The Granby Animal Clinic, Inc.
The thyroid gland is located in the neck and plays a very important role in regulating the body's rate of metabolism. 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 non-malignant change (benign). Less than 2% of hyperthyroid cases involve malignancy.
Many organs are affected by this disease, including the heart. The heart is stimulated to pump faster and more forcefully; eventually, the heart muscle enlarges to meet the increased demand for blood flow. The increased pumping pressure leads to a greater output of blood and high blood pressure. About 25% of cats with hyperthyroidism have hypertension.
A cat is at increased risk for hyperthyroidism with advancing age. Environmental and dietary risk factors have been investigated and may play a role in predisposing cats to hyperthyroidism, though the specific mechanisms are not known. Siamese appears to have a somewhat increased risk of developing hyperthyroidism than other breeds.
The typical cat with hyperthyroidism is middle aged or older; on the average, affected cats are about 12 years of age. The most consistent finding with this disorder 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 lose weight. The weight loss may be so gradual that some owners will not even realize it has occurred or it may be quite rapid. Affected cats often drink a lot of water and urinate more. They may also have periodic vomiting or diarrhea, and their hair coat may be unkempt. In some cats, anorexia develops as the disease progresses.
Two secondary complications of this disease can be significant. These include hypertension and a heart disease called thyrotoxic cardiomyopathy. Hypertension develops as a consequence of the increased pumping pressure of the heart. In some cats, blood pressure can become so high that retinal hemorrhage or detachment will occur resulting in blindness. Heart problems develop because the heart must enlarge and thicken to meet the increased metabolic demands. Both of these problems are reversible with appropriate treatment of the disease.
In most instances, diagnosis of hyperthyroidism is relatively straight forward.
The first step is to determine the blood level of one of the thyroid hormones, called thyroxine (or T44). Usually, the T4 level is elevated and 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 cats. When this occurs, other tests such a free T4 or a T3 Suppression Test, is performed. If this is not diagnostic, a thyroid scan can be performed at a veterinary referral center or the T4 can be measured again in a few weeks.
Because less than 2% of hyperthyroid cats have cancerous growths of the thyroid gland, treatment is usually very successful. Since there are multiple choices for treatment, many factors must come into consideration when choosing the best therapy for an individual cat.
Before deciding on a treatment option, the overall health of the cat needs to be assessed with particular attention paid to kidney and heart function. In addition to a serum chemistry analysis, complete blood count and urinalysis, blood pressure should be checked and if significant heart disease is suspected radiographs, echocardiogram and an EKG may be indicated.
Treatment options for hyperthyroidism include:
Radioactive iodine therapy is a very effective way to treat hyperthyroidism.
Radioactive iodine is given by injection and destroys all abnormal thyroid tissue without endangering other organs. Treatment requires several days of hospitalization at a veterinary hospital licensed to administer radioisotope therapy. If the cat is healthy and has good kidney function, this is considered the preferred therapy since it returns the cat to a euthyroid (normal thyroid function) state and additional medication is rarely needed.
Surgical removal of the affected thyroid lobe(s) is also very effective. Because
hyperthyroid cats often have underlying cardiac disease, it is necessary to use anti-
thyroid medications to control the disease prior to surgery. If the disease involves
both lobes of the thyroid gland, two surgeries may be required. In cats with unilateral
thyroid enlargement, only the one gland is removed, but the cat should be monitored
for recurrence of thyroid disease.
Oral medication can also be used. Administration of an oral drug, methimazole,
can control the effects of the overactive thyroid gland by blocking the formation of the
thyroid hormone. The medication does not cure the disease and must be given to the
cat for the remainder of its life. Over time the dose of methimazole often has to be
increased. Most cats tolerate the mediation well, however side-effects are possible.
Potential side-effects include vomiting, lethargy, anorexia, fever, anemia,
facial pruritus and hepatitis. Periodic blood tests must be done to keep the dosage
regulated and monitor for possible side-effects. Oral medication is sometimes used
disease that is potentially masked by the hyperthyroidism.
Dietary therapy with Hill’s prescription diet y/d, a specially formulated diet for
hyperthyroidism, can also be used to control the disease. If fed exclusively, most cats
will return to a euthyroid state. The down fall of the diet is that cats cannot eat
anything else. In addition, the diet is not very high in protein and some cats are
unable to maintain or regain adequate muscle mass on the diet.
Many owners of cats with hyperthyroidism are hesitant to have radiation therapy or surgery because of their cat's advanced age. But remember, old age is not a disease. The outcome following any of the treatment options is usually excellent. Most cats have a very good chance of returning to a healthy state and can live many years after diagnosis.
There are no know measures to prevent hyperthyroidism, but early detection can help avoid complications of the disease. Middle-aged and geriatric cats should all receive a complete physical examination by a veterinarian every 6-12 months. Special attention should be given to thyroid enlargement and the typical clinical signs of hyperthyroidism.
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