Hyperthyroidism in Cats

What are the thyroid glands?

The thyroid glands are located in the neck and play a vital role in regulating the body's metabolism. Hyperthyroidism is a disorder characterised by the overproduction of thyroid hormone and a subsequent increase in the pet's metabolism. This is a commonly-diagnosed condition in older cats. Although the thyroid gland enlarges, it is usually a benign or non-malignant (non-cancerous) change. However, a small proportion of hyperthyroid patients have malignant thyroid gland tumors. Many organs are affected by hyperthyroidism, including the heart. The heart is stimulated to pump faster and more forcefully; eventually, the heart enlarges to meet these increased demands for blood flow. The increased pumping pressure leads heart murmurs and/or abnormal heart rhythms, and high blood pressure. A good proportion of cats with hyperthyroidism have high blood pressure and concurrent heart disease.

 

Are certain cats more likely to develop hyperthyroidism?

Older cats are at increased risk for developing hyperthyroidism. Environmental and dietary risk factors have been investigated and may play a role in predisposing some cats to hyperthyroidism, although the specific mechanisms are not known. No individual breed is known to be at increased risk, although the Siamese appears to have a somewhat increased incidence of hyperthyroidism compared to other breeds.

 

What causes hyperthyroidism?

Some of the risk factors for hyperthyroidism have been defined above. A specific cause forhyperthyroidism has not been identified. The possible role of dietary iodine continues to be investigated as a dietary influence on development of hyperthyroidism.

 

What are the clinical signs of hyperthyroidism?

The typical cat with hyperthyroidism is middle aged or older. The most common clinical sign of hyperthyroidism is weight loss secondary to the increased metabolic rate. The cat tries to compensate for this with an increased appetite. In fact, some of these cats have a ravenous appetite and will literally eat anything in sight! Despite the increased intake of food, most cats continue to lose weight.

Affected cats often drink a lot of water and urinate more frequently. There may be periodic vomiting or diarrhoea, and the fur may appear unkempt. In some cats, anorexia develops as the disease progresses.

Two secondary complications of this disease can be significant. These include hypertension (high blood pressure) and heart disease called thyrotoxic cardiomyopathy. Hypertension develops as a consequence of the increased pumping pressure of the heart. In some cats, blood pressure can become so high that retinal hemorrhage or detachment will occur and result in blindness. Heart problems develop because the heart muscle would have had to enlarge and thicken to meet the increased metabolic demands. Both of these problems may be manageable, although not fully reversible, with appropriate treatment of the disease.

 

How is hyperthyroidism diagnosed?

The first step is to determine the blood level of one of the thyroid hormones, called total thyroxine (or TT4). Usually, the TT4 level is so high that there is no question as to the diagnosis. Occasionally, a cat suspected of having hyperthyroidism will have T4 levels within the upper range of normal. When this occurs, a second test, usually either a Free T4 by Equilibrium Dialysis (FT4 by ED) or a T3 Suppression Test, may be performed. If these tests are not diagnostic, a TT4 can be measured again in a few weeks. If a thyroid tumour is suspected, your veterinarian may recommend an ultrasound examination of the thyroid glands and a referral to a specialist facility.

 

How is hyperthyroidism treated?

Because only a small number of cats with hyperthyroidism have cancerous growths of the thyroid gland, medical treatment is usually very successful. Several tests are performed before choosing any form of treatment. These tests are needed to evaluate the overall health of the cat and predict the chances for treatment complications. Such tests may include blood tests, urinalysis, and xrays, blood pressure determination. Echocardiography (ultrasound of the heart) may be recommended based on your cat's condition.

There are several options for treatment; any one of them could be the best choice in certain situations. Many factors must come into consideration when choosing the best therapy for an individual cat. 

The treatment options for hyperthyroidism are:

1. Radioactive iodine
A very effective way to treat hyperthyroidism is with radioactive iodine therapy (I-131). It is given by injection and destroys all abnormal thyroid tissue without endangering other organs. Treatment requires one to three weeks of hospitalisation at a veterinary hospital licensed to administer radiation therapy. This is a definitive cure for 95.4% of patients undergoing radioactive iodine treatment.

