What is hyperthyroidism?
Hyperthyroidism is the most common endocrine (hormonal) disorder in cats. It is rarely seen in cats under eight years of age, and there is no sex or breed predisposition. It is due to increase use in the production and secretion of thyroid hormone by the thyroid glands (there are two of them) in the neck.

It occurs when the thyroid start starts producing too much thyroid hormone. This causes cats’ metabolism to increase, and cats will start burning calories like crazy, causing them to lose weight. They try to compensate by eating more food, but they usually cannot keep up, and cats will lose weight despite having an excellent (often ravenous) appetite.

What are the clinical signs of hyperthyroidism?
Cats may present with any one or a combination of the following clinical signs (which tend to develop gradually):

1. Weight loss
2. Increased appetite
3. Hyperactivity and restlessness
4. Increased heart rate, with a variety of cardiac rhythm irregularities and heart murmurs
5. Increased frequency of defecation, with abundant, bulky stools
6. Increased thirst and urination
7. Occasional vomiting
8. Panting
9. Matted, greasy, and unkempt coat
10. Behaviour change – increased aggression and feistiness

How is hyperthyroidism diagnosed?
In many hyperthyroid cats, a nodule is usually palpable in one or both of the thyroid lobes. As the enlarged lobe may be freely movable and can slide along and behind the trachea, it may be difficult to detect and require careful palpation. In the normal cat, the thyroid lobes are either not palpable or are small and symmetrical.

However – NOT being able to palpate (feel/detect) a nodule on the thyroid does NOT eliminate this disease.

Once hyperthyroidism is suspected on the basis of clinical signs, the diagnosis is confirmed by detecting elevated serum thyroid hormone levels (a T4 test). Other laboratory tests may also be abnormal, such as elevation of the liver enzymes (very common) or changes on an electrocardiograph (ECG).

At HIGHlands Veterinary Hospital, we run these tests in-house,’ and results are typically available within 20-30 minutes.

How can hyperthyroidism be treated?
There are FOUR therapeutic options for the treatment of hyperthyroidism. Which treatment option is most suitable for your cat depends on a number of factors, and together we will discuss and decide on the BEST treatment for every patient.

1. Anti-thyroid drug therapy

Anti-thyroid drugs are readily available and reasonably economical. They do not destroy the thyroid gland but act by interfering with production and secretion of thyroid hormone. Their use does not result in a cure, but rather controls the condition.

Carbimazole (Neo Mercazole) is the most commonly used medication. Initially it is given three times daily. It is reduced to a maintenance dose once the thyroid hormone levels have returned to normal. When used as a long term treatment, twice daily dosage is usually required….

For many owners, medicated their cat morning and night can be difficult indeed.

Mild (and often transient) side effects to Neo Mercazole are seen quite commonly in cats on this medication (~15% of patients), and can include poor appetite, vomiting and lethargy.

More serious side effects are seen less frequently (~5% of patients) and can include a fall in the number of white blood cells, clotting problems, or liver disorder.

Regular 6-monthly bloods tests should therefore performed on any cat which is on this medication to monitor for potential side effects.

In patients the occurrence of severe adverse reactions (persistent vomiting or liver damage) may necessitate withdrawal of the drug.

Some cats simply can’t tolerate this medication and therefore we need to choose one of the other options below.

2. Surgical thyroidectomy

Surgical thyroidectomy (removal of the thyroid glands) has the immediate advantage over drug therapy in that it provides a cure – an immediate cure.

Because surgical skill and experience are necessary to minimise potential side effects, many practices do not offer this treatment and instead will refer the patient off to a specialist surgeon – which can be quite expensive.

Over the last 30 years, I have performed many 100s of successful surgical thyroidectomies and therefore we do not have to refer our patients for specialist surgery – unless they specifically request for this to be done.

Anaesthesia can be a challenge in hyperthyroid patients; both as a direct result of the body changes which have occurred due to the over-active thyroid (heart and blood pressure issues), and also because a number of patients have other concurrent diseases e.g. (most commonly) chronic renal disease.

To reduce hyperthyroid-related surgical risks, many patients are pre-treated with anti-thyroid drugs for 3 to 4 weeks prior to surgery to reduce their thyroid hormone levels back to normal. Any associated cardiac disease also needs to be carefully controlled.

Side effects of the surgical procedure are not common, but may include nerve damage, or hypoparathyroidism (lack of the hormone that controls the level of calcium in the blood). The parathyroid glands are located very close to the thyroid glands, and so can be inadvertently damaged (bruised) when the thyroid glands are being removed. The resultant hypocalcaemia (low blood calcium level) can result in muscle twitching, weakness and convulsive seizures. Typically a ‘bruised’ parathyroid gland will regain normal function within a few days. Because of this potential, patients under-going surgical thyroidectomy are observed closely for the first 2-3 days after surgery.

