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Hyperthyroidism in Cats

By: Dr. Michael Bernstein

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Updated: June 26, 2014

Feline hyperthyroidism, also referred to as thyrotoxicosis in cats, is one of the most common endocrine disorders in cats. If your adult cat suddenly begins to lose weight despite a voracious appetite, he may have a hormone problem, specifically the hormone produced by the thyroid gland. You should probably get your pet to a veterinarian to be checked for feline hyperthyroidism.

Feline hyperthyroidism has become a widely recognized disorder in cats. It is caused by an unregulated overproduction of thyroid hormone by the thyroid glands, which is usually related to a benign enlargement (growth or tumor) of one or both thyroid lobes. Cancer of the thyroid gland is found in less than 2% of cats. This enlargement of the thyroid gland(s) is referred to as thyroid adenoma or thyroid adenomatous goiter. It is unknown what causes the thyroid to become enlarged.

The thyroid gland consists of two flat lobes shaped like a butterfly and located on either side of the trachea, or windpipe, just below the voice box. These lobes are flattened and cannot be easily palpated. The thyroid gland acts as the thermostat for the metabolic rate of the body, controlling how fast or slow the body functions. Hyperthyroidism can have effects on multiple organ systems, since the increased thyroid hormone levels increase the cat's metabolic rate.

Hyperthyroidism is diagnosed in cats from age 4 to 20 + years, although this disease is usually diagnosed in older cats (95% are at least 8 years of age). There is no recognized breed or sex predilection for this disease, although purebred cats seem to be less likely to be hyperthyroid.

There is no known cause for hyperthyroidism although canned food and ectoparasiticide exposure has been theorized.

What to Watch For

The classic signs are:

  • Weight loss in spite of an increased appetite
  • Restlessness and/or hyperactivity
  • Increased activity levels or irritability

    Other signs include:

  • Increased thirst
  • Vomiting and or diarrhea
  • Decreased grooming activity
  • Seeking cool areas

    Please note approximately 10% of cats have what is referred to as "apathetic hyperthyroidism" with atypical symptoms. Clinical signs may include lethargy, weight gain and decreased appetite.


    Diagnosis can be made by a simple blood test that measures the level of the thyroid hormone (T4).

    Your veterinarian may also perform other diagnostic tests to exclude other diseases, including:

  • Complete medical history and physical examination
  • Complete blood count (CBC) and serum chemistry profile
  • Thoracic radiographs
  • Blood pressure
  • Echocardiogram
  • Electrocardiogram (if an abnormal or irregular heart rate is suspected)
  • A T3 suppression test in hard-to-diagnose cases
  • In some cases a radionuclide scan (used in diagnosis for whole body or individual organ scanning)


    Treatment is directed at controlling excessive secretion of thyroid hormones and can involve a variety of approaches depending on several factors. These include your cat's overall health, availability of radioactive iodine therapy and cost considerations. There are three main methods of treatment:

  • Radioactive iodine therapy
  • Surgical removal of the abnormal thyroid lobes
  • Medical therapy with Tapazole® (methimazole) and beta-adrenergic blockers (such as atenolol) to reduce some of the symptoms of hyperthyroidism
  • Iodine restricted diet such as Hills y/d

    Home Care and Prevention

    At home, be sure to administer any medications prescribed by your veterinarian. If your cat is taking Methimazole, a potential adverse effect is loss of appetite, which may be related to liver complications.

    There is no prevention because the cause is not known. However, examination of the thyroid area should be a regular part of any veterinary examination in older cats. If weight loss occurs in your older cat, your veterinarian may recommend a thyroid blood test to screen for this condition.

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