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Hyperthyroidism in Cats

Hyperthyroidism is commonly found in middle-aged and older cats.

This disease is caused by too much thyroid hormone in the bloodstream.

The increase in thyroid hormone is most often secondary to enlargement of the thyroid gland.

Fortunately, in most cats, the enlargement is due to a non-cancerous tumor called an adenoma.

Rarely, cats may develop hyperthyroidism from thyroid adenocarcinomas, which are cancerous and can spread.

Although hyperthyroidism can make cats very ill, treatment options are available.

What Happens in Hyperthyroidism?

Too much thyroid hormone causes an increase in metabolic rate.

Metabolic rate is how fast or slow the body’s organs work.

An increase in metabolic rate places additional demands on all the organs of the body and can cause a cat to feel sick.

What Are Some Symptoms of Hyperthyroidism?

  • Weight Loss
  • Increased appetite
  • Increased thirst
  • Increased urination
  • Vomiting
  • Diarrhea
  • Hyperactivity
  • Aggression
  • Matted or greasy coat

Initial signs of hyperthyroidism are often mild, but worsen as the disease progresses.

What Is The Thyroid?

The thyroid is a glandular, butterfly-shaped organ that secretes thyroid hormones called T3 and T4.

Where Is The Thyroid?

The thyroid is located in the front of the neck. Veterinarians will often move their fingers down the front of a cat’s neck in an effort to detect an enlargement of the gland.

Additional functional thyroid tissue may also be located throughout the neck and upper thoracic regions called an accessory or ectopic thyroid tissue.

What Does The Thyroid Do?

Both the primary thyroid and the accessory tissue secrete hormones. These hormones regulate how the body utilizes food and converts it to energy.

How Does It Work?

The thyroid gland is controlled by a small structure located at the base of the brain called the pituitary gland.

The pituitary gland regulates the number of thyroid hormones found in the bloodstream.

The pituitary signals the thyroid gland to make more hormones when blood levels of the hormone are low and signal the thyroid to decrease hormone release when the blood levels are high.

How is Hyperthyroidism Diagnosed?

A blood test showing an increase in the thyroid hormone, T4 is the most common method of diagnosis.

A few hyperthyroid cats will have a normal T4 and additional blood tests to look at alternative forms of the thyroid hormones may be suggested to help confirm a diagnosis.

Why Treat Hyperthyroidism?

Cats with hyperthyroidism often suffer additional cat health problems including heart disease and hypertension, or high blood pressure. Hypertension can damage the heart, kidneys, and brain.

Treating hyperthyroidism may resolve or lessen these secondary problems.

What Are the Options for Treatment?


Surgical removal of the thyroid gland. The thyroid gland consists of two parts or lobes.

Some vets remove only the visibly diseased lobe, others recommend removing both since there is a high probability of the other lobe becoming diseased.

Make sure that you have a skilled, recommended veterinary surgeon, and your kitty will be under general anesthesia for the surgery.

A careful preoperative evaluation must be completed prior to surgery to assess kidney, liver, and hyperthyroidism.


  • Long-term treatment or cure in most cats
  • Hyperthyroid medication is typically not needed after the gland is removed


  • General anesthesia required
  • Risk of damage to nearby parathyroid glands, critical to the regulation of calcium levels in the blood
  • A supplemental hormone may be needed in cats without adequate functional accessory thyroid


Medications for hyperthyroidism work by limiting the production and release of thyroid hormone.

Methimazole is in the form of a pill that needs to be given by mouth one to three times daily, depending on each cat’s case.

Alternatively, compounding pharmacies can create a tasty liquid medication or a paste that is applied to the inside of the ears for absorption if pilling the cat is difficult or not an option.

This drug works by suppressing the thyroid gland’s production of thyroid hormone but does not cure the disease. If treatment is stopped, the hyperthyroid condition will recur.


  • Medications are readily available
  • Medications are shown to be effective when given by mouth or on the skin (transdermal)


  • Lifelong treatment is required to control the disease and is not a cure
  • Most require a minimum of twice-daily dosing
  • Side effects can include anorexia, vomiting, fever, anemia, and lethargy
  • Transdermal formulations may result in localized reactions
  • The drug dose will likely need to be modified using routine blood work

Radioactive-Iodine Therapy

Radioactive iodine is absorbed by the thyroid gland and destroys the tissue.

Treatment is via a single injection of radioiodine under the skin.

The hyperactive thyroid tissue takes up large amounts of this substance via the bloodstream, and the diseased thyroid cells die.


  • The therapy can be curative
  • After one to two weeks of treatment, most cats have normal thyroid levels
  • Does not require anesthesia
  • No collateral damage to the nearby parathyroid glands
  • Side effects are few and not serious


  • Radioactive substances do require specialized licensing and training to administer
  • Cats are hospitalized for the therapy and visitors are typically prohibited
  • In rare, persistent cases a repeat treatment may be suggested
  • Thyroid hormone levels may become too low and the cat may need to be given a supplement


A prescription diet available as Hill’s® y/d Feline Thyroid HealthTM has severely restricted amounts of iodine, without which the thyroid cannot produce excess thyroid hormone.

The use of this diet is very controversial.

The food is supposed to improve your cat’s thyroid in three weeks as it supports kidney health with controlled phosphorus and low sodium.

While food can be expensive, it is a nice alternative to the other treatments.


  • Exclusive feeding of this diet can result in normal thyroid hormone levels in eight to 12 weeks
  • Cats that are not candidates for surgery or radioactive-iodine therapy and cannot be medicated may benefit from this diet
  • Available in dry and canned forms


  • Iodine, an essential element, is below recommended levels for daily intake by adult cat
  • Long-term effects of insufficient iodine levels are unknown.
  • Not curative
  • Cannot be combined with any other form of food to be effective
  • The cat must remain on the diet permanently to avoid relapse
  • Households with multiple cats may need to offer multiple diets or supplements
  • Some cats refuse the food

Before beginning any treatment, a full blood panel and urinalysis are recommended.

These tests are likely to be repeated during and after treatments to ensure thyroid levels are in the normal range and to monitor for damage to other organs.

Commonly, hyperthyroid cats have concurrent kidney disease which may only become apparent during treatment for hyperthyroidism.

The presence of kidney disease may alter the recommended course of treatment.

Most veterinarians consider radioactive-iodine therapy the best choice when the cat is a good candidate. Surgery also has the potential to be curative but has increased risk.

Lifelong medication and potentially dietary management are also avenues to consider.

Surgery and radioactive-iodine treatments have the largest upfront costs, but long-term management with medical therapy or specialized food has the potential to be as expensive over the cat’s lifespan.

Treating hyperthyroidism in cats is potentially curative or manageable for long periods of time.

Hyperthyroidism in Cats

Depending on the treatment, treating hyperthyroidism can become expensive. This is why you should ALWAYS consider buying pet insurance for your cat.

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