Thyroid tumors account for 1.2 % to 3.8 % of all tumors in the dog and typically develop in older dogs with a median age of 9-11 years. There is no gender predisposition but Golden retrievers, Boxers, and Beagles are over represented. Most thyroid carcinomas are non-functional (meaning they don’t produce thyroid hormones); 60% of patients have normal thyroid function; 30% are hypothyroid (under active thyroid); 10% are hyperthyroid (over active thyroid). Approximately 30-40% of thyroid carcinomas will have already metastasized (spread) at the time of diagnosis and ~80% will ultimately develop metastasis.

What causes thyroid cancer?

The cause of thyroid cancer in dogs is largely unknown.

Common signs of thyroid cancer in dogs

• Most dogs have a mass that is noticed on the ventral neck
• Most patients are asymptomatic (not signs of disease)
• Less common signs that some patients will present with include: coughing, change in quality of their bark, trouble swallowing, trouble breathing, facial swelling/edema
• Patients may have signs related to hypo- or hyper-thyroidism if the thyroid carcinoma is functional 

How is thyroid cancer diagnosed?

• A CBC; chemistry panel; urinalysis are needed to assess total body function
• Thyroid panel is needed to determine if thyroid function is normal, under active, or over active.
• 3 view thoracic (chest) radiographs.
• Aspiration and cytology of the regional lymph node(s).
• Ultrasound and CT scan of the mass can be used to help evaluate how invasive it is into the surrounding tissues/structures and if the tumor can be removed surgically.
• A fine needle aspirate and cytology can be performed but is unfortunately not very accurate when it comes to making a diagnosis of thyroid carcinoma.
• A biopsy (taken at the time of surgery) is required to make a definitive diagnosis of thyroid carcinoma.

• The treatment of choice is surgical resection when the mass is freely movable and not invading surrounding structures.
• Thyroid carcinomas that cannot be completely removed can be treated with radiation therapy.
• Patients with incomplete surgical resection, metastatic disease, or evidence of more aggressive tumors can be treated with chemotherapy (in addition to surgery and/or radiation therapy). Signs of tumors that have a higher chance of spreading include capsular/vascular invasion, large tumors, or bilateral tumors (masses of both the right and left thyroid glands). 
• Anti-angiogenic (metronomic) chemotherapy is also an option for reducing tumor growth
• Low dose thyroid supplementation may also help by inhibiting growth of the cancer cells.
• Radioactive iodine, a common treatment for thyroid cancer in people, is infrequently used to treat thyroid cancer in dogs. 

What is the prognosis for dogs with thyroid cancer?

• The median (average) survival time after complete surgical excision for non-invasive, freely movable masses is about 3 years.
• The median survival time after surgical excision alone for more invasive masses is 6-12 months.
• Dogs treated with radiation therapy without evidence of metastatic disease can have median survival times of over 2 years. 

What is on the horizon for this cancer?

Targeted therapies are currently being evaluated In the therapy of this tumor.