|
THYROID TUMORS IN DOGS
Overview
The thyroid glands are paired structures located along the windpipe (trachea), about one halfway down the neck of dogs. The thyroid glands are responsible for producing hormones that are vital for normal body function. Thyroid growths in dogs can be benign (adenoma) or malignant (carcinoma). Benign growths tend to get larger and may produce excess hormones; malignant growths will also spread to other parts of the body. While benign adenomatous changes of the thyroid gland are common in cats, the majority of dogs have malignant neoplasia. Thyroid tumors are commonly seen in middle aged to older large breed dogs such as boxers and golden retrievers.
Symptoms
Dogs may have no symptoms or may develop a lump in the neck region (Figure 1). If the mass compresses the windpipe (trachea), these dogs may present with difficulties breathing or coughing (Figure 2). If the mass is pressing on the esophagus your dog may gag or have difficulty swallowing. Although the majority of canine tumors are malignant, they rarely produce excessive hormones that are associated with clinical signs of hyperthyroidism (restlessness, hair coat abnormalities or drinking and urinating more than usual).
Figure 1. This dog’s neck has been shaved in preparation for surgery; its head is to the left. The large thyroid masses (blue arrows) caused the neck to appear swollen.
Figure 2. The large thyroid tumor (blue arrows) in this dog’s neck forced the trachea (green arrows) over to the side and flattened it. This dog presented with coughing and difficulty swallowing because of pressure from the tumor.
When to Seek Veterinary Advice
If you feel a growth or lump anywhere along the neck of your dog from his chin to his front legs you should make an appointment to see a veterinarian. If you observe that your dog is having distress or difficulty breathing you should seek immediate veterinary care.
Exam and Diagnosis
These masses occasionally cause neck swelling on radiographs (x-rays) but other imaging techniques such as ultrasound or CAT scan may be better for assessing the size and invasiveness of the tumor. Definitive diagnosis is based on microscopic examination of a tissue sample. Due to the highly vascular nature of the tumor coagulation parameters (blood clotting tests) should be assessed prior to biopsy.
Treatment Options
Removal of these tumors can be difficult because the tumors can invade local blood vessels or other tissues. Radiation or chemotherapy is often recommended for masses that are incompletely resected or are too large for surgical removal.
When to Contact an ACVS Veterinary Surgeon
Because tumors that are large or invasive can be difficult to remove, most veterinarians will refer dogs with tumors over 1 inch in diameter to an ACVS Veterinary Surgeon.
Complications
There are always risks associated with generally anesthesia. Complications specific to removal of thyroid tumors in dogs include bleeding or damage to the recurrent laryngeal nerve, which is responsible for movement of the larynx (upper airway cartilages) during breathing and swallowing (Figure 3, 4). Dogs that have both thyroid glands removed may experience low calcium or, rarely, thyroid hormone levels (hypocalcemia or hypothyroidism). Finally, there can be some swelling of the incision on the neck after surgery.
Figure 3. The thyroid tumor (blue arrows) in this dog was located along the trachea (green arrows) and was attached to the recurrent laryngeal nerve (white arrows). Damage to this nerve could cause laryngeal paralysis, resulting in coughing and gagging after surgery.
Figure 4. This thyroid tumor grew down into the jugular vein, one of the major vessels that drains blood from the head. Tumors cells released into this blood vessel can spread to the lungs; however, this 10-year-old dog lived for 2 years after surgery without any signs of illness.
Aftercare
After surgery your dog may have a soft bandage around his neck. You should avoid putting any leashes or collars around your dogs’ neck until he has healed from surgery (usually 10-14 days). During this time period you should keep your dogs activity limited and follow the advice of your ACVS Veterinary Surgeon regarding any medication that may be needed after surgery. If both thyroid glands are removed, your veterinarian may need to check your dog’s calcium levels several times during recovery.
Prognosis
Surgical resection of the tumors has been associated with the best response if the mass is freely moveable, less than 4cm in size, nonmetastatic (has not spread) and can be completely removed. Long-term survival (1 to 3 years) may be achieved in dogs, depending on histologic features and early diagnosis prior to local invasion or metastatic (spreading) disease.
Speak to a board certified surgeon in your area to see if surgery is an option for your dog.
—Janet R. Kovak, DVM
Diplomate ACVS
Posted 2/13/2006
| 
