Hyperthyroidism. Don’t ignore the signs!

If you have a cat you must be aware of how common thyroid issues are with them. Hyperthyroidism is a disease that is most common in middle-aged and older cats.

It’s the increased production of thyroid hormones (T3 and T4) from the thyroid gland that causes the increase. The thyroid gland, located on your cat’s neck, controls the metabolism. Hyperthyroidism is, in most cases, caused by a benign tumor called adenoma, but the reason why the disease occurs is not yet fully understood.

A cat with hyperthyroidism can exhibit the following symptoms:
– increased thirst
– increased urination
– increased appetite
– weight loss
– a greasy coat
– diarrhea
– vomiting
– behavioral changes – depression, hyperactivity and/or aggression

If your cat is experiencing these you should see your vet who should take a detailed history before continuing with a clinical examination of your cat. Before beginning the clinical examination, a blood pressure measurement is recommended. It’s important to palpate the neck area because sometimes the thyroid gland can be enlarged. On heart auscultation, cats often have an increased heart rate and sometimes a heart murmur.

Hyperthyroidism is diagnosed by measuring the amount of thyroid hormone (T4) in the blood. Given the age of the patient, the blood sample, together with a urinalysis, should also be screened for other diseases to get an overall picture of your cat’s health.

Hyperthyroidism can hide a kidney disease, so it is important to recheck the kidney values after treatment is started.

Pro Tip! It is normal to see increased liver enzymes in cats with hyperthyroidism, but they often level out when treatment is instituted. Hyperthyroidism can hide a kidney disease, so it is important to recheck the kidney values after treatment is started.

There are several different treatment methods for hyperthyroidism.


First of all, there is a tablet treatment that causes the production of thyroid hormones to decrease. The tablets are given once to twice a day for the rest of the cat’s life. Your vet should take regular blood samples to check that the dose is still appropriate, and also to screen for kidney disease and possible side effects. Side effects of the tablets may include decreased appetite, vomiting/diarrhea, itching or bone marrow suppression.


Surgery is a long-term or permanent treatment option where the thyroid glands are removed. However, your cat would need to undergo a general anaesthetic. There is also the risk of damage to the parathyroid glands which lie close to the thyroid gland and help the body to maintain the blood calcium levels.

Radioactive Iodine Therapy

However, the treatment of choice, for cats with hyperthyroidism, is radioactive iodine therapy. Radioactive iodine is injected and quickly absorbed into the bloodstream. The iodine is taken up by the thyroid gland and destroys the abnormal thyroid tissue without damaging the surrounding tissues or the parathyroid glands. The side effects are few but initially you can notice a temporary voice change and some difficulty swallowing. The treatment requires special personnel and equipment with the majority of cats treated with radioactive iodine returning to normal hormone levels within one to two weeks of treatment.

Iodine Free Food

Yet another treatment option is to give your cat iodine free food. Iodine helps to form the hormone so feeding your cat iodine free food will help to naturally reduce the thyroid hormone. If you feed your cat this particular diet it shouldn’t eat anything else, not even treats. As with the tablets, regular blood samples are a good idea to measure your cat’s response to the treatment.

I hope you’ve found this helpful and feel free to share with anyone you know who has a cat. Spotting the signs early on can make a huge difference to the quality of life of your favourite feline friends.

I’m a veterinarian with almost 10 years experience in small animal practice. I’m originally from Sweden but moved to Copenhagen to fulfill my dream of becoming a vet. I’ve been working in Denmark, United Kingdom and back home in Sweden. Animals, especially dogs, has always been a big part of my life. Being a vet and helping animals and their owners, feels more like a privilege than a job.
Alvin, my Cavalier King Charles Spaniel follows me anywhere I go.

Caroline Edvinsson


Future telemedicine for pets.


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