Last Updated: 18/02/2026

Hyperthyroidism In Cats

Has your cat been diagnosed with an overactive thyroid? Check out our Vets guide to the causes symptoms and treatment for this common condition.

Author: Dr Jennifer Lau BVSc

Reading Time: 4 minutes - short read

tabby and white cat sitting and looking a camera

If you’ve recently been told that your cat has hyperthyroidism, it’s completely understandable to feel worried or overwhelmed. The good news is that this condition is common- especially in middle-aged and senior cats- and with proper diagnosis and management, many cats continue to live happy, comfortable lives.

Understanding hyperthyroidism in cats

Hyperthyroidism (not to be confused with hypothyroidism) is the most common hormonal disorder in cats. It occurs when the thyroid glands become enlarged and overactive, producing excessive amounts of thyroid hormones. These hormones play a vital role in regulating your cat’s metabolism, body temperature, heart rate, blood pressure, and digestive function. When too much hormone is produced, the body essentially goes into overdrive.

The thyroid produces two main hormones:

T3 (triiodothyronine): The active hormone that directly influences the body’s metabolic rate and affects nearly every cell.
T4 (thyroxine): An inactive hormone produced by the thyroid gland that is converted into T3 within body tissues. T4 is the hormone most commonly measured in blood tests when diagnosing hyperthyroidism.

In most cases, hyperthyroidism is caused by an increase in thyroid hormone production due to enlargement of the thyroid gland. This enlargement is typically the result of a benign (non-cancerous) tumour called an adenoma. In rare cases, however, hyperthyroidism can be caused by a malignant tumour known as an adenocarcinoma.

Symptoms of hyperthyroidism in cats

There are several classic symptoms of hyperthyroidism in cats:

Weight loss
Increased appetite
Increased thirst and urination
Vomiting
Diarrhoea
Hyperactivity
Poor grooming resulting in an unkempt coat

How is hyperthyroidism in cats 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.

Treatment of hyperthyroidism in cats

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 hyperthyroidism affects your cat's quality of life through poor muscle condition and bodyweight, ongoing vomiting, diarrhea and damage to other organs.

Radiotherapy using Iodine: Radiotherapy 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 to four days. In this time, the radiation levels will reduce to an amount safe enough to allow 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 the T4 hormone from the thyroid gland. 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 therapy: 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.

Hills Prescription Diet Y/D Thyroid Care

Hill’s Prescription Diet y/d is a specially formulated veterinary diet designed to help manage hyperthyroidism in cats. It works by being extremely low in iodine, a mineral that the thyroid gland requires to produce thyroid hormones.

By carefully restricting iodine intake, this diet helps reduce the production of excessive thyroid hormones and can assist in bringing hormone levels back toward normal. Hill’s y/d must be fed as the cat’s sole source of nutrition- no other foods, treats, or flavoured medications- because even small amounts of additional iodine can reduce its effectiveness.

Available in both dry and canned forms, it offers a non-invasive management option for cats who may not be suitable candidates for medication, radioactive iodine therapy, or surgery.

Regular veterinary monitoring is still essential to ensure the diet is working effectively and to assess overall health.

Hills Prescription Y/D Diet

Looking after your hyperthyroid cat

Provide a comfortable bed: Since the majority of cats diagnosed with hyperthyroidism are senior cats over the age of 12, providing them with a warm and comfortable bed to sleep and rest is important. For more information, take a look at our article Which Cat Bed Should I Choose? for tips and advice on choosing the right bed for your cat.
Extra grooming: Cats with Hyperthyroidism are prone to unkempt, matted and greasy coats. You can help to look after their coat with regular, gentle brushing, and moisturising skin products. For more information on cat grooming, take a read through What are the Best Cat Brushes? and our Cat Grooming Guide.
Regular vet check ups: Keep up to date with regular vet checkups so your vet can monitor your cat's condition. Your vet may take a blood test, urine test and blood pressure reading to monitor your cat's thyroid levels, kidney health and blood pressure.