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The thyroid gland is located in the neck and plays an 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.
Clinical Signs
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. 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 a lot. There may be periodic vomiting or diarrhea, frequent bowel movements, restlessness, frequent crying or vocalizing, rapid heart beat, and the hair coat may be unkempt. In some cats, anorexia develops as the disease progresses. These cats usually have increased energy.
Two secondary complications of this disease can be significant. These include hypertension (high blood pressure) and a heart disease called thyrotoxic cardiomyopathy.
Hypertension develops as a consequence of the increased pumping by the heart. In some cats, blood pressure can become so high that retinal hemorrhage or detachment will occur and result in blindness. The heart problems develop because the heart must enlarge and thicken to meet the increased metabolic demands. Both of these problems are reversible with energy and appropriate treatment of the disease.
Diagnosis
In most instances, diagnosis of this disease is relatively straightforward. The first step is to determine, by feeling, if the thyroid is enlarged, this is them verified by performing a blood level test of one of the thyroid hormones, called thyroxine (or T4). Usually, the thyroid is enlarged in size and the T4 level is elevated so there is no question as to the diagnosis. Occasionally, a cat suspected of having hyperthyroidism will have a normal T4 level. When this occurs, a second test, either a T3 Suppression test or a TRH Stimulation 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.
Treatment Options
Because less than 2% of these cats have cancerous growths of the thyroid gland, treatment is usually very successful. There are three 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. When possible, tests are done before adopting any form of treatment. These tests are
needed to evaluate the overall health of the cat and predict the chances for complications. Such tests include blood tests, urinalysis, and x-rays, and if available, EKG, blood pressure determination, and cardiac ultrasound can be performed.
Oral Medications
Administration of an oral drug, methimazole, can control the effects of the overactive thyroid gland. Some cats have reactions to the drug but that number is fairly small (less than 20%). However, the side effects may begin as late as six months after the beginning of treatment and can include vomiting, lethargy, anorexia, bone marrow suppression, fever and anemia. Methimazole does not destroy the abnormal thyroid tissue but rather prevents the production of excess thyroid hormone. 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. This type of treatment is appropriate for the cat who is a poor surgical risk due to other health problems, is exceptionally old, when limited finances are available, or when other options are not available.
Cats will need to be closely monitored at first when taking this medication. Owners can expect monthly examinations, CBC, and thyroid testing for the first three to four months of therapy. Maintenance testing will involve examination, CBC, and thyroid testing every six to twelve months.
SurgerySurgical removal of the affected thyroid lobe(s) is also very effective. Because hyperthyroid cats are usually over 8 years of age, 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. If surgery is the treatment method chosen, the cat is usually treated with an anti-thyroid medication for several weeks prior to the operation. During that time, the ravenous appetite should subside and the cat will probably gain weight. Some cats also have a very fast heart rate and high blood pressure; these problems can be managed with medication before surgery. The T4 level is measured on weekly or biweekly intervals until the cat is ready for surgery. Thyroid hormone levels are then tested after surgery.
Radioactive Therapy
A very effective way to treat hyperthyroidism is with radioactive iodine therapy. This is the ideal method of treatment with minimal risk. It is given by injection and destroys all abnormal thyroid tissue without endangering other organs. Treatment requires one or two weeks of hospitalization at a veterinary hospital licensed to administer radiation therapy. This treatment is often limited to veterinary teaching institutions and referral centers because of government regulations regarding radioactive materials. There is a specialist in Houston who does this treatment.
Recurrence of the disease is a possibility in some cats. It is uncommon after radioactive iodine therapy. When surgery is done, recurrence is possible if abnormal thyroid cells are left in the cat. Remaining cells can grow causing the disease to recur. However, this occurs less than 5% of the time and usually after 2-4 years. Another possibility for disease recurrence is that one lobe of the thyroid gland was normal at the time of surgery so it was not removed. Then, months or years later, it became abnormal.
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 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.©This information sheet was donated by Claws and Paws Veterinary Hospital. To find out more about this hospital click here
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