Feline Hyperthyroidism

Feline hyperthyroidism was first recognized in 1979 and is now the most frequently diagnosed endocrine disease in cats. Recent studies have shown that a flame retardant chemical containing polybrominated diphenyl ethers (PBDEs), that is widely distributed in furniture such as couches and bedding, induces this disease.

In most cases we use a blood test for thyroid hormone levels(T4 ) to make a diagnosis. We sometimes need a second test using T4 or free T4 because many factors may alter the T4 concentrations and the initial levels are at a questionable level. Complicating factor to a clear diagnosis include, age, illness, stress, drugs, and even the time of day the blood sample is taken. So unless the values on the screening tests are very high a second test that provides a direct measurement of free T4 (the active state of the hormone) may be needed. Using these two tests provides an accurate evaluation of thyroid status with values less influenced by extraneous factors

There are currently two main methods for the treatment of hyperthyroidism:

1. Medical management using an anti-thyroid drug treatment is readily available at our hospital and is economical. Life long treatment, usually involving once or twice daily oral or topical dosage, is required to control the production of thyroid hormone. Routine blood tests are checked periodically during treatment to monitor your cat for potential side effects. This is adequate treatment for most cats and is the initial method we use to see if problems develop from the correction of the excess thyroid hormone.  Sometimes the consequences of lowering thyroid levels decreases blood pressure and causes a problem with the kidneys.

 2. Radiation using radioactive iodine (I-131) often provides an effective cure for hyperthyroidism. Occasionally multiple treatments are need. The procedure consists of an injection of radioactive iodine that is taken up by the overactive thyroid tissue, but not by any other body tissues. This radiation usually destroys all affected thyroid tissue, including any ectopic thyroid tissue that would be inaccessible to surgery.


Medical Management

 This site has been designed to help answer questions that you may have about hyperthyroidism and how it is treated.
It will provide you with information about the condition as well as some guidance on how best to help your cat during treatment.
With daily medication and careful monitoring your cat will soon be feeling a lot better. Many of the signs associated with hyperthyroidism will start to disappear and will be kept under control.
What is Hyperthyroidism?
A BRIEF DESCRIPTION

Cats with hyperthyroidism have a thyroid gland that is producing excessive amounts of thyroid hormones. This causes a dramatic increase in the body’s metabolic rate.
Hyperthyroidism occurs when the thyroid gland over–produces thyroid hormones. Your cat’s thyroid gland is located in its neck. It is divided into two parts called lobes which lie on either side of the windpipe (trachea).
When a cat has hyperthyroidism either one or both of the thyroid lobes become en-larged (both lobes are affected in 70% of cases). This leads to excessive production of the thyroid hormones T4 (also known as thyroxine) and T3 (also known as triiodothy-ronine).
Thyroid Hormones
Are essential to proper growth of body cells and the development of these cells for spe-cific roles in the body.
Help regulate the metabolism of protein, fat and carbohydrate by cells.
Are involved in the regulation of heat production and oxygen consumption and therefore a wide–range of metabolic processes.
As a result, an excess of thyroid hormones affects the function of virtually every organ system.
EXCESS THYROID HORMONES
In cats with hyperthyroidism the excess of T4 and T3 in the blood causes a large in-crease in the metabolic rate. As the metabolism of each cell increases they require more energy to function. This means that a cat with hyperthyroidism has to eat a lot of food to provide this energy.
As the condition progresses it becomes increasingly difficult for a cat to eat enough to provide the huge amount of energy required so they start to lose weight. This is why the most common clinical sign in cats with hyperthyroidism is weight loss despite a rav-enous appetite.
The increased metabolic rate also causes other signs including restlessness and a rapid heart rate.

Diagnosis
The early stages
Hyperthyroidism is more often seen in older cats.
RECOGNIZING THE SIGNS OF HYPERTHYROIDISM
Typically, you will notice that your cat is suffering from severe weight loss whilst main-taining a ravenous appetite.
Cats with hyperthyroidism produce thyroid hormones in excess, which leads to a dra-matic increase in their metabolic rate.
Other common signs to look for include:
Increased restlessness and irritability
An unkempt coat
Diarrhoea
Occasional vomiting
Frequent urination
Increased fluid intake
Anxious facial expression
Rapid heart rate
Not all cats will react to the disease in the same way and your cat may not necessarily display all of these signs. Wherever possible, it is always a good idea to keep a note of the changes you see in your cat’s habits, behaviour and appearance. When hyperthy-roidism is suspected, diagnosis is normally very straightforward.

PHYSICAL EXAMINATION
In most cases, your veterinarian will be able to feel the enlarged thyroid lobes in your cat’s neck. Occasionally, the thyroid gland cannot be easily felt. This happens when the thyroid tissue is located elsewhere, usually inside your cat’s chest.
DIAGNOSTIC TESTS
To confirm the diagnosis, your veterinarian will arrange a blood test in order to measure levels of the T4 hormone. Although both the T3 and T4 hormones are produced in excess in a cat suffering from hyperthyroidism, total T4 measurement is the most accurate test available.
Since most cats with hyperthyroidism are older, it is recommended that your cat has other blood tests to assess their organ function and make sure that they do not suffer from any other problems. This is important because other conditions might affect the successful treatment of hyperthyroidism.


Managing Hyperthyroidism

It is common to prescribe Methimazole (Tapazole) for an initial period of three weeks. This will ensure that your cat’s condition becomes more stable.
TREATMENT
Hyperthyroidism can be treated very successfully, especially since very few cats develop cancerous growths of the thyroid gland. Treatment will involve bringing the levels of thyroid hormones in the blood back to normal.

STABILIZATION
In order to get your cat’s thyroid hormone levels back to normal, it is common for your veterinarian to prescribe Methimazole (Tapazole) for an initial period of three weeks. This will ensure that your cat’s condition becomes more stable.
Your veterinarian will then assess and monitor your cat’s response to the therapy and discuss the most suitable long term treatment options.
LONG–TERM MEDICAL TREATMENT
Methimazole (Tapazole) contains the active ingredient Thiamazole and is given to your cat every day to reduce the production of thyroid hormones. If your cat’s hyperthyroid-ism is treated using Methimazole (Tapazole) this will not cure the condition. Your cat will need to be treated for the rest of it’s life.
Methimazole (Tapazole) blocks the synthesis of T3 and T4 and leads to a return to normal metabolic rate by reducing the production of T3 and T4.
If your veterinarian recommends long–term use of Methimazole (Tapazole), monitoring of the condition is very important. Your cat will need to have regular blood tests. These tests will ensure that your cat is receiving the correct amount of medication. It will also allow your veterinarian to make sure your cat remains healthy as it gets older.

Continuous Care
 
Your cat will need to be carefully monitored and returned for follow–up appointments on a regular basis.
A series of blood tests will need to be carried out before starting treatment and after 3 weeks, 6 weeks, 10 weeks and 20 weeks. Assessing the response to treatment in this way enables the veterinarian to monitor for possible side–effects to Methimazole (Tapazole) therapy or check for the development of any unrelated complications.
After these initial appointments, and once your veterinarian is happy that your cat is stable, you will normally just need to visit every three months to monitor your cat’s condition. At some stages it may be necessary to adjust the dose of Methimazole (Tapa-zole) prescribed.

Methimazole (Tapazole) will resolve the outward signs of hyperthyroidism whilst your cat is being treated but it does not cure the disease. Even if you notice dramatic physical improvements to your cat’s health this does not mean that you should stop treatment.



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CAUSE of HYPERTHROIDISM

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