Hyperthyroidism in Dogs

Summary

Canine hyperthyroidism results from the excessive production of thyroid hormones due to a functional thyroid gland tumor. Thyroid cancer is rare in dogs. Canine thyroid tumors are usually initially detected as a lump in the throat area that may cause coughing or a change in the bark. Symptoms associated with excessive thyroid hormone production include weight loss with increased appetite and excessive thirst. Size and spread of the tumor determines treatment and influence outcome. Treatment options include surgical removal of the tumor, chemotherapy, and radiation therapy. Average life expectancy after treatment may be as long as 1-3 years. The cause of functional thyroid tumors in dogs is unknown.

Risk Factors

Not all canine thyroid tumors are functional. Thyroid hormone levels remain normal or may even drop with a non-functional thyroid tumor that destroys normal thyroid tissue. 

Functional thyroid tumors are rare in dogs, and may produce no obvious symptoms at all. The most common initial symptom is a lump in the throat area. If the lump is large enough, coughing and  difficulty breathing may result, and this indicates a need for more urgent care. Over 80% of canine thyroid tumors are malignant. Early detection and prompt treatment can contribute to better outcomes.

Possible Causes

While the cause for functional thyroid tumors is unknown, a predisposition in middle-aged to older, medium-size to large dogs is suggested.

Main Symptoms

When a functional thyroid tumor produces enough excess thyroid hormones, symptoms associated with hyperthyroidism result:

• Weight loss • Increased appetite• Hyperactivity• Excessive thirst and urination (polydipsia/polyuria)

• Changes in hair coat

Detailed Characterization

If excessive thyroid hormone production is not sufficient to cause symptoms, general symptoms associated with any type of thyroid mass are expected:

• Lump in throat area • Difficulty swallowing • Difficulty breathing • Exercise intolerance • Cough

• Change in bark

Some thyroid tumors are asymptomatic.

Testing and Diagnosis

Regardless of if they are functional or not, thyroid tumors usually present as a lump in the throat. In addition to a thorough physical examination, further diagnostics include bloodwork, thyroid profiles (additional blood tests), imaging of the neck (x-rays, CT scan, MRI, ultrasound), and potentially a fine needle aspirate to microscopically examine cells from the lump. X-rays of the chest are helpful to determine whether or not the tumor has spread to the lungs (metastasis).

Steps to Recovery

Surgical removal of the tumors/thyroid gland is an option when there are no metastases and the tumor is not too large or deeply attached. Radiation and chemotherapy are additional options. Treatment with radioactive iodine (T-131) is an option for tumors that are large or vascularly invasive, or when the dog is a poor surgical candidate. Most thyroid tumors in dogs are malignant. With surgical removal and chemotherapy, a survival time of around 1-3 years is expected.

Prevention

Functional thyroid tumors are not contagious to people or other animals. Their cause is unknown so there is no known prevention.

Is Hyperthyroidism Common in Dogs?

Functional thyroid tumors are rare in dogs.

Typical Treatment

Treatment options for hyperthyroidism include surgical removal of the tumor, chemotherapy, radiation therapy, or palliative care.