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Hyperthyroidism was
first recognized in cats in 1979 and is now one of the most
commonly diagnosed disorders of older cats. The cause of this
condition is still unknown and is likely to be multifactorial. A
recent study has suggested a relationship to environmental
contaminants. Feline hyperthyroid disease is comparable to toxic
nodular goiter of people. Thyroid glands are located beside the
trachea produce hormones which control the body’s metabolic rate. In
most cats (98%), overproduction is the result of abnormal increase in
cells in the thyroid tissue. Malignant thyroid tumors in cats are
rare. The cat’s metabolic rate increases when the thyroid manufactures
excessive thyroid hormones. This affects many organs
including the kidneys, liver, heart, and brain. Hyperthyroidism is a
progressive disease that, if left untreated, will be fatal. The good
news is that it is a treatable disease and most cats tolerate
treatment very well.
Clinical Signs
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Diagnosis
The appearance of the clinical signs listed above and /or the presence
of enlarged thyroid glands are indications that the patient may be
hyperthyroid. Blood tests are then needed to confirm the diagnosis.
Occasionally a cat with obvious signs of hyperthyroidism will not have
elevated thyroid hormone levels. In such cases, additional testing may
be needed.
Treatment
Spontaneous remission of hypothyroidism does not occur. Failure
to institute therapy will result in insidious progression to
emaciation, severe metabolic and cardiac dysfunction and ultimately
death.
There are currently three treatment options: drug therapy, surgery,
and radioactive iodine. The most common treatment is the
administration of anti- thyroid medication (methimazole: Tapazole
®
or generic). This medication
concentrates in the thyroid gland and inhibit the synthesis of thyroid
hormones. When the medication is not given for a day the thyroid level
will rise and clinical signs will return. This is the best
treatment for cats with pre-existing kidney disease.
Hyperthyroid
disease increases the blood flow to the kidneys and delays the
clinical signs of kidney failure. Kidney disease can be revealed when
the level of thyroid hormone returns to normal with any type of
treatment. When the treatment is oral medication it can be stopped or
reduced as needed to manage kidney disease. These medications have no
effect on the underlying pathologic lesion so the thyroid nodule will
continue to grow and the dose of medication commonly increases.
Medication is given daily and blood samples are drawn every 1-6 months
to evaluate and adjust therapy. Up to 25% of patients receiving this medication
will develop some adverse reactions. For these patients and others
choosing not to give daily medication, other treatment options should
be explored. Radioactive iodine in an excellent treatment choice for
many cats and in some cases surgery may be considered. Hyperthyroid
disease responds well to therapy and these patients can live for many
years.By:
Annette Cowell, DVM
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