Hyperthyroidism in Cats


This article is written by Pet Circle veterinarian, Dr Jennifer Lau BVSc

What is Hyperthyroidism?

If your cat has just been diagnosed with Hyperthyroidism (not to be confused with Hypothyroidism), they are not alone. In fact, hyperthyroidism is the most common hormone imbalance in cats. It commonly affects middle-aged and older cats. An enlarged, overactive thyroid gland can affect the functioning of your cat’s metabolism, body temperature, blood pressure, heart rate and gastrointestinal function.

There are two types of thyroid hormones; T3 and T4. T3 is an active hormone that has many functions such as setting the body's metabolic rate, which affects every cell in the body. T4 is an inactive hormone produced by the thyroid glands, this is most likely the hormone that your veterinarian has been monitoring.

What causes Hyperthyroidism?

Hyperthyroidism is caused by an increased in production of thyroid hormones from the thyroid glands. This enlargement is usually a non-cancerous, benign tumour called an adenoma, however there are rare cases of hyperthyroid caused by malignant adenocarcinomas.

    Symptoms of Hyperthyroidism

  1. Weight loss
  2. Increased appetite
  3. Increased thirst and urination
  4. Vomiting
  5. Diarrhoea
  6. Hyperactivity
  7. Poor grooming resulting in an unkempt coat

How is Hyperthyroidism diagnosed?

After your vet takes a thorough history, taking note of possible symptoms of hyperthyroidism. Then they will physically examine your cat, they might feel an enlarged mass in the neck area and hear an increased heart rate. They may also take your cat's blood pressure. A blood test with analysis of thyroid hormone levels is needed to diagnose hyperthyroidism. Most cats with hyperthyroidism will have levels of the thyroid hormone T4 above the reference range. However, a small number of cats may be the exception and have T4 levels within normal levels and may require further testing.

Hyperthyroidism affects other organs in the body, particularly the heart and kidneys. The blood biochemistry panel and urine test will also help your veterinarian assess your cat's overall general health.

How is Hyperthyroidism treated?

There are a few treatment options for Hyperthyroidism in cats and your veterinarian will work through the options with you to find one that works well for you and your kitty. The treatment you and your vet decide on will take into consideration of how severe the disease is, your cat's health and what works for you as well. Treatment is important because otherwise hyperthyroidism affects their quality of life through poor muscle condition and bodyweight, ongoing vomiting, diarrhea and damage to other organs.


This is an effective long term curative treatment for most cats. For this treatment, your kitty will be given an injection of radioactive iodine and kept in the clinic or hospital for three or four days. The radiation levels will reduce to a safe amount in this time for them to come home with you. You will need to take care with contact and cleaning after your kitty in the week or two following the treatment. You will need to minimize contact with them and confine them to their own room. This is because they will still emit low levels of radioactive iodine. This treatment is limited to hospitals or clinics with radioisotope permits and is not available at every clinic. Your veterinarian will give you a referral if you decide to go with this treatment.

Medical treatment

This treatment involves the daily application of anti-thyroid medication, in a pill or paste, that stops the production of thyroxine. These medications are usually very effective but your cat will need to be on them for the rest of their life. Regular ongoing checkups with your vet are necessary to monitor the condition and make sure the dosage is adequate.

Surgical treatment

Surgical removal of the thyroid gland is a procedure done under a general anesthetic. There may be general anesthetic risks if your older cat has kidney or heart issues.

Dietary treatment

Iodine is necessary for thyroxine hormone production by the thyroid gland. Hill's Prescription Diet Y/D limits the amount of iodine present in the diet which then prevents the production of excess thyroxine. If your kitty is on an iodine-restricted diet, you need to ensure that you don't feed them any other foods and treats (this includes hunting and scavenging!) They will need to be on this diet lifelong.

Nutrition for Hyperthyroid Cats

Top Therapeutic Diets for Hyperthyroid Cats

The following diets are prescription veterinary diets, which means you will need to check with your vet prior to ordering.

Hill's Prescription Diet Thyroid Care y/d Cat Dry Food Yd Thyroid Care is clinically proven to improve thyroid health within 3 weeks when fed as the sole source of nutrition. It also supports kidney health with controlled phosphorus and sodium levels, promotes bladder health with controlled minerals and pH, contains high level of omega 3 and omega 6 fatty acids for healthy skin and coat.

Hill's Prescription Diet Thyroid Care y/d Cat Canned Food Hill's Prescription is a tasty wet food formulation of thyroid care y/d with limited iodine to decrease hormone production.

Top Tips for looking after your Hyperthyroid cat

1. Give them a comfortable bed. Make sure they have a warm and comfortable bed to sleep and rest. View all comfy cat beds here.

2. Help out with grooming. Look after their coat for them as the condition can cause unkempt, matted and greasy fur. View all cat brushes here.

3. Keep up with regular check ups. Keep up to date with regular vet checkups so your vet can monitor their condition. Your vet may take a blood test, urine test and blood pressure reading to monitor your cat's hormone levels, kidney health and blood pressure.

Product recommendations:

Further Reading

Exploring Grain-Free Pet Food

Can Pets Get or Transmit COVID-19?

Why Do Cats Purr?

How to Treat Hairballs in Cats

9 Best Indoor Cat Toys

What is the Best Grain Free Cat Food?