What is Hyperthyroidism?

Hyperthyroidism is a disorder characterized by the overproduction of thyroid hormone and a subsequent increase in the metabolic rate. This is a fairly common disease of older cats. Although the thyroid gland enlarges, it is usually a benign or non-malignant change. Less than 2% of hyperthyroid cases involve malignant thyroid gland tumors.

Many organs are affected by the increase in metabolism caused by hyperthyroidism.  The heart is stimulated to pump faster and more forcefully; eventually leading to heart enlargement and high blood pressure.  Most cats with hyperthyroidism have high blood pressure.

The increased circulation causes the kidneys to work harder at filtering the blood, and this wears out the kidneys.  Kidney function may appear normal at first, but when the hyperthyroidism is controlled and the heart rate and blood pressure return to normal, some of the patients are found to have underlying kidney disease.

Many hyperthyroid cats also have increased liver enzymes in the blood, due to accelerated death and replacement of liver cells.  These values usually return to normal when the hyperthyroidism is treated.

What causes hyperthyroidism?

A specific cause for hyperthyroidism has not been identified.  Older cats are at increased risk for developing the disease. 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 are the clinical signs of hyperthyroidism?

The typical cat with hyperthyroidism is middle aged or older. The average age of affected cats is approximately twelve years. The most common clinical sign of hyperthyroidism is weight loss secondary to the increased rate of metabolism. 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. The weight loss may be so gradual that some owners will not realize it has occurred, or the weight loss may be quite rapid. Affected cats often drink a lot of water and urinate more frequently. There may be periodic vomiting or diarrhea, and the fur may appear unkempt. In some cats, anorexia develops as the disease progresses.

The secondary complications of this disease can be significant. The heart can develop thyrotoxic cardiomyopathy, and it can literally wear out from pumping so hard and fast.  Hypertension (high blood pressure) can cause hemorrhage in the eye and retinal detachment, leading to sudden onset blindness. Both of these problems are potentially reversible if treated quickly, but if kidney damage has occurred, that is usually permanent, leaving the cat with diminished ability to maintain hydration, balance electrolytes and clear the blood of toxins.


How is hyperthyroidism diagnosed?

In most instances, diagnosis of this disease is relatively straightforward. 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, is performed. If these tests are not diagnostic, a thyroid scan can be performed at a veterinary referral center or the TT4 can be measured again in a few weeks.


How is hyperthyroidism treated?

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 include blood tests, urinalysis, and x-rays, EKG, blood pressure determination. Cardiac ultrasound or echocardiography may be recommended based on your cat’s condition.

There are four choices 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 cure hyperthyroidism is with radioactive iodine therapy (I131). It is given orally or by injection and destroys all abnormal thyroid tissue without endangering other organs. Treatment is quite safe, but requires three to five days of hospitalization at a veterinary center licensed to handle radioactive materials.  At one time this procedure was only offered at the veterinary universities, but today it is available in the Atlanta area through private facilities.  Though this treatment is somewhat expensive, it usually cures the disease, so that ongoing therapy is not necessary.  Radioactive iodine is the procedure most commonly used in people with hyperactive thyroids.

2. Surgery. Surgical removal of the affected thyroid lobe(s) is also very effective. Because hyperthyroid cats are older patients, there is a degree of risk involved. However, if the cat is otherwise healthy, the risk is minimal. If the disease involves both lobes of the thyroid gland, two surgeries may be required, depending on the surgeon’s choice of procedures. In many cats, only one thyroid lobe is abnormal, so only one surgery is needed.  Surgery for hyperthyroidism is becoming less common due to the success of the less invasive radioactive iodine therapy.

3. Medical management. Administration of an oral drug, methimazole, can control the effects of the overactive thyroid gland. Methimazole 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 every three to six months for cats receiving methimazole. This type of treatment is appropriate for the cat that is a poor surgical risk due to other health problems or for the cat that is exceptionally old. It may also be used for a few weeks to stabilize the patient, get some weight gain and assess for other underlying disease prior to going on to another type of treatment. Most hyperthyroid cats can be controlled quite well for months or years on medication, but some cats cannot or will not take methimazole.  Side-effects might include vomiting, lethargy, anorexia, fever, facial swelling or anemia, and these might occur within days or as late as six months after beginning treatment.  Methimazole is available as a pill, but it can also be compounded into an oral solution or a transdermal gel.

4.  Dietary management.  In 2011, Hill’s Pet Nutrition released a Prescription Diet y/d for the control of hyperthyroidism in cats.  Like methimazole, it prevents the production of excess thyroid hormone, but it does it by restriction of the dietary iodine necessary to produce the hormone.  Limiting iodine appears to be a safe and effective way to control hyperthyroidism, but the cat must eat y/d exclusively, without getting so much as a single treat, for the diet to work.  The y/d is a good maintenance diet for most cats, so it is possible to put all cats in a household on y/d in order to gain compliance.  The diet is available in both caned and dry forms.


What is the prognosis for hyperthyroidism?

Many owners of cats with hyperthyroidism are hesitant to have radiation therapy or surgery because of their cat’s advanced age, but 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.  Recurrence of the disease is a possibility in some cats following I131 treatment or surgery, but this occurs in less than 5% of patients.  Cats managed medically or with diet control also usually do very well as long as the protocol is administered routinely and follow-up blood and diagnostic test schedules 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 ten to detect hyperthyroidism before potentially irreversible damage occurs.