What you should know about hyperthyroidism in cats

The hyperthyroidism is a common condition in older cats of 10 years or more due to an increase in the production of thyroid hormones from the thyroid gland , which is located in the neck of the cat. It is characterized by accelerating the metabolic rate and energy expenditure of the cat; you would be burning calories from food faster than you can consume on a daily basis.

The thyroid gland , located in the neck, is responsible for the metabolism of the feline body. In people, the thyroid gland is shaped like an “H” and is located at the base of the throat. Its position is the same in cats, but they lack the crossbar of the H, so there are actually two glands, one on each side of the trachea.

In cats, an underactive thyroid is very rare. However, an overactive thyroid is quite common. Many older cats with an overactive thyroid have a truly enlarged gland that the vet can feel in the neck.

In addition to unexplained weight loss , your cat may also become more restless, hyperactive, vocal, and very hungry.

Symptoms of feline hyperthyroidism

The most common symptoms of hyperthyroidism are:

  • Increased appetite
  • Anxiety or nervousness.
  • Weight loss (often despite eating more)
  • Increased heart rate.
  • Vomiting
  • Meows at night.
  • Poor quality of hair, unkempt coat.
  • Increased thirst and urination.

Not all cats will have these symptoms, and about 20% of hyperthyroid cats will be lethargic and depressed rather than hyperactive.


Diagnosis of hyperthyroidism in cats

Although the thyroid glands enlarge with hyperthyroidism, they are usually not visible. Its detection will require a careful palpation (tactile examination) by the veterinarian. However, in some cats there is no clear increase, because the overactive tissue is in an unusual location.

To confirm a diagnosis, a blood test is needed, to measure the level of thyroid hormones in the blood. In general, measuring the thyroxine (T4) level  will be essential to confirm a diagnosis, although additional tests may sometimes be necessary.

If the cat does not have elevated T4 levels but the vet suspects that the cat has hyperthyroidism, additional tests may be necessary. These tests are; for example, an EKG, chest x-ray, or technetium scan .


Complications of hyperthyroidism

Uncontrolled hyperthyroidism has consequences on the heart by causing an increase in heart rate, but also changes in the muscle wall of the heart that will eventually cause heart failure if left untreated. As the disease progresses, the muscle in the largest chamber of the heart (left ventricle) enlarges and thickens, which is known as left ventricular hypertrophy .

Hypertension (high blood pressure) is another potential complication of hyperthyroidism, although less frequently, and this can cause damage to various organs (eyes, kidneys, heart and brain).

Kidney disease (chronic kidney disease) does not usually occur as a direct consequence of hyperthyroidism, but the two diseases often appear together because both are common in older cats. Caution is needed when both diseases are present, as hyperthyroidism can sometimes have adverse effects on kidney function.


Treatments applied to hyperthyroidism

Most cats with thyroid problems do very well when given treatment. There are four types of treatment, each with advantages and disadvantages:

  1. Antithyroid drug therapy

The most widely used and effective antithyroid drugs are thioamides ( methimazole and carbimazole ). These drugs reduce both the production and the release of thyroid hormones from the thyroid gland. They do not provide a cure, but they do allow short- or long-term control of hyperthyroidism, but it must be administered daily.

  1. Surgical removal

Surgical removal of affected tissues ( thyroidectomy ) can produce a permanent cure and is a common treatment for many hyperthyroid cats. In general, this usually allows for a long-term or forever cure in most cats.

Sometimes signs of hyperthyroidism develop again later if previously unaffected thyroid tissue becomes diseased.

Because there is a small risk of temporary interference with calcium regulation, it is generally recommended that cats remain hospitalized for a few days after surgery, and blood calcium concentrations are monitored during this time.

After surgery, occasional blood tests are recommended to ensure normal thyroid hormone levels are maintained.

  1. Radioactive iodine therapy

Radioactive iodine is a very safe and effective treatment for hyperthyroidism. It has the advantage of being curative in most cases without continuous treatment.

Radioactive iodine is injected under the skin. Iodine is absorbed only by active (abnormal) thyroid tissue, resulting in selective local accumulation of radioactive material in abnormal tissues. The radiation destroys the affected abnormal thyroid tissue, but does not damage the surrounding tissues or the parathyroid glands.

No significant side effects, but because cats are temporarily radioactive, they should be kept hospitalized for a short period after treatment as a precaution.

A single injection of radioactive iodine cures in nearly 95% of all hyperthyroid cats, but occasional blood tests are recommended after treatment to ensure normal thyroid hormone levels are maintained.

  1. Diet treatment

What should a cat with hyperthyroidism eat?

A recent treatment is to feed them a special diet that controls iodine levels . Since the thyroid gland uses iodine to make thyroid hormones, adding just enough iodine to the diet to produce normal levels of these hormones can help control the disease.

The advantages of diet therapy include low cost and ease of application. Disadvantages include complications if the cat has other diseases or conditions, takes other medications or supplements, or does not like the taste.


In general, all cats with hyperthyroidism need some type of treatment.

The goal of therapy is to restore normal thyroid function and minimize the side effects of treatment without causing lower-than-normal thyroid hormone levels (known as hypothyroidism ).

Cats usually do quite well with treatment, and if the disease is recognized quickly and treated correctly, the cat’s prognosis will be very positive.

Treatment will be more complex when the cause of hyperthyroidism is a malignant tumor known as adenocarcinoma . Luckily this is rare, and is usually the cause of 1-2% of cases.

Continuous monitoring of the cat after any treatment is very important, as well as routine checks with the veterinarian.



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