Thyroid tumors in dogs: what are we talking about?
A thyroid tumor develops due to an anarchic multiplication of the cells that make up the thyroid, this bilobed gland located in front of the trachea in the neck region.
In dogs, there are benign (non-cancerous) tumors as well as malignant (cancerous) tumors of the thyroid. And, although relatively rare in dogs, most thyroid tumors are unfortunately cancerous in dogs. These are adenocarcinomas in 90 to 95% of cases and adenomas in 5 to 10% of cases.
Adenocarcinomas often invade neighboring structures such as the esophagus, trachea, larynx, nerves, vessels and cervical muscles. They frequently metastasize to the lungs, lymph nodes, cervical vertebrae, kidneys, adrenal glands, myocardium, liver and brain.
In the vast majority of cases, these tumors are non-secreting, i.e. they are not the cause of excessive production of thyroid hormones.
What causes thyroid tumors in dogs?
The reason why a particular pet may develop this type of tumor is not yet precisely known. This type of cancer, like most cancers, appears to be caused by a complex mix of risk factors.
In the specific case of thyroid tumors, there seems to be a genetic component, as certain breeds are more commonly affected, including Golden Retrievers, Beagles and Boxers for adenocarcinomas.
What are the symptoms associated with a thyroid tumor in dogs?
Dogs with a thyroid tumor usually present with a mass on the underside of the neck.
If this mass compresses the trachea, it can cause coughing or difficulty breathing. If it presses on the esophagus, it can cause dysphagia (difficulty eating properly) and swallowing normally. If she presses on her vocal cords, she may be causing a change in their bark.
Some dogs may also have cervico-facial edema and bleeding disorders.
Rarely, one may observe symptoms of hyperthyroidism which include increased appetite, polyuro-polydipsia (the dog drinks and urinates more), weight loss, restlessness and tachycardia (heart rate fast and abnormal).
How are these diseases diagnosed?
This type of tumor is diagnosed from a thorough physical examination during which the veterinarian may palpate a cervical mass.
A biopsy of this mass is generally necessary to establish a definitive diagnosis. Since most tumors in dogs are cancerous and metastasize, further imaging tests may be needed to determine the stage of the disease.
The veterinarian can also request blood tests that will help define the dog's thyroid status.
What treatments are available?
Various treatment options are available for dogs with thyroid tumors. The choice of treatment will depend on the size of the tumour, the extent to which it has invaded nearby tissues and whether there are any metastases.
The surgery consists of a thyroidectomy which involves removing the entire thyroid gland.It is especially indicated for dogs with mobile thyroid tumors without invasion of neighboring tissues or with partially adherent tumors with limited invasion of surrounding tissues. In the second case, the surgeon also removes the invaded tissue, as far as possible.
When the tumors are too large, very invasive or firmly attached, surgery is not an option and radiotherapy with radioactive iodine (I-131) or chemotherapy is generally used.
With proper treatment, the prognosis for dogs with thyroid tumors is good to excellent when the tumors have been successfully resected. On the other hand, the prognosis is worse for dogs that have large tumors and/or very adherent to neighboring tissues.