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Hyperthyroidism in cats is a very common disorder, especially in more senior cats, and is caused by an over-production of thyroid hormones from the thyroid gland.

The thyroid glands are located in the neck and consist of two separate lobes on either side of the windpipe. One or both of the lobes becomes enlarged and leads to an excessive production of T4 (Thyroxine) and T3 (Triiodothyronine). In 98% of cases, the enlargement is benign (non-cancerous), the remaining 2% are caused by a malignant (cancerous) growth of the thyroid gland. 

An Example of Hyperthyroidism with Merry and Mungo Jerry

Very recently two 14 year old siblings, “Merry” and “Mungo Jerry”, visited our Penistone surgery for their booster vaccinations. Both had lost a little weight and after our vet asked a few questions their owner thought that Mungo Jerry was drinking perhaps a little more than normal. On examination, both cats had enlarged thyroid glands in their neck and Merry’s heart rate was slightly higher than normal. Routine bloods were taken, including checking their Total Thyroid (TT4) levels which revealed that they both had high TT4 levels confirming both cats had Hyperthyroidism.

What do Thyroid Hormones do?

Thyroid hormones are responsible for the:

  • Regulation of the protein, fat and carbohydrate metabolism by cells
  • Regulation of heat production and oxygen consumption, which in turn affects the regulation of a wide range of body processes

Excessive thyroid hormones cause a dramatic increase in your cat’s metabolic rate and therefore affect nearly every body system. They tend to burn up energy too rapidly and typically suffer weight loss despite having an increased appetite and food consumption.

Common Symptoms of Hyperthyroidism in Cats

  • A ravenous appetite (polyphagia)
  • Weight loss despite this increase in appetite
  • Increased thirst (polydipsia)
  • Increased urination (polyuria)
  • Increased heart rate (tachycardia)
  • Agitated, restless (“hyper”) or irritable
  • Occasional vomiting
  • Diarrhoea
  • A scruffy unkempt or matted coat

Not every cat will show all of these signs. As you can see in the case of Merry and Mungo Jerry, this feline pair were not your typical hyperthyroid cats. They didn’t have any of the other classic symptoms, they were still healthy weights and were in good condition.

Diagnosing Hyperthyroidism in Cats

Hyperthyroidism is usually suspected on clinical signs and physical examination. A gentle touch of your cat’s neck may reveal an enlarged thyroid gland, however, in some cats, there is no obvious enlargement because the overactive thyroid tissue is in an unusual site such as the chest cavity. The disorder is then confirmed with:

  1. A blood test to check the cat’s Total Thyroid (TT4) level. In borderline cases, a more specific test, called the Free T4 (FT4) may also need to be checked
  2. Routine bloods are also advised to check for concurrent disease, such as kidney (renal) disease
  3. Your Vet may suggest urine testing, a heart scan or chest x-ray (if heart disease is suspected)
  4. A blood pressure check

Treatment of Hyperthyroidism in Cats

There are several different treatment options available if your cat is diagnosed with hyperthyroidism. Each treatment option has its advantages and disadvantages. Your Vet will be able to discuss these with you and decide which is the most appropriate for your cat. It is important to treat the hyperthyroidism as more serious complications, such as heart conditions and high blood pressure (hypertension), could develop if left untreated.

  • Medical management : daily or twice daily tablets, a new liquid formulation or a transdermal gel; the drug gets absorbed through your cats skin - handy if your cat is difficult to pill!

If your cat is maintained on medication, regular blood tests will need to be carried out to ensure your cat is on the correct dose. Medical management will resolve the outward signs of hyperthyroidism, does not cure the disease and needs to be continued for the rest of your cat’s life. The same applies for the iodine-restricted food. It has also been reported that in up to 20% of cases, previously benign changes can become cancerous as time goes on; medication would not treat this.

  • Surgical management : thyroidectomy, to remove one or both of the affected thyroid glands

Unilateral or bilateral thyroidectomy is commonly performed and generally very successful. It can provide remission from the disease and even produce a permanent cure. However, surgery does come with its risks and complications, which would need to be discussed with your Vet before it was undertaken.

Radio-iodine therapy is safe and effective, and will provide a cure in most cases with no ongoing treatment. Your cat would have to be referred to a referral centre for this, where they would be hospitalised for the duration of their treatment.  

Merry and Mungo Jerry

We are pleased to see that Merry and Mungo Jerry have been back for their regular health checks and are doing very well. They have gained weight and are eating and drinking normally. Here are photos of them both following their recent blood samples.