Hyperthyroidism in Cats

Written by: Alison Laurie, Senior Consultant, Crown Vets Inverness

Hyperthyroidism is usually seen in middle aged to older cats, generally in cats over 7 years of age. Male and female cats seem to be affected equally. In the vast majority of cases hyperthyroidism is caused by a benign, non-cancerous, tumour, an adenoma, causing enlargement of one or both thyroid glands. Rarely, in less than 2% of cases, a malignant thyroid tumour, an adenocarcinoma, can be the underlying cause.

A wide variety of symptoms are noted with hyperthyroidism. Initially these signs can be subtle, becoming progressively more severe over time. As older cats are generally affected, other disease processes common in this age of cat can complicate or even mask the presenting clinical signs.

 The classic clinical symptoms of hyperthyroidism are:

  • Weight loss
  • Usually an increased appetite
  • Increased thirst
  • Increased activity
  • Increased vocalisation
  • Restlessness and irritability
  • An increased heart rate
  • A noted poor, greasy, unkempt coat
  • Mild to moderate diarrhoea and occasional vomiting
  • Panting more when stressed 
  • In severe cases generalised weakness, lethargy and loss of appetite
  • Muscle tremors
  • Behavioural changes

Uncontrolled hyperthyroidism can cause secondary changes within the heart and heart muscle, which can eventually cause heart failure if left untreated. High blood pressure is another potential complication of hyperthyroidism, which can in turn cause damage to other organs such as the eyes and kidneys. Kidney disease can commonly occur concurrently with hyperthyroidism as the two diseases are common in older cats. Continuing monitoring and check-ups and regular blood checks are required to monitor these cases as the medical management of hyperthyroidism can sometimes have an adverse effect on the patients kidney function.

Although one or both thyroid glands usually enlarge with hyperthyroidism, the enlarged gland/s are not always palpable. To confirm a suspected diagnosis of hyperthyroidism, Blood tests are required and it is also important to do further blood tests to check that there is no other concurrent disease, for example kidney disease or diabetes.

Hyperthyroidism caused by non-cancerous thyroid adenomas causing a thyroid enlargement/ hyperplasia can be managed by medical management treatments, by surgical thyroidectomy or with radioactive iodine therapy, which is available at some veterinary hospitals. Cancerous adenocarcinomas of the thyroid gland are aggressive tumours, but are thankfully rare, these tumours are generally treated by surgical removal and follow up radiotherapy and chemotherapy treatments.

If you think your cat may have any of the aforementioned symptoms of hyperthyroidism, then do contact your vet for an appointment for an initial consultation and investigations as may be required after a thorough check up. Some vets run regular “senior” cat clinics where your older cat can have a clinical check and Bloods tests to screen them for any evidence of any health problems, so also enquire about such "senior" cat health clinics.  The sooner that hyperthyroidism is diagnosed the better as appropriate treatments can mean a longer, happier life ahead for your older cat.