2. Oral medication
Administration of an oral drug, Carbimazole (Vidalta®), can control the effects of the over-active thyroid gland. Some cats have reactions to the drug, but that number is fairly small (less than 20%). However, the side-effects may begin as late as six months after the beginning of treatment and can include vomiting, lethargy, anorexia, fever, and anemia. Carbimazole blocks the production of excess thyroid hormone rather than destroying the abnormal thyroid tissue. Therefore, the drug must be given for the remainder of the cat's life. Periodic blood tests must be done to keep the dosage regulated and to ensure that no adverse side-effects are developing. Blood tests are usually performed one month after any dose change or routinely every three to six months for cats receiving Carbimazole. Your vet will advise you when an appropriate time to schedule follow-up blood tests.

3. Topical medication
Methimazole gel works in a similar fashion to the carbimazole tablets, except that the gel is applied to the hairless aspect of the ear flap. This drug must be given for the remainder of the cat's life. Periodic blood tests must be done to keep the dosage regulated and to ensure that no adverse side-effects are developing. Blood tests are usually performed one month after any dose change or routinely every three to six months for cats receiving methimazole gel. Your vet will advise you when an appropriate time to schedule follow-up blood tests.

4. Prescription diet (Hill's y/d)
This diet is the only clinically proven nutrition to manage feline hyperthyroidism without medication, surgery or radioactive iodine therapy. When feeding this diet, it is important that you understand that it must be the only food that your cat receives, so you may need to keep you cat confined to your backyard to prevent eating any of the neighbour's food. Blood tests will still be required every three to six months when using this diet to make sure that the thyroid levels are being controlled appropriately.

5. Surgery (last resort and/or for thyroid tumours)
Surgical removal of the affected thyroid lobe(s) may be considered for cats with cancerous growths of the thyroid glands. Because hyperthyroid cats are usually over eight years of age, there is a degree of risk involved. However, if the cat is otherwise healthy, the risk is minimal. If surgery is the treatment method chosen, the cat is often treated with an anti-thyroid medication for several weeks prior to the operation. During that time, the ravenous appetite should subside and the cat will probably gain weight. Some cats also have a very fast heart rate and high blood pressure; these problems can be managed with medication before surgery. Possible complications include low thyroid hormone (which may require life-long supplementation) and/or low calcium levels post-operatively, but this varies from patient to patient.

 

What is the prognosis for hyperthyroidism?

Recurrence of the disease is a possibility in some cats. It is uncommon after radioactive iodine therapy. When surgery is done, recurrence is possible if abnormal thyroid cells are left in the cat. The remaining cells will likely grow causing the disease to recur.

One possible side effect of treating hyperthyroidism is kidney disease. High blood pressure can mask signs of kidney disease. When we treat hyperthyroidism and lower blood pressure, the kidney disease is revealed, this is another reason for recommending regular blood tests to monitor the cat's health status.

Many owners of cats with hyperthyroidism are hesitant to have radiation therapy or surgery because of their cat's advanced age. But remember, age is not a disease. The outcomes following both surgery and radiation therapy are usually excellent, and most cats have a very good chance of returning to a normal state of health. Cats managed medically also often do very well, as long as the medication is administered routinely and follow-up blood and diagnostic tests are performed.

 

Can hyperthyroidism be prevented?

There are no preventive measures for hyperthyroidism, but middle-aged and senior cats should receive a complete physical examination by a veterinarian every six to twelve months. Special attention should be given to thyroid enlargement and the typical clinical signs of hyperthyroidism. Annual blood and urine tests are important in all cats over age six to detect hyperthyroidism before potentially irreversible damage occurs. Do phone us at (06) 3588675 for more information or if you have any further enquiries regarding hyperthyroidism.

 

This client information sheet is based on material written by Ernest Ward, DVM.
© Copyright 2005 Lifelearn Inc. Used with permission under license. February 10, 2008, sourced from Diagnostic Imaging Atlas

 

Published by Cahill Animal Hospital on 28 May 2016

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