Hyperthyroidism in Dogs: Causes, Signs and Management

Hyperthyroidism in dogs is rare but has serious consequences. Recognize the signs that can be associated with this excess in thyroid hormones and act early for the best outcome.

Portrait Mixed Breed Adult Dog Drinking Water Outdoor On Sunny Summer Day
Increased thirst is one sign of hyperthyroidism in dogs. If your dog is drinking more water than usual, plus shows signs of the other symptoms on the list, contact your vet. © Elena Popova/ Getty Image

Dogs have a thyroid gland in their necks that produces hormones. When excess thyroid hormones are released, hyperthyroidism occurs and results in negative effects in the body. This condition is most common in older dogs but is quite rare overall.

Causes of hyperthyroidism in dogs

A cancerous thyroid mass is the most common cause of hyperthyroidism in dogs. Less commonly, a benign mass called an adenoma can develop in the thyroid gland and cause symptoms.

Giving too much levothyroxine to a hypothyroid dog is also a common cause of hyperthyroidism. This can be prevented through monitoring and dose adjustments.

Signs of hyperthyroidism in dogs

Hyperthyroidism in dogs causes various symptoms, which include the following:

  • Enlargement of thyroid gland
  • Weight loss despite an increased appetite
  • Increased thirst and urination
  • Increased heart rate
  • Abnormal heart rhythms
  • Dull hair coat
  • Hair loss
  • Hyperactivity
  • Vomiting
  • Diarrhea
  • Behavioral changes, such as aggression
  • Difficulty breathing and swallowing
  • Coughing
  • Facial swelling

If you notice any of these symptoms, bring your dog to the veterinarian right away to determine the underlying cause and begin treatment.

Management of hyperthyroidism in dogs

Management depends on the underlying cause. For dogs who have received too much levothyroxine, simply adjusting the dose can return thyroid hormone levels back to normal.

In dogs with thyroid masses, surgery may be an option. Radiation therapy can be used alone or in combination with surgery as another form of treatment. Radioactive iodine therapy also destroys thyroid tissue and reduces hormone levels. Your veterinarian can advise you on the best approach based on your dog’s overall health and needs.

Complications of hyperthyroidism in dogs

Hyperthyroid dogs are at an increased risk for other health conditions, such as heart or kidney disease and high blood pressure. Because thyroid cancer has usually spread by the time of diagnosis, most dogs experience a shortened lifespan. However, with the proper management, dogs with hyperthyroidism can still live happy lives.

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