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Key takeaways

Canine hyperthyroidism results from the excessive production of thyroid hormones due to a functional thyroid gland tumor.

  • Thyroid cancer is rare in dogs
  • The cause of functional thyroid tumors in dogs is unknown
  • 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 determine 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
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A closer look: Hyperthyroidism in Dogs

The thyroid is a gland located in the throat and is responsible for regulating metabolism and other related functions. Metabolism is the process of breaking down food into nutrients and energy for use by the body. Changes in metabolism generally affect appetite, energy levels, and body weight. Hyperthyroidism refers to overactive thyroid resulting in increased appetite, heightened energy levels, and weight loss.

In dogs, hyperthyroidism is very rare and is linked to having an active tumor in the gland causing excess secretion of thyroid hormone which over-stimulates metabolism.

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.

Excessive thyroid hormone production may not be sufficient to cause symptoms.

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Risk factors

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.

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

Possible causes

Hyperthyroidism in dogs is caused by functional thyroid tumors which secrete excess thyroid hormone, leading to abnormally high metabolism. The cause of functional thyroid tumors in dogs is unknown. As with all forms of cancer, environmental and genetic factors are suspected.

Main symptoms

Some thyroid tumors are asymptomatic.

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

Testing and diagnosis

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 tumor/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.


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

Is Hyperthyroidism in Dogs common?

Functional thyroid tumors are rare in dogs.

Typical Treatment

  • Surgical removal of the tumor
  • Chemotherapy
  • Radiation therapy
  • Palliative care


Mark E. Peterson , DVM, DACVIM-SAIM / Janice E. Kritchevsky , VMD, DACVIM-LAIM - Writing for Merck Veterinary Manual
Wendy Brooks, DVM, DABVP - Writing for Veterinary Partner
No Author - Writing for American College of Veterinary Surgeons
No Author - Writing for Lloyd Veterinary Medical Center

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