SHARED on 28 Aug, 2018
Canberra Cat Vet
16-18 Purdue Street
Belconnen ACT 2617 AU
02 6251 1444

Kate King


Hyperthyroidism is common in older cats and treatment is usually straightforward.


Hyperthyroidism is the most common endocrine condition in middle-aged to senior cats. It occurs in 10% of the cat population over 10 years of age. Hyperthyroidism is caused by excess production of thyroid hormones by the thyroid gland.


Common signs of hyperthyroidism:

  • Weight loss
  • Increased appetite
  • Irritability or aggression
  • Increased drinking and urinating
  • Vomiting or diarrhoea
  • Rough hair
  • Panting
  • An enlarged thyroid gland
  • Rapid heart rate

An oversupply of thyroid hormone makes all body systems work harder.


Common causes of hyperthyroidism:

  • A benign tumour of the thyroid gland
  • Rarely, a malignant tumour of thyroid gland

The majority of cases are due to the benign form and respond well to treatment


Diagnosis is based on history, clinical signs and blood tests to determine thyroid hormone levels. The health of other organs like the liver and kidneys is also assessed and treatment tailored to the individual.


Treatment can involve one or more of the following:

  • Radioactive iodine therapy
  • Antithyroid medical management
  • Surgery if there is a tumour
  • Diet therapy

Radioactive iodine therapy

Radioactive iodine therapy means that the cat has low-level radioactivity when discharged from the veterinary speciality centre. Therefore, it is important to minimise exposure to the cat's bodily secretions such as saliva, urine for 3-4 weeks after treatment.

Advantages Disadvantages

Cures hyperthyroidism

Cure rate >95%

Relapse rate 5%

Simple treatment of one injection or an oral capsule

Serious side effects are rare

Minimal risk of hypothyroidism

Requires specialty facility with relevant licenses

Hospitalisation stay can be days to weeks

Owner cannot visit the hospital

Strict home confinement for 2 weeks after discharge

Owner must collect wastes for 2 weeks after discharge

Owner cannot cuddle cat for long intervals for 2 weeks after discharge

Not reversible

Medical management

Medical management requires life-long treatment. Tablets once or twice a day or a gel applied to the inner ear are available. Occasionall there are side effects such as loss of appetite, vomiting, lethargy, skin rashes and face swelling, but mostly medication is well tolerated.

Advantages Disadvantages

Response rate >95%

Medication formula comes in tablets, liquids or topical gels

Requires no hospitalisation

No risk of permanent hypothyroidism

Best if kidneys not functioning well

Relapse 100% if medications stop

Requires medications once or twice  a day

Frequent blood tests are required to monitor effectiveness and safety

Drugs reactions like facial itching, vomiting, liver failure, and abnormal blood cells may occur

A benign tumour may become malignant



We don't recommend surgery.

Diet therapy

A special diet which slows the thyroid down but doesn't cure the condition is available. Malignancy is a risk on the diet. We only recommend it if other options are not affordable.

Advantages Disadvantages

Only a change in diet is required

Control rate >80% 

Safe in cats with kidney disease

Cannot eat any other foods

Strict low-iodine diet

Relapse is 100% when off the diet

Your veterinarian will discuss the various options available with you.


Treatment improves a hyperthyroid cat's quality of life and life expectancy.