Hyperthyroidism in Cats

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

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 (triiodothyronine) and T4 (thyroxine). 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, which is converted to T3 when absorbed by the tissues of the body. T4 is most likely the hormone that your veterinarian has been monitoring.

What causes Hyperthyroidism in Cats?

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 in Cats

There are several classic symptoms of hyperthyroidism in cats:

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.

Diet and Nutrition for Hyperthyroid Cats

If radiotherapy, medical or surgical treatment of your cat's hyperthyroidism are not available or are not appropriate for your cat, your vet might prescribe Hill's Prescription Y/D. If your cat is prescribed this diet, you need to ensure that you don't feed them any other foods and treats (this includes hunting and scavenging!). Any foods or treat other than the prescription diet will reduce it's effectiveness.

A full response to the prescription hyperthyroid diet, in the form of normal thyroxine levels, can take up to 12 weeks, and it will be ineffective in about 10% of cats. For cats who respond to the diet, it needs to be fed life-long.

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.