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Key takeaways

Hyperthyroidism is the most common hormonal disease in cats over the age of 10. It results when the thyroid gland produces excessive thyroid hormone.

  • Overactivity of the thyroid in cats leads to significant weight loss despite an increased appetite
  • Other symptoms include vomiting, diarrhea, restlessness and irritability, and excessive thirst and urination
  • Cats with hyperthyroidism usually have normal energy levels and appear to feel good, which often leads to a delay in diagnosis and treatment -Delayed diagnosis may result in complications like high blood pressure, kidney disease, and heart failure
  • There are four major treatment methods for hyperthyroidism: radioactive iodine, surgical removal, long-term medication, and lifelong dietary changes
  • Potential outcomes vary, but In most cases, the prognosis for hyperthyroidism is excellent
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A closer look: Hyperthyroidism in Cats

Located in the throat, the thyroid gland is the hub of metabolic regulation. Metabolism is the process of breaking down nutrients from digested food for use by the body’s systems. The thyroid secretes a series of thyroid hormones which stimulate metabolism, among other secondary functions.

Hyperthyroidism refers to a state of overactive thyroid hormone production. Sustained levels of abnormally high thyroid hormone stimulate the body to increase metabolism for prolonged periods, leading to weight loss and increased appetite.

Rarely, cats show symptoms “opposite” to the traditional signs of hyperthyroidism. These cats are lethargic, have a reduced appetite, and seem depressed.

Left untreated, more serious complications develop and become life threatening.

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Risk factors

Hyperthyroidism is a common disease in cats, affecting 3-8% of cats over the age of 10. Annual screening blood tests are useful for identifying the disease before symptoms develop. Early diagnosis and treatment not only keep the disease from progressing, but also help prevent complications like high blood pressure, kidney disease, and heart disease.

As a result of their excessive urinary output and voluminous stools/diarrhea, many cats with hyperthyroidism are less consistent about using the litter box.

Several risk factors for hyperthyroidism have been identified:

  • Aging: Cats over the age of 8 are more likely to be hyperthyroid
  • Canned food diets: Certain types of pop-top aluminum cans are lined with a compound that can affect the thyroid gland
  • Indoor lifestyle: The reason indoor cats are more likely to develop hyperthyroidism is unknown

Possible causes

Non-cancerous growths in the thyroid gland are the most common cause of hyperthyroidism. These produce large amounts of thyroid hormone constantly, without responding to the body’s natural systems of regulation. In about 1-2% of cases, the growth in the thyroid gland is cancerous. These cats have a poor prognosis.

Main symptoms

In addition, there may be behavioral changes due to increased energy levels, such as hyperexcitability, restlessness, irritability, pacing, or aggression.

Testing and diagnosis

Diagnosis of hyperthyroidism starts with a thorough physical examination. Key findings that indicate hyperthyroidism include an increased heart rate, elevated blood pressure, and an enlarged thyroid gland.

To confirm a diagnosis of hyperthyroidism, a blood test to measure thyroid hormone levels is used. If hormone levels are high, a diagnosis is confirmed. Some cases require further testing to measure other factors involved in thyroid hormone production.

Steps to Recovery

There are several treatment options for hyperthyroidism in cats, including radioactive iodine, surgical removal of the gland, long-term medication, and lifelong dietary changes.

The treatment of choice for hyperthyroidism is radioactive iodine administration, if available and the cat is a suitable candidate. In many cases, only a single treatment is required to produce a cure.

Surgical removal of the thyroid growth is another effective treatment option.

Lifelong, daily administration of methimazole or carbimazole are used as pharmaceutical treatment of hyperthyroidism. These medications are available as oral tablets or compounded into a topical gel applied to the cat’s ear.

Low-iodine prescription foods can reduce hormone levels when fed as an exclusive diet. In some cases, low-iodine diets are recommended in conjunction with other treatment protocols.

Regardless of which treatment option is chosen, ongoing monitoring of thyroid hormone levels is necessary for successful management of feline hyperthyroidism.

With the exception of dietary therapy, these treatment options all have the potential to lower thyroid hormone levels too far, leading to hypothyroidism.

The overall prognosis for hyperthyroidism is excellent. Surgical removal and radioactive iodine treatments often completely resolve hyperthyroidism, with no further treatment required. Cats who are managed through medication or dietary changes have a good prognosis, as long as their treatment is consistent and monitoring is maintained.


Hyperthyroidism in cats is not contagious nor can it be prevented. It is associated with aging in cats.

Is Hyperthyroidism in Cats common?

Hyperthyroidism is the most common hormone disease of cats, affecting 3-8% of cats over the age of 10.

Typical Treatment

  • Surgical removal
  • Radioactive iodine
  • Medication
  • Dietary management


Wendy Brooks - Writing for Veterinary Partner
Wendy Brooks - Writing for Veterinary Partner
Wendy Brooks - Writing for Veterinary Partner
Wendy Brooks - Writing for Veterinary Partner
Wendy Brooks - Writing for Veterinary Partner
Mark E. Peterson - Writing for Merck Veterinary Manual
Smith, F.W.K., Tilley, L.P., Sleeper, M.M., Brainard, B.M - Writing for Blackwell’s Five-Minute Veterinary Consult: Canine and Feline. Seventh Edition.

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