Note for Pet Owners:

This information is provided by Provet for educational purposes only.

You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

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Hyperthyroidism is the presence of excess thyroid hormone (triiodothyronine (T3) and thyroxine (T4)) in the blood. Hyperthyroidism is one of the most common diseases of older cats - even though it was only first recognised in 1979, and hyperthyroidism is rare in dogs.

The vast majority (over 98%) of feline hyperthyroidism cases are caused by benign tumours (called adenomas, or adenomatous hyperplasia) of the thyroid gland. Malignant cancer (thyroid carcinoma) account for the other cases.

In dogs thyroid carcinoma is the only reported cause.

Breed Occurrence
Cats develop the disease when they are over six years of age. No particular breed is predisposed to develop hyperthyroidism and males and females are equally affected.

Because thyroid hormones
have an effect on many organ systems it isn't surprising that there are a wide range of clinical signs that might be seen in affected animals. The most common signs, which have been reported in at least one study to be present in over 50% of cats with hyperthyroidism are :

  • Enlargement of the thyroid gland (can be felt in the neck, and sometimes it can be seen) - called goitre
  • Weight loss
  • Increased appetite (polyphagia)
  • Increased activity (hyperactivity)
  • Increased heart rate (tachycardia - over 240 to as high as 360 beats/minute)
  • Increased urine production (polyuria)
  • Increased thirst (polydipsia)
  • Heart murmurs
  • Vomiting
  • Diarrhoea
  • Aggressiveness
  • Anxiety
  • High blood pressure (hypertension) and secondary affects may be present eg retinal haemorrhage

Less frequently reported signs include :

  • Inappetance (anorexia)
  • Panting
  • Fever (intermittent)
  • Lying on cold surfaces
  • Generalised muscle weakness
  • Generalised muscle tremors
  • Depression
  • Heart failure (uncommon)
  • Excessive nail growth
  • Increased breathing rate (tachypnoea)
  • Difficulty breathing (seen as mouth breathing or laboured breathing)- dyspnoea
  • In dogs inspiratory difficulty may be experienced because of compression of the windpipe (trachea) by a large thyroid tumour.
  • Sneezing
  • Flexion of the neck
  • Skin disease
  • Voice changes
  • Increased blood supply to ears and visible mucous membranes

Congestive cardiac failure in severe, untreated cases.

Diagnosis is based upon the clinical history and presenting signs. Thyroid enlargement may be unilateral or bilateral. Sometimes enlarged thyroid tissue extends down the neck to the thoracic inlet, and tumour tissue can even extend into the chest making identification and surgical removal difficult. 

Non-specific tests include :

Specific tests :

  • High basal T3 and T4 blood concentrations
  • Thyroid biopsy/ histopathology on excised tissue
  • Technetium imaging - helps to identify whether both lobes are involved or just one, and whether metastatic spread has occurred


The objective of treatment is to return circulating thyroid hormone concentrations to the normal range (called the "euthyroid state").

There are 3 methods of treatment for hyperthyroidism :

    1. Surgical removal of the affected gland(s)

Renal complications are common in animals with hyperthyroidism so screening for renal disease should be performed prior to surgery. Some authors administer aqueous iodine (Lugols solution) orally 3-5 drops daily for 1-2 weeks pre-surgery as this reduces thyroid size

Thyroidectomy is relatively simple surgery, however care must be taken to preserve the parathyroid glands and to avoid damage to local adjacent structures such as major nerves and blood vessels. Carcinomas are much  more invasive locally than adenomas and they can be more difficult to remove. If thyroid tissue extends into the chest this too can be difficult to remove.

Surgery can be followed by hypoparathyroidism if the parathyroid glands have been removed/damaged and for this reason circulating calcium concentrations should be monitored carefully for 4-5 days during the post-operative period. If concentrations fall below 1.2 mmol/l supplementation is needed with intravenous 10% calcium gluconate at a dose of 1-1.5 ml/kg body weight, followed by oral maintenance doses of 500-750mg/kg/day calcium gluconate tablets .

Horner's syndrome may occur if the cervical nerves have been damaged and voice changes can occur due to altered pressures around the larynx.

    2.   Antithyroid drugs

These drugs reduce the production of thyroid hormone by the thyroid gland.. They all can produce side-effects such as itchiness, bleeding disorders, jaundice, vomiting and anorexia.

Carbimazole is the drug of choice in the UK to stabilise a patient  pre-thyroidectomy or for the long term control of hyperthyroidism. This drug is metabolised to to the active ingredient methimazole (which is available in other countries).

Dogs 10-15mg/day  orally divided into 2-3 doses. Increase dose to maintain normal T3/T4 in circulation

Cats 10-15mg/day orally given in divided doses 2-3 times daily for 1-3 weeks. Then maintain euthyroid state with minimal dose needed in 1-2 daily doses 

Propylthiouracil is less popular because is causes more side-effects including immune-mediated haemolytic anaemia.:

Cats 50mg three times daily orally. Then adjust dose to maintain euthyroid state

Sometimes propranolol is used in combination with anti-thyroid drugs to control the cardiovascular effects of hyperthyroidism.

    3.   Radioactive iodine

In licensed centres radioactive iodine (I131) can be used to destroy the neoplastic thyroid tissue. Over-dosage can result in hypothyroidism, but it has been used successfully in cats.


4. Nutritional intervention 

Recently a new special diet (Hill's Precription diet Feline y/d) has been introduced which is very low in iodine content (below NRC minimum requirements) . If this is    fed    exclusively and the cat does not get any other source of iodine it will induce regression of the benign thyroid adenoma responsible for hyperthyroidism. Clinical signs resolve in a few weeks of starting the diet.

The prognosis is good for most cases...except malignant carcinomas which will often have spread before treatment is initiated

Long term problems
Cardiac changes, renal complications and the secondary effects of hypertension are all problems if the condition is not adequately treated.


Updated October 2013