Feline Hyperthyroidism

What is Hyperthyroidism?

Hyperthyroidism is caused by an excess of thyroid hormone in the body.  Most often this is the result of a small tumor in the thyroid gland which secretes excess amount of thyroid hormone into the system.  Thyroid hormone is responsible for many functions in the body.  It affect the rate of metabolism and tend to speed up the metabolic rate.  As a result, the heart beats faster, the body uses more energy than normal, the pet may lose weight despite a good appetite.

What are the symptoms of Hyperthryoidism?

The hallmark symptoms of Hyperthyroidism are:

Which animals can get Hyperthryoidism?

Hyperthyroidism is predominantly seen in cats over the age of 6 years.  Certain breeds such as the Siamese and Burmese cat seem to be predisposed to get this disease.  Hyperthyroidism is more likely to occur in older pets.

Why do pets get sick from Hyperthyroidism?

When the thyroid hormone levels are in excess, they force the body to a higher metabolic rate.  This causes excess wear on the organs particularly the heart.  Thyroid hormone can cause increased blood pressure (hypertension) which can reduce the blood and oxygen supply to the organs.  Both of these factors may lead to heart failure and strokes.

Can Hyperthyroidism be prevented?

We know that hyperthyroidism is caused by tumors in the thyroid gland.  In most cases these are not malignant tumors and will not spread throughout the body.  Unfortunately, we do not know why these tumors develop and therefore have no way to prevent it at this time.

How is Hyperthryoidism Treated?

There are three approaches to the  treatment of Hyperthyroidism: Radioactive Iodine, Medical & Surgical.

Radioactive Iodine Therapy:

The thyroid gland selectively takes up iodine in order to form thyroid hormone.  If we inject a radioactive isotope of iodine (I¹³¹) it is taken up into the thyroid gland where the radiation can then kill the tumor.  In addition, any thyroid tumor located elsewhere in the body will also take up the isotope and be killed.  The success rate with a single injection of radioactive iodine is about 90%.  Cats must stay at the facility for a few days until the radiation has left their bodies.  This is generally the preferred and most successful form of treatment for this disorder.  More information is available at http://www.avmi.net/NewFiles/Hyperthyroidism/Hyperthyroidism.html

Medical Management:

We can give a drug (Tapazole) which will reduce the amount of circulating thyroid in the blood stream and therefore relieve the symptoms of the disease.  The drug must be given for the remainder of the pet’s life and monitoring of the blood must be done regularly.  Once we have determined the appropriate dose of medication, the thyroid level will need to be checked every 6 weeks to 3 months. 
This method of management is attractive to some owners due to the age of their pet. The long term cost of the drug can be expensive however and there are possible side effects from the drug itself.

Surgical  Management:

Most often, Hyperthryoidism is caused by a tumor in one or both of the thyroid glands (located in the neck region).  The tumors can be readily removed by surgery which will eliminate the problem in most cases.  The surgery is relatively safe and can be performed in our facility.  Due to the other possible problems related to high thyroid levels, pre-operative testing would be needed to ensure safety.  These tests would include: blood and urine tests, x-rays of the chest , electrocardiogram and/or ultrasound to evaluate the heart.  If everything checks out in advance, the surgery should go very smoothly.

Occasionally, there may be extra thyroid tissue in the chest cavity which can also develop into tumors.  Therefore, removal of a single gland will not always cure the problem. In about 20-30% of the cats, a tumor may develop later in life on the other thyroid gland. If this should occur, a second surgery or medical management would have to be considered.