Overactive Thyroid (Hyperthyroidism)

Overactive Thyroid (Hyperthyroidism) in Cats


It is not uncommon in our consultations to get older cats that the owners think are just ‘getting old’. Of course cats are like people and slow down, sleep more, get stiffer as they age and often, as the organs are not working a efficiently as when they were young, can start to loose some muscle and bodyweight.

Some of these cats (but not all!) we can actually help quite a lot. Hyperthyroidism is where the cat gets a benign tumour on the thyroid gland which pumps out thyroid hormone uncontrollably. Humans commonly get an underactive thyroid which causes lethargy and weight gain so conversely if a cat is getting an overactive hormone you will see it as weight loss and being slightly more active than a normal cat of this older age.


Common symptoms of overactive thyroid are:

  • Weight loss
  • Eating more (often just seen as a very good appetite)
  • Vomiting or diarrhoea a bit more frequently than normal
  • Sometimes drinking more
  • Increased activity or sometimes irritability
  • Unkempt coat

Some of these symptoms understandably when they come on slowly over months can trick you into thinking it is a normal part of the ageing process but an examination and blood test by the vet can help to see if we can treat this problem.

When we examine an older cat we look for a higher heart rate, weight loss, enlarged thyroid gland (goitre) or signs of increased breathing rate (or even panting in extreme cases) as signs of potential overactive thyroid.
Unfortunately sometimes we are presented with a cat who is panicing for breath or a cat who is in excruciating pain because a blood clot has blocked the blood supply to the back legs.

In the early stages the excessive thyroid hormone increases the body’s metabolism and means that weight loss and an increased appetite are usually first seen. (strangely about 20% of cats with overactive thyroid can have a lower appetite – not all cats read the same text book!) As the problem progresses this increased level of thyroid hormone makes the heart work harder and harder and, like any muscle who is made to work harder, gets thicker and cant pump the blood as efficiently. Eventually heart disease will strike and fluid can build up in the lungs causing laboured breathing or panting, or a blood clot can form in the heart and a small piece can fly off and block a blood vessel somewhere else in the body.


This is another disease, just like high blood pressure, that seems innocent in the early stages, but stopping it progressing can help your cat to live a longer life or better quality.

Treatment can be with tablets, strict use of a special diet, an operation to remove the overactive gland or special radioactive iodine treatment. This is all where a good chat with your vet will help to work out what is best for the individual patient.

I hope you can see now the importance of at least one, ideally two, health checks per year. As your vet examines your cat they are not ‘just’ doing the vaccinations, he or she is looking closely for early clues about potential problems that might be coming on. All our vets are animal owners too and we want your older pets, just like ours, to have a good quality of life in older age.

To book an appointment with our team of animal experts, call our Lancaster surgery on 01524 32696, our Morecambe surgery on 01524 410867 or our Milnthorpe surgery on 01539 562770. Alternatively, send us an email to info@bayvets.co.uk and we will get back to you within 24 hours.