Overview?

The thyroid gland secretes hormones to regulate many metabolic processes, including growth, development and energy expenditure. Hyperthyroidism (also known as thyrotoxicosis) is caused by the overactivity of the thyroid gland, which produces more than the normal level of thyroid hormones T4 & T3, which are released into the bloodstream. Most often, hyperthyroidism is the result of a benign tumour (called an adenoma) involving one or both of the thyroid lobes. Feline hyperthyroidism is the most common disease of the endocrine system.

What is the endocrine system?

The endocrine system is composed of glands which secrete hormones into the bloodstream. Hormones regulate many body processes.

What does the thyroid do?

Consisting of two lobes (left & right), the thyroid is located in the neck on either side of the windpipe. It produces two hormones T3 triiodothyronine & T4 thyroxine. Both hormones help control metabolism.

What are the symptoms of Hyperthyroidism in cats?

Some symptoms of hyperthyroidism may include;

• Weight loss despite an increased appetite
• Increased thirst & urination
• Vomiting
• Diarrhea
• Behavioural changes (nervous/jittery behaviour, aggression, overgrooming & or bald patches, hyperactive, increased vocalisation)
• Rapid heartbeat
• Poor coat
• Weakness (occasionally)
• Decreased appetite (occasionally)
• Hypertension (high blood pressure)

What effects does hyperthyroidism have on the cat?

Increased levels of the thyroid hormones cause an increased heart rate. Cats may also have a heart murmur. It can lead to heart failure & kidney damage. High blood pressure, which can lead to blindness. It can be fatal if left untreated.

Secondary hypertrophic cardiomyopathy can occur as a result of hyperthyroidism.

How is Feline Hyperthyroidism diagnosed?

Your veterinarian will observe your cat's clinical signs, palpitate the thyroid gland. Other diseases such as renal failure & feline diabetes have similar symptoms, so it is necessary to take routine & specific blood tests to detect elevated levels of the hormones T3 & T4 are performed. Some cats with hyperthyroidism may show normal levels of these hormones in their blood test. If this is the case then a T3 suppression test may be performed. This involves taking a blood test to check the levels of T3 & T4, 7 oral doses of the thyroid hormone T3 & a blood test after the hormone was given. In a normal cat, the level of T4 will drop, in a cat with hyperthyroidism the T4 levels will stay the same or increase slightly.

Thyroid imaging are another method. A small dose of a radionuclide which concentrates in the thyroid tissue.

Are some cats prone to it?

No specific breeds are prone to Feline Hyperthyroidism, however is most often seen in older (8 years plus) cats.

There has been talk of a possible link between the consumption of canned food & hyperthyroidism in cats, but this doesn't appear to be the only cause.

What is the treatment of Feline Hyperthyroidism?

There are three options to treat hyperthyroidism in cats. Each treatment has it's pros & cons. Treatment should be depends on other medical conditions your cat may have (heart disease, kidney failure), availability of a nuclear medicine facility & cost.

Drugs (Methimazole/Tapazole): The use of medication doesn't cure hyperthyroidism, only controls it. Administered daily for the rest of your cat's life. There may be side effects from the use of drugs including vomiting & lethargy. Short term this is the cheapest option, but in the long run it can prove costly having to pay for daily medication for your cat. Methimazole can be given either in oral or transdermal (applied to the skin inside the ear) form. Advantages of medication are that treatment is reversible if side effects occur (see precaution at the end of this article). Other side effects may include; anorexia & vomiting.

In some cats, hyperthyroidism & kidney failure run concurrently. The hyperthyroidism can act to 'protect' the kidneys. This is because hyperthyroidism produces a hyperdynamic cardiac state (increased blood flow), which increases glomerular blood flow (GBF) and glomerular filtration rate (GFR), improving renal function.  Once treatment begins & the hyperthyroidism is brought under control, kidney failure may become worse. If kidney failure is suspected, your veterinarian will recommend using drugs to control the hyperthyroidism & closely monitor kidney function. That way, if the kidneys do begin to deteriorate, medication can be re-evaluated or stopped immediately, opposed to surgery or radioactive iodine treatment which is permanent.

Surgery: Removal of the enlarged thyroid lobe(s), known as thyroidectomy. Pros of this option are that it is a permanent cure. Cons include increased anaesthetic & surgical risks due to the effect the disease may have had on the cat's heart & kidneys. If both glands are removed, there is increased risk of accidentally removing the small parathyroid glands, (which among other things regulates the calcium supply in the body), which can cause hypocalcemia (low blood calcium). If both lobes are removed you will have to give your a daily thyroid supplementation.

Radioactive Iodine Treatment: This involves a single injection of radioactive iodine (radioiodine I-131). This concentrates in & destroys the diseased thyroid tissue, while leaving the normal thyroid tissue intact. Pros of this option are that the cat doesn't require anaesthesia, there is no need to give your cat medication for the rest of it's life and in the majority of cases it is a permanent cure. In a few cases treatment will need to be repeated. No damage is done to the parathyroid glands. Cons are that it is expensive in the short term, and can only be performed at a specialist veterinary centre. After treatment your cat will be slightly radioactive & will have to stay at your veterinarian's until the radioactive levels drop.