-----Hyperthyroidism
River Road Veterinary Clinic
Box 309
Norwich, Vermont 05055
Phone: 802-649-3877 Fax: 802-649-1345
email: RRVC@Valley.net.com
www.rrvetclinic.com
Dr. Christine Pinello - Dr. JoAnne Giel - Dr. Kathy Ling Newcomb
HYPERTHYROIDISM
Hyperthyroidism is the most commonly diagnosed endocrine disease of cats. It is most commonly seen in cats 8 years of age or older.
Clinical signs associated with hyperthyroidism include weight loss with normal or increased appetite, tachycardia (increased heart rate), vomiting, and poor hair coat. Diarrhea, polyuria (increased frequency of urination), and polydipsia (increased water intake) may also be noted. A palpable thyroid nodule may be noted at the thoracic inlet during physical exam, and is present in 90% of hyperthyroid cats. While less common, cats may present for weakness, lethargy, anorexia (not eating), or GI disturbances.
Cats with untreated hyperthyroidism may develop a hypertrophic
or, less frequently, dilated cardiomyopathy. (In hypertrophic cardiomyopathy
the heart muscle becomes thickened, leaving little room for blood in
the chambers. A dilated heart has thin, weak walls.) A heart murmur
or gallop rhythm may be noted in these cats on physical exam. Hypertrophic
cardiomyopathy may resolve after correction of hyperthyroid state
.
It is important to remember that hyperthyroidism is a polysystemic disease,
affecting many body systems and therefore causing many different clinical
signs.
Diagnosis
Any cat suspected of having this disease should have a complete blood
count, chemistry panel, and serum T4 concentration performed. A T4 of
greater than 3.9 on our machine is diagnostic for this disease. Due
to the cyclic secretion of this hormone, a normal or high normal T4
does not rule out this disease in cats with clinical signs or exam findings
consistent with hyperthyroidism.
All cats 8 years old or greater should have a T4 included in their annual senior workup.
Treatment
Hyperthyroidism may be managed with oral antithyroid medications, surgery
or radiation therapy.
All patients require a thorough medical workup prior to treatment with a recheck in 2 weeks. It is important to note that hyperthyroidism may mask kidney failure by increasing blood flow to the kidneys. Successful treatment of the thyroid condition will decrease the metabolic rate and renal blood flow will decrease, potentially sending the animal into renal failure. Clients should be counseled about this and know the clinical signs of renal failure.
Medical Treatment of Hyperthyroidism
Tapazole is the safest, most effective antithyroid drug currently in
use and is available in both pill and chewable form. It works by inhibiting
the formation of thyroid hormone.
Therapy should be staged with gradual increase in dosage and frequent monitoring of patient to minimize side effects and identify dosage necessary to normalize serum T4.
· Weeks 1 to 2:
2.5 mg Tapazole PO SID. Recheck QBC to monitor red cells and platelet
count. Recheck BUN and Creatinine. Monitor patient for side effects
including anorexia, vomiting or lethargy. If no hematologic changes
or adverse reactions are noted dose may be increased as follows.
· Weeks 3 and 4:
Perform serum T4 4 to 6 hours after morning dose. If still elevated,
increase Tapazole to 5 mg PO in morning and 2.5 mg PO in evening. Repeat
QBC. If T4 is still elevated in two weeks, the dose may be increased
every 2 weeks in 2.5 mg/day increments.
If QBC is normal and serum T4 is within or near reference range the animal should remain on this dosage for an additional 4 weeks before recheck of QBC and T4.
Serum T4 levels decline to normal within 1 to 2 weeks after the animal is receiving an adequate dose of Tapazole. Clinical improvement is noted 4 to 6 weeks after initiation of effective dose.
Periodic reevaluation of patient, including QBC and chemistry
should be performed every 3 to 6 months.