Hyperthyroidism in Pets and Companion Animals

Hyperthyroidism does affect pets, but to differing degrees; it is the single most common glandular disorder in cats, while it is rare in dogs. In cats, it is caused by an overproduction of the thyroid hormone thyroxine, and increases the animal’s metabolism. In dogs, it is usually caused by either a tumor of the thyroid, or as a result of medication given to them to combat underproduction of thyroid hormones (hypothyroidism).

Normally, the thyroid gland produces thyroid hormones for use in the body as it is signaled to do so by the pituitary gland. These thyroid hormones regulate the chemical processes occurring in the cells throughout the animal’s body.

hyperthyroidism in pets

If there are too many of the thyroid hormones being produced, as in the case of hyperthyroidism, it throws the body’s processes into an accelerated state, resulting in increased metabolism and a host of other symptoms, including anxiety, weight loss, diarrhea and many others.

Signs and Effects of Hyperthyroidism in Pets

The most common symptom of hyperthyroidism in pets and companion animals is increased appetite combined with weight loss. Other symptoms can include:

  • Excessive thirst
  • Panting
  • Hyperactivity
  • Unkempt appearance
  • Vomiting
  • Increased urination
  • Increased shedding
  • Poor body condition
  • Rapid breathing
  • Difficulty in breathing
  • An enlarged thyroid gland (which can be felt as a lump on the neck in some cases), and
  • Rapid or abnormal heart rate.

In some cases, dogs with hyperthyroidism exhibit the opposite of some of these symptoms. They can seem to suffer from general apathy with a loss of appetite, weakness, and depression.

Diagnosis of Hyperthyroidism in Pets

In dogs, a preliminary diagnosis may sometimes be made based on the veterinarian palpating the enlarged thyroid gland through the animal’s skin. Following this, a battery of tests, including complete blood work and a urinalysis, will most likely be ordered.

Finding a high concentration of thyroxine in the blood will usually be enough to confirm a diagnosis of hyperthyroidism, though in some cases of hyperthyroid (especially early cases), high concentrations of the hormone may not be present, making a definitive diagnosis more problematic. Your vet may also perform a variety of other tests, including a thyroid gland scintigraphy, and take X-rays to determine the potential metastasis of a thyroid tumor.

In cats, hyperthyroidism can cause similar symptoms as a host of other common diseases affecting older cats, including inflammatory bowel disease, kidney failure, diabetes, and intestinal cancer.

Your vet will most likely order a complete blood chemistry panel and urinalysis to rule out several other potential problems prior to arriving at a positive diagnosis through a simple blood test looking for elevated thyroxine in the blood. In a small percentage of cats, hyperthyroidism is very challenging to diagnose as its effects are masked by other concurrent conditions or illnesses.

Treatment Options for Hyperthyroidism in Pets

In dogs with elevated thyroid hormone levels due to over-medication for hypothyroidism, a simple adjustment of medication may be enough to solve the problem. In cases where a tumor on the thyroid is determined to be the cause of the issue, surgical removal of the affected gland, sometimes in conjunction with radiation therapy depending on the size and invasiveness of the tumor, will be necessary.

In cats, three treatment options are typically used. Oral administration of a medication like Methimazole can regulate the output of the thyroid gland for the remainder of the pet’s life is typically the first option considered. Unfortunately, These medications are typically very well tolerated in animals.

As a second option, surgical removal of the affected gland or a tumor on the gland may be the best course of treatment. The third and most effective treatment option involves the administration of radioactive iodine, which concentrates in the thyroid naturally, and irradiates the benign tumor causing the issue from within.


Author: Giano Panzarella