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Hyperthyroidism occurs when your dog’s body produces too much of the thyroid hormone, increasing your dog’s metabolic rate to dangerous levels. This disease is rare in dogs and is much more common in cats, but when it does affect dogs, is it very much serious.

The thyroid is a gland located in your pup’s neck that is responsible for producing a number of important hormones which help to regulate body temperature and metabolism. When your dog’s thyroid hormones become too high or too low they can cause serious health issues.

A type of cancer, called thyroid carcinoma, is the primary cause of hyperthyroidism in dogs. Once diagnosed, your veterinarian will discuss the treatment options and prognosis for your dog. These options will depend on the stage of the cancer, the size of the tumor, and the extent of nearby tissue involvement.

Causes of Hyperthyroidism in Dogs

The most common cause of hyperthyroidism in dogs is thyroid carcinoma, which is a type of cancer. This cancer causes the thyroid glands to overproduce the hormone thyroxine.

Another cause is overly aggressive treatment of hypothyroidism. Dogs with hypothyroidism take oral medication (usually levothyroxine) to increase thyroxine production. If the dose is too high, the thyroid glands overproduce thyroxine.

Symptoms of Hyperthyroidism in Dogs

  • Depression. 
  • Diarrhea. 
  • Enlarged thyroid gland. 
  • Excessive thirst. 
  • Forced breathing. 
  • Heavy, rapid breathing. 
  • Increased appetite. 
  • Hyper-activity
  • Increased urination. 
  • Nervousness. 
  • Rapid heart rate. 
  • Shaggy hair texture. 
  • Vomiting. 
  • Weight loss.


Hyperthyroidism is diagnosed via blood tests that measure the levels of thyroid hormones. Your vet may order additional testing if there’s evidence of a thyroid tumor.

Treatment of Hyperthyroidism in Dogs

Most thyroid problems in dogs are associated with low levels and so a simple supplementation or addition of thyroid hormones will result in normal levels quickly. Response is generally dramatic.

Hyperthyroidism is uncommon in dogs but may occur with active thyroid cancer. Surgical removal of thyroid tumors is possible but radiation therapy is more likely to be effective.


Little can be done to prevent thyroid tumors in dogs. If hyperthyroidism was caused by a medication overdose, frequent rechecks and giving medication exactly as prescribed will help prevent additional overdoses.


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