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Mission Pet Hospital

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Medical Information

While we have a discussion of some frequently asked questions, a wealth of information about common veterinary issues can be found at: www.veterinarypartner.com

Hyperthyroid T4

What is hyperthyroidism?

Hyperthyroidism is a very common disease affecting middle-aged and older cats. Hyperthyroidism is caused by an increase in production of thyroid hormones (T3 and T4) from enlarged thyroid glands in the neck. In most cases, enlargement of thyroid glands is caused by a benign tumor called an “adenoma”. Some cases of Hyperthyroidism are caused by a malignant tumor called an “adenocarcinoma” but this is very rare. Thyroid hormones affect nearly all the organs in the body. Therefore, thyroid disease can sometimes cause secondary problems.

 

What are the signs of hyperthyroidism?

Cats afflicted with Hyperthyroidism usually develop a variety of signs. The most common clinical signs of Hyperthyroidism are weight loss, increased appetite, and increased thirst and urination. Hyperthyroidism may also cause vomiting, diarrhea, and hyperactivity. The coat may appear matted or greasy.

 

What are the complications of hyperthyroidism?

Over time, cats with hyperthyroidism may develop an enlargement and thickening of the heart. If left untreated, these changes will eventually compromise the normal function of the heart and can even result in heart failure. This means that in some cats with Hyperthyroidism, additional treatment may be required to control secondary heart disease. Once the underlying Hyperthyroidism has been controlled, the cardiac changes will often improve or may even completely resolve.

Hypertension (high blood pressure) is another potential complication of Hyperthyroidism and can cause additional damage to several organs, including the eyes, kidneys, heart and brain. If hypertension is diagnosed along with Hyperthyroidism, drugs may be needed to control the blood pressure. As in the case of heart disease, after the Hyperthyroidism has been successfully treated, the high blood pressure will often resolve, and permanent treatment for it may not be required.

 

How is hyperthyroidism diagnosed?

Hyperthyroidism is diagnosed through bloodwork. Most cats with Hyperthyroidism will have elevated levels of the thyroid hormone T4 in their bloodstream. However, a small percentage of cats with Hyperthyroidism will have T4 levels within the normal range.If Hyperthyroidism is still suspected, further testing may be needed.

 

How can hyperthyroidism be treated?

Generally, the treatment a cat receives for Hyperthyroidism will depend on specific circumstances, including heart and kidney function. Concern about kidney failure is a major determinant of the course of treatment, and may eliminate some of the options. In most cases, one of two treatment options is chosen and these include either medication or radioactive-iodine therapy. Surgery is usually only considered if an adenocarcinoma is suspected.

 

What is the medication used to treat hyperthyroidism?

Medication includes the use of anti-thyroid medications (either oral or transdermal forms) that act by reducing the production and release of thyroid hormones from the thyroid gland. These medications do not provide a cure for the disease, but they do provide control of Hyperthyroidism. The medications are readily available and relatively inexpensive. However, some cats may experience side effects that can include vomiting, anorexia, diarrhea, lethargy, anemia or allergic reactions (itchy ears/face). Lifelong treatment (usually involving twice- daily dosing) will be required. Routine blood tests should be done periodically during treatment to evaluate the effectiveness of therapy, to monitor kidney function and to detect any side effects.

 

What is Radioactive Iodine?

Radioactive-iodine therapy is another treatment option for cats with Hyperthyroidism. This therapy is recommended only once the cat has been evaluated and determined to be an ideal candidate for the treatment. During treatment, radioactive-iodine is administered as a subcutaneous injection and is absorbed into the bloodstream. The iodine is taken up by the overactive thyroid cells but not by other body tissues. The radiation destroys the abnormal thyroid tissue but does not damage the surrounding tissues or the parathyroid glands.

The radioactivity carries no significant risk for the cat, but precautionary protective measures are required for those who come into close contact with the cat within two weeks after the procedure. A treated cat has to remain hospitalized until the radiation level has fallen to within acceptable limits. The majority of cats have normal hormone levels within one to two weeks of treatment. Radioactive-iodine is curative is approximately 95% of all cases. For the few cats where Hyperthyroidism persists, the treatment can be repeated. Rarely, a permanent reduction in thyroid hormone levels (Hypothyroidism) occurs after radioactive-iodine treatmentthen thyroid-hormone supplementation may be required.

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