumors that are freely moveable in the neck.
• Radiation therapy is used for thyroid tumors that are fixed to underlying tissues, because surgery may cause extensive damage to critical structures such as important nerves, esophagus, windpipe and large blood vessels.
• Chemotherapy is frequently used to treat thyroid tumors as they have a high tendancy to spread.
• After surgery, exercise should be limited to minimize the risk for development of a fluid pocket in the neck called a seroma. If a seroma develops, it typically will resolve on its own with time.
• Offer your pet soft food for about one week while the neck may be sore and swallowing may be more difficult.
• Use a harness instead of a collar to walk your dog to minimize pressure on the incision.
• If your pet scratches the incision, return to us to have the neck bandaged.

Prognosis

• Average survival times have been reported to be 3 years when freely moveable tumors have been excised and no evidence of spread of the tumor to lymph nodes and lungs is present.
• Average survival times with radiation of fixed tumors is also favorable with 72% of the patients still alive 3 years after treatment.
• Patients that have thyroid tumors that are greater than 100 cubic centimeters have a spread rate of the tumor being close to 100%; tumors that are less than 27 cubic centimeters have a relatively low spread rate.
• A less common thyroid tumor called a medullary thyroid tumor, although still malignant, has been shown to have a better prognosis than thyroid carcinomas.