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Clinical Picture of Feline Hyperthyroidism

There are many clinical signs associated with feline hyperthyroidism. The hallmark sign is:

• Weight loss despite excellent appetite.
• Enlarged Thyroid Glands - In the normal cat, the lobes of the thyroid gland cannot be felt with one’s fingers. In the hyperthyroid cat at least one lobe is usually prominent and may be detected by your veterinarian during a physical exam.

Hyperthyroidism is generally a disease of older cats. The average age at diagnosis is about thirteen.

How to Tell if a Cat is Hyperthyroid

A blood panel is often ordered in the diagnostic work-up of any significant clinical signs. The blood panel surveys hepatic, renal, hematologic, and other body systems. Although there are multiple changes which fit hyperthyroidism, mild increases in liver enzymes are most commonly observed indicating mild (usually clinically insignificant) damage to the liver. The elevated T4 level forms the basis for diagnosis; there can be no other diagnosis in such a case.

*If T4 is markedly elevated, the diagnosis is clear. Some cats have borderline results and require further testing.

When T4 Levels are Borderline

In older cats, the normal range for T4 is much lower than it is for young adult cats and thus sometimes it is not obvious whether a cat is truly hyperthyroid. Further, T4 levels are readily reduced by other disease states and there is some normal (but unpredictable) fluctuation in T4 throughout the day.

The easiest tests for your veterinarian to perform are called the T3 SUPPRESSION TEST and the EQUILIBRIUM DIALYSIS FREE T4. To do the T3 suppression test, the owner must be able to give the cat a pill three times a day for two days and return to the vet’s office for a blood test on the third day. In other words, some specific scheduling is necessary and the cat must be able to take pills.

The pill given is a T3 pill, active thyroid hormone. After two days of exposure to this medication, a normal thyroid will respond by reducing its T4 production. In a hyperthyroid cat, this negative feedback mechanism is not operating and the T4 level will not drop. Even borderline cases can be sorted out.

The Equilibrium Dialysis Free T4 represents a special form of thyroid hormone, the part that is not bound to blood proteins (as measured by a laboratory technique called “equilibrium dialysis.”) Unlike T3 and total T4 measurements, the Equilibrium Dialysis Free T4 is not as subject to the influence of non-thyroidal illness. For example, it is normal for total T4 levels to drop when one is sick; however, the free T4 portion remains unchanged. In this way, a hyperthyroid cat that is sick from another disease might have a T4 in the normal or borderline range, but the Equilibrium Dialysis Free T4 would still be high enough to confirm the hyperthyroid condition. The Equilibrium Dialysis Free T4 is a simple blood test that can be run on any serum sample, though sometimes it must be sent to a special laboratory, thus requiring a few days for results.

Of course a nuclear medicine scan will also identify a hyperthyroid cat readily. The equipment necessary is only available in a limited group of facilities. For more information click here.

What Causes Hyperthyroidism?

Hyperthyroidism is caused by a benign growth in the thyroid gland that is over-producing T4. It is important to realize that these tumors are almost always benign and represent a form of goiter rather than a form of cancer. Only 3-5% of hyperthyroid cats have a cancerous thyroid growth.

Many people want to know what caused their cat’s thyroid gland to grow a tumor, benign or otherwise. A study published in the Journal of the American Veterinary Medical Association in March of 2004 shows some interesting information. There is a strong correlation between eating canned food and developing hyperthyroidism later in life; in fact, cats who eat only canned foods from “pop-top” type cans have five times the risk of developing hyperthyroidism relative to cats who eat only dry food. Cats whose diet is 50% canned food have 3.5 times the risk of developing hyperthyroidism relative to cats who eat only dry food. It has been speculated that pop-top type aluminum cans are lined with a substance called Bisphenol-A-diglyciddyl ether, which is transferred into food containing oils or fats. In areas of the world where this type of can is not used for cat food, hyperthyroid is not a common disease. Still, it is important to realize that a good 25% of hyperthyroid cats have never eaten canned foods in their lives, so there is clearly more than one factor at work.

Why is it so Important to Treat Hyperthyroid Cats?

Hyperthyroid cats frequently experience reduced quality of life through weight loss, muscle deterioration, chronic vomiting or chronic diarrhea. Not all cats experience these signs at the time of diagnosis but there are less visible reasons to treat: heart disease and high blood pressure. These problems can result in heart failure, sudden blindness, or sudden death and all can be prevented with timely treatment for thyroid disease.