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Feline Hyperthyroidism
CLINICAL PICTURE OF FELINE HYPERTHYROIDISM
There are many clinical signs associated with feline hyperthyroidism. The hallmark sign is:
* weight loss despite excellent appetite.
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.
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.
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.
MEDICATION FOR HYPERTHYROIDISM
The most common medication prescribed to treat feline hyperthyroidism is called METHIMAZOLE (TRADE NAME TAPAZOLE).
All these medications block the production of T4 and T3. It is important to understand what the potential side effects of methimazole and its relatives are and the monitoring that is generally recommended.
ADVANTAGES OF USING METHIMAZOLE/TAPAZOLE:
- Medication is inexpensive relative to radiotherapy or surgery.
- Control of thyroid disease is achieved only while the pet is on medication so that if there is any problem with exacerbation of poor kidney function, treatment can be discontinued.
- No hospitalization is required.
- Side effects are relatively uncommon.
- If an occasional dose is skipped, no harm is done.
- If after the first 3 months of therapy no side effects have been encountered, the chance of side effects occurring thereafter is substantially reduced.
DISADVANTAGES OF USING METHIMAZOLE/TAPAZOLE:
- Medication must be given at least daily (usually twice daily).
- Approximately 15% of cats will experience some kind of side effect. The usual side effects are: lethargy, loss of appetite, and vomiting. If one of these side effects occurs, medication is discontinued until the symptoms resolve. Medication is then restarted at a lower dose and gradually increased to the former dose. These side effects do not generally recur if medication is increased gradually in this way.
- Facial itching is a more serious side effect. This side effect also resolves with anti-itch medication and discontinuation of methimazole. Cats who have this side effect can be expected to have it again if medication is restarted and another form of treatment should be instituted. Facial itching occurs in less than 4% of cats on methimazole.
- Serious liver failure results in a extremely small number (less than 4%) of cats taking methimazole. This toxicity can be expected to resolve after discontinuation of the medication but, again, alternative therapy should be considered.
- Bone marrow changes can also result from methimazole administration. Blood tests evaluating white blood cell patterns should be periodically performed to monitor for these changes. This side effect occurs in less than 4% of cats on methimazole and necessitates a change in therapy.
MOST SIDE EFFECTS OCCUR DURING THE FIRST 3 MONTHS OF METHIMAZOLE THERAPY. Periodic blood testing to examine T4 level, white blood cell patterns, kidney function, and liver enzymes should be periodically performed. Be sure to ask your veterinarian to review an appropriate schedule for your cat.
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