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Feline Hyperthyroidism

The thyroid glands are a pair of glands which are located on either side of the larynx. They produce several hormones which play a role in the regulation of many processes which occur around the body. Thyroxine (T4) is one of the more important hormones and is the one involved in Hyperthyroidism, the most common hormonal disorder of cats. Overproduction of T4 by the thyroid glands leads to a variety of changes occurring throughout the body.

The Cause of Feline Hyperthyroidism

Hyperplasia (excess growth) of the thyroid gland or benign functional tumours of the thyroid are the most common cause of excess T4 production. Very rarely, malignant tumours are the cause. In most cases the glands on both sides are effected. This "over-growth" of the thyroid gland produces T4 at an uncontrolled rate, leading to the clinical signs we see.
Hyperthyroidism is predominantly seen in cats over ten years of age.

Clinical Signs

Some or all of the following signs may be seen:
  • Polyphagia – excess eating
  • Weight Loss
  • Polydipsia – excess drinking
  • Behavioural Changes – may become hyperactive or irritable
  • Diarrhoea
  • Coat Changes – cats may groom to much or not at all, scaly or oily skin may be seen
  • Increased Heart rate
  • Increased Respiratory rate
  • A mass may be palpable in the area of the thyroids


The definitive way to diagnose Hyperthyroidism is to measure the level of T4 in the serum.

There are however many other conditions in the older cat which can cause similar signs, so initially we will usually order a more general blood test (Body Function Test). This will allow us to see if there are any concurrent problems occurring (e.g. renal problems) and also assess the effect the thyroid problems may be having on other organs (e.g. the liver). The changes we see will usually help to indicate the thyroid may be the cause and we can then order the T4 level.

One of the problems of an increased T4 is the effect it has on the heart. The muscle of the heart can become enlarged leading to heart problems. If we suspect this may be the case, an Xray can be performed and treatment may be required for the heart problems. Heart failure is a common cause of death in untreated hyperthyroid cats


There are three options for treatment of hyperthyroidism:
  1. Radioactive Iodine: I 131 has been used to destroy the overactive areas of the thyroid gland. The success rate is ~80-90%. Specialised facilities are required for this form of treatment.
  2. Surgery: Removal of the gland at fault is an obvious option for treatment. However this can present us with further problems: 1. Surgery is of most use if the disease is in a single gland however hyperthyroidism usually effects both and often special imaging equipment (nuclear scintigraphy) is usually required to differentiate this. 2. There are small glands within the capsule of the thyroid (the parathyroids) which are vital to life. If these glands are removed as well as the thyroid severe problems with calcium levels can occur.
  3. Medical Treatment: There are several drugs available which will block the synthesis of T4 by the thyroid glands leading to reduced levels of T4 in the blood. Carbimazole may be given as a tablet as a long-term treatment or to prepare the patient for one of the above methods of treatment

Your Veterinarian will decide what the best treatment for your cat's case will be.