» Hyperthyroidism






Hyperthyroidism is a disease of the thyroid gland, which is located in the neck, on either side of the trachea. It is the most frequent endocrine disease in elderly cats. The average age of affected cats is 13 years. There is no breed or sexual predisposition.

The hyperthyroidism is caused by over-secretion of thyroid hormones. These hormones are involved in the body’s general metabolism and body temperature regulation. If the thyroid secretes an overabundance of hormones, the animal’s metabolism goes into overdrive. Hyperthyroidism is usually caused by nodules on the gland.

They are difficult to locate accurately because the thyroid extends along the trachea and can be found in various locations. Thyroid tissue has been found scattered in the thorax, for example, especially along the mediastinum.


The symptoms of hyperthyroidism follow an insidious course of gradual deterioration. The main reasons for consultation are weight loss, polyphagia (the cat eats a lot), polyuria and polydipsia (the cat drinks and urinates a lot), and diarrhoea and vomiting. The cat may also appear agitated, nervous, and even become aggressive. On the other hand, the cat may be weak or lethargic when the disease is already advanced. Its coat is dull and patchy.

The thyroid is not palpable in a healthy cat but becomes palpable in 90% of hyperthyroid cats.


Several suspicious criteria detected during the physical examination may orient the diagnosis, to wit:

  • Weight loss ;
  • Presence of a goitre (90% of cases) ;
  • Hyperactivity ;
  • Polyuria-polydipsia ;
  • Digestive disorders such as vomiting and diarrhoea, and cardiac disorders (tachycardia, heart murmur, and galloping heart sounds).

Blood tests should include hormone assays, particularly the baseline concentration of thyroxine, total T4. If the total T4 level is too high, the diagnosis of hyperthyroidism can be made. However, certain cases are dubious, within the normal range. In these cases, other tests are required, such as assaying circulating T4 or TASH (thyroid-stimulating hormone, which is produced by the pituitary, the role of which is to stimulate hormone production by the thyroid).

Other abnormalities in the blood work may exist, such as an excessively high haematocrit level (50% of the cases), an altered complete blood count known as a « stress profile », and moderate rises in the hepatic enzymes.

The urine test results may likewise show changes. In this case there will be a decrease in the urine density (dilution of the urine) and proteinuria (presence of proteins in the urine). Ultrasound examination of the thyroid will reveal small cysts and sometimes whether involvement is unilateral or bilateral. However, ultrasound will not provide grounds for ruling out an ectopic location of the thyroid.

Scintigraphy is the technique of choice for both diagnosing hyperthyroidism and choosing the treatment in cats. Indeed, it is the only technique that enables one to identify with certainty all the hormone-secreting nodules and to orient the diagnosis towards a thyroid adenoma or carcinoma.


There are three types of treatment for feline hyperthyroidism:

  • Medical treatment based on antithyroidal substances. These substances block thyroid hormone synthesis. This treatment must be taken for life and has the advantages of being reversible. However, adverse reactions are not rare and the treatment must be adjusted at regular intervals. Follow-up every 2-3 weeks the first three months of treatment is advisable. One must add to this drug regimen a low-iodine diet (no fish or seafood products). Warning, however: It does not prevent thyroid tumour proliferation ;
  • Surgical treatment: This consists in excising the thyroid tissue responsible for the excessive secretion of hormones. It is curative and definitive if all of the tissue involved is removed, which is not always possible. If the area responsible for the high secretion level is too extensive, surgery is impossible, because it runs the risk of resulting in hypothyroidism and hypoparathyroidism. Pre-surgical scintigraphy must be performed to determine the area involved ;
  • Metabolic radiotherapy: This is the targeted destruction of the thyroid hormone-secreting nodules by injecting radioactive iodine. This treatment has a 90-95% cure rate after one injection. However, it is not readily available because very few veterinary centres perform it. It requires several days’ hospitalization to ensure that the cat is no longer radioactive and is not inexpensive.


Dr Isabelle de Grand Ry
DMV | GPCert In Small Animals Surgery
Vet Center l’ETOIL

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