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Hyperthyroidism (toxic goiter) in cats

Hyperthyroidism is the most common hormonal disorder in the middle-aged to older cats. Hyperthyroidism is caused by changes in the thyroid gland, it involves elevated levels of thyroid hormones that control metabolism. Here you can read about which symptoms you should pay attention to and which different treatment methods are available.
Close up of a tabby and white cat

Cat's thyroid glands

The thyroid gland controls the body's metabolism and forms, among other things, the hormone thyroxine (T4). The thyroid gland is located on the underside of the neck below the larynx. It is divided into two lobes located on either side of the trachea, in the midline of the throat. In a healthy cat, the thyroid gland should not be felt.

Symptoms of Hyperthyroidism (Toxic Goiter)

Hyperthyroidism often affects older cats and symptoms usually appear from about 8 years of age, but cats from 4 years of age and older can also be affected. Hyperthyroidism means elevated levels of metabolic hormones. Symptoms to look out for include:

  • Weight loss despite good appetite
  • Enlarged thyroid gland (In a healthy cat, the thyroid gland should not be felt.)
  • Behavioural changes such as increased activity, anxiety and aggressiveness
  • Impaired fur quality such as matted fur and hair loss
  • Increased thirst and urination
  • Diarrhoea and vomiting
  • Faster heart and breathing rate
  • Intense yawning
  • In the long course of the disease, fatigue and reduced appetite may be noticed

Many cats become more active, but there are cats who, on the contrary, withdraw, become inactive, eat less and become weak. Since cats that suffer from hyperthyroidism are usually older, you should bear in mind that there are several diseases that can have similar symptoms.

How is hyperthyroidism diagnosed in cats?

Vets diagnose hyperthyroidism by taking a blood sample to measure the amount of the thyroid hormone, thyroxine (T4). Since the disease affects other organs, such as the kidneys and heart, and other diseases can occur at the same time, a number of parameters are checked.

It is common to have high liver values ​​and they are usually due to hyperthyroidism. If the cat also has problems with the kidneys, the kidney values ​​can still be falsely normal due to the effects of hyperthyroidism. The heart may be affected and need to be examined with an ultrasound and it is common to measure the blood pressure.

In a majority of cases, the cause of hyperthyroidism is benign changes in the thyroid gland, so-called adenomas. Malignant changes occur very rarely. In approximately 70% of cases there are changes in both parts of the thyroid gland. In about 80-90% of cases, the enlarged thyroid gland can be felt with the fingers.

Cats with suspected hyperthyroidism can be very sensitive to stress in connection with sampling and examinations. They should therefore be handled in as calm an environment as possible and sometimes sedatives may need to be given.

Treatment for hyperthyroidism (toxic goiter)

Once a diagnosis has been made, your vet will consult with you about the best treatment. There are advantages and disadvantages to all methods. Your cat's age can play a role, as can other diseases, etc. The goal of the treatment is to get the thyroid hormones to function normally. Treatments can include:

  • Medicine
  • Iodine-free feed
  • Iodine syringe
  • Surgery

Medicines that reduce the production of the thyroid hormone

One treatment method is to give medication twice daily, it is available as tablets or in liquid form. The medication blocks excessive production of the thyroid hormones and is given daily, for the rest of the cat's life. During the treatment, regular visits are made to the vet to check the dose and detect any side effects. Side effects can be vomiting, diarrhoea, reduced appetite, itching or effects on the bone marrow.

Iodine-free food for cats

Iodine helps form the thyroid hormone, so iodine-free food can help reduce the thyroid value. If your cat is treated with this method, it must not eat anything else, this includes treats and other food, for the rest of its life. Regular return visits to the vet are made to check it's working.

Iodine syringe

An injection with radioactive iodine is another treatment method. Cats are given an injection equivalent to a vaccination, containing a low dose of radioactive iodine which is absorbed by the thyroid gland and destroys the diseased cells there. Cats may need to stay a few days at the vet surgery after the treatment and certain restrictions apply at home for a short time afterwards. This treatment is available at major animal hospitals.


Surgery of the overactive lobes may be curative. Surgery is not as common today, due to other treatment methods being developed. Surgery also requires that the cat's general condition is able to handle anaesthesia. There is some risk that the parathyroids, which are very close, are not damaged.

The four small lobes of the parathyroid (parathyroid glands) must be left intact during surgery as it's vital for the body's calcium balance. Therefore, the cat's calcium levels are always checked after surgery. Common symptoms that the cat has a calcium deficiency are anxiety, behavioural changes and convulsions. If necessary, calcium supplements can be given.

Other diseases should be investigated

A cat with suspected hyperthyroidism should also be screened to rule out other diseases.

Diseases with similar symptoms to hyperthyroidism

  • Diabetes - increased thirst, appetite, urination and weight loss
  • Kidney diseases - emaciation, dull fur, increased thirst and urination
  • Liver disease - emaciation, vomiting, elevated liver values
  • Tumour disease - emaciation

Frequently asked questions about hyperthyroidism

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