There is generally a low rate of recurrence of hyperthyroidism following surgery, although some cases do recur. This can happen when a case of bilateral hyperthyroidism (i.e. where both thyroid lobes are affected) is mistakenly treated as a unilateral case (where only one side is affected) – the differentiation of normal from abnormal thyroid tissue is not always straightforward.

Around 70% of hyperthyroid cases are bilateral, and in unilateral disease the gland on the opposite side of the neck is normally reduced in size. Occasionally, adenocarcinoma (malignant tumours) are present, and although they do not usually spread through the body, local invasion may prevent satisfactory surgical excision.

3. 131I (radioactive iodine) therapy

This treatment can only be used in and by veterinary practices which have a special license and are at liberty to use radioactive isotopes.

This uses radioactive iodine (I131) which is administered subcutaneously (injected under the skin) or given by mouth, and is selectively concentrated within the follicles of the thyroid gland.

131I selectively destroys the affected thyroid tissue, including any areas of thyroid tissue which may be inaccessible to surgery, and spares adjacent normal tissue, including the parathyroid glands.

An initial tracer-dose of 131I may be given in order to precisely calculate the correct treatment-dose of 131I for any individual. However, recent experience suggests that a standard dose (150-250MBq/cat) is likely to be curative in around 90% of cases. Where hyperthyroidism persists after treatment, a second dose can be given. Very occasionally permanent hypothyroidism (lack of thyroid hormone) has been seen after 131I treatment, but this can be easily managed with thyroid hormone replacement therapy.

The primary advantages of 131I treatment are that it is curative, has no serious side-effects (no toxicity, no hypoparathyroidism), does not require an anaesthetic or sedation, is associated with a low recurrence of hyperthyroidism and the location of the tumour is unimportant.

The cost of treatment however is of concern to many owners as it is typically MUCH more expensive then surgical treatment – and as mentioned above, you would need to take your cat to a facility licensed to use this treatment. Currently this involves going to Sydney.

It is important to note that large doses of 131I are the only effective treatment for thyroid adenocarcinoma, which is responsible for around 1 to 2% of feline hyperthyroid cases.

The problems of 131I treatment include:-

a) Poor availability, due to safety regulations that cover the use of radioactive products (need to go to Sydney).
b) Hospitalisation for between 1 and 2 weeks following treatment, which is necessary to allow adequate decay of the 131I (adds extra cost).
c) It is not suitable for use with patients requiring intensive care as, particularly in the early days following treatment, excessive handling of the cat must be avoided.
d) The radiation risk to personnel treating the cats.
e) Expense

4. Special Diet therapy

There is now a special diet made by the Hills Company – it is called Y/D.

The diet works by severely limiting the amount of dietary iodine intake. By restricting the iodine, you deprive the cat of one of the essential building blocks for production of thyroid hormone.

The only real caveat: the cat is allowed to eat Hill’s y/d, and ONLY Hill’s y/d. Absolutely NOTHING else can be eaten – no treats, and no people food. The iodine level in non-y/d food is enough to nullify the effects of the y/d.

Most cats do find the food to be palatable. If the cat doesn’t like the food, or starts to show disinterest, you cannot mix any other type of food with the y/d to make it more enticing. Hill’s y/d comes as a canned and a dry formulation.

When discussing the use of Y/D, I find that most clients will ask me one of two questions;

“I have a young cat (early diagnosis of hyperthyroidism), won’t my cat get bored with eating the same food for the next – 5, 7, 10 years….”

“I have an older cat that is very set in his/her ways – how can I get him/her to eat a new diet….”

That’s It
You now have an excellent insight into this very common disease of the older cat.

My take-home message is this; if your cat is showing ANY of these clinical signs, weight loss in spite of ravenous appetite, excessive thirst and urination, vomiting, diarrhoea, panting, restlessness or hyperactivity, cranky or even nasty behaviour and excessive vocalization at night, then one of the first things to consider is that your cat has an overactive thyroid…

Bring your cat in to see us so that we can run a very simple blood test to rule this disease in or out. And then if s/he has an overactive thyroid, we can discuss which form of treatment will work best for you and you feline companion.

The good news is that very soon you’ll have your ‘old’ friend back!

If you have ANY questions at all, please ring me at 02-4872-1144 or email me at info@highlandsveterinaryhospital.com.au – I’m always happy to help.