Hyperthyroidism in Cats

What is Hyperthyroidism?

Hyperthyroidism is a common condition of older cats and most commonly due to an enlargement of the gland. The thyroid gland produces a hormone called Thyroxine that controls metabolism. Excess production of the thyroid hormone makes cells and organs work harder and faster. Hyperthyroidism is a common condition of older cats and very variable in its presentation.

What are the symptoms?

The classic signs of hyperthyroidism may include one or all of the following:

  • Weight loss

  • Increase in appetite

  • Increase in water intake

  • Diarrhoea

  • Vomiting

  • Blindness

  • Increased activity

  • Increased vocalisation


Bloods to test Thyroxine levels (T4), which are elevated in Hyperthyroid cases.

Concurrent diseases

Concurrent diseases may also be present, particularly as this condition is mostly found in older cats. It is therefore common to carry out further tests, these will vary from case to case but may include assessment of kidney function, blood pressure and eye examination.


There are various options which include the following:

  • Medical management with tablets which work by inhibiting the production of the thyroid hormone. This form of management is life-long, and regular blood checks are required to ensure adequate control, particularly at the initiation of treatment. Each animal will requires a unique regime and it may take several months to achieve adequate control.

  • Surgery removal of the thyroid gland which produces the hormone. Surgery has the potential to resolve hyperthyroidism. There are several risks associated with surgery included the general anaesthetic and damage to the parathyroid glands which controls calcium. This may be a temporary problem and can be managed with long term supplements.

  • Radioactive iodine is another option. The radioactive iodine is taken up by the thyroid gland and destroys the thyroid cells. This method can of treatment can also lead to permanent resolution of hyperthyroidism. Additional information on radioactive therapy is available at the clinic.