Hyperthyroidism in Cats


Hyperthyroidism is the most common endocrine disease of cats. Hyperthyroid cats have pathologically raised metabolisms which affect the muscles, heart, gastrointestinal tract, kidneys and the nervous system. The constant excess of thyroid hormones in the bloodstream, if left untreated, can lead to congestive heart failure, intractable diarrhoea, kidney damage, and retinal damage (due to high blood pressure).

What is the function of the thyroid gland?

The thyroid gland, which is located at the base of the neck, secretes thyroid hormones that are required for growth and reproduction. The general effect is to increase the rate of metabolism, with effects throughout the body such as increases in heart rate, blood pressure, body temperature, gut movement, and brain activity.

What causes hyperthyroidism?

About 98-99% of cases are the result of a benign tumour of the thyroid gland which causes the thyroid gland to increase greatly in size. The enlarged thyroid gland then secretes excessive and unhealthy amounts of thyroid hormone. Cats with enlarged thyroid glands usually have a palpable goitre that can be detected during a veterinary examination.

Who is at risk?

While hyperthyroidism occurs frequently in cats, it is an extremely rare disease in dogs. Cats are usually middle-aged to older (average 13 years of age) when they develop disease.

How can I recognize the signs of hyperthyroidism in my cat?

This disease has a gradual onset, and your pet may have been ill for some time before obvious symptoms are present. The most common symptoms, in decreasing order of frequency are:

•    Weight loss
•    Eating too much
•    Drinking too much water/excessive urination
•    Rapid heart rate
•    Hyperactivity
•    Diarrhoea
•    Respiratory problems
•    Poor body and coat condition
*A small percentage of cats will actually have the opposite signs: depression, lethargy, decreased appetite, etc.  (referred to as “apathetic hyperthyroidism”).

If you are concerned that your cat may be ill with hyperthyroid disease, make a prompt appointment with a veterinarian.

How does the veterinarian diagnose cases of hyperthyroidism?

The veterinarian will make the diagnosis based on a clinical history, physical exam, blood test and urine test. The amount of thyroid hormone in the blood can easily be measured, and is a good indicator of disease. Your pet may require further tests (e.g. chest x-rays, ultrasound of the heart) if your vet feels that the hyperthyroidism has resulted in heart disease.

Can hyperthyroidism be treated?

There are a number of treatment options for cats diagnosed with hyperthyroidism. Your vet will help you decide what the best option for your cat is.

1. Medication: Carbimazole (or its metabolite methamizole) are the drugs of choice for long-term management or pre-operative treatment of feline hyperthyroidism. They inhibit thyroid hormone production.

•    Vomiting, anorexia, and depression are common side-effects of medication that often resolve over time

•    While medication is an effective short-term plan, there is poor control of the condition in the long-term

2. Surgery: Thyroidectomy (surgical removal of the thyroid gland) is often the best treatment option available.

•    Surgery does not always effect a cure, as regrowth of thyroid can occur (uncommon)

•    Possible complications of surgery are hypocalcaemia (inadequate levels of calcium in the blood), hypothyroidism (inadequate thyroid hormone), and paralysis of the larynx

3. Radioactive Iodine therapy: this is the treatment of choice

•    Therapy is safe and effective

•    Regrowth of thyroid tissue is uncommon

•    The lack of facilities offering radioactive iodine therapy means this treatment is of very limited availability to most patients

(photo: lacabezagrande)