Hyperthyroidism, also referred to as an overactive thyroid, is a disorder commonly seen in elderly cats. It arises as a result of excessive production and secretion of thyroid hormone from the thyroid gland.

Where is the thyroid gland? What do thyroid hormones do?


The thyroid gland is located within the neck on either side of the windpipe. It is responsible for the production and secretion of the thyroid hormones; T4 and T3 (AKA Thyroxine). These hormones have an important role in regulating metabolic rate- the rate at which the body burns energy. It also has generalised effects on growth, maturation and has an effect on the heart and blood vessels within the body.

Could your pet have hyperthyroidism?

Hyperthyroidism is frequently seen in cats. This is in contrast to dogs where hypothyroidism, a reduction in thyroid hormone is more commonly recognised. Hyperthyroidism is the most common endocrine (hormone related) disease to affect cats older than 8 years of age. It can be seen in cats ranging from 4-20 years, however fewer than 5% of cats with hyperthyroidism are younger than the age of 8.

The classical signs of hyperthyroidism are listed in the list below. Because of the multi-systemic effects of the disease and the variable clinical signs, hyperthyroidism should be suspected in any mature to elderly cat with a medical complaint. The most common clinical signs are those highlighted with an * in the list.

Clinical signs of Hyperthyroidism in Cats

  • Weight loss*
  • Polyphagia (excessive eating or appetite)*
  • Unkempt haircoat, patchy alopecia (hair loss)*
  • Polyuria/polydipsia (excessive urinating/drinking)*
  • Vomiting*
  • Nervous, hyperactive
  • Diarrhoea, bulky stools
  • Decreased appetite
  • Tremor
  • Weakness
  • Difficulty breathing
  • Decreased activity, lethargy
  • Anorexia (loss of appetite)