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Hyperthyroidism is one of the most common diseases diagnosed in
older cats. It is caused by a growth of the thyroid gland, which
is completely benign in the vast majority of cases, but causes the
thyroid gland to produce more thyroid hormone than usual. Thyroid
hormone has effects all over the body, and causes a general
increase in metabolism. The clinical signs of hyperthyroidism are
therefore many and varied, and can include effects on the heart,
gut and urinary tract.
The most common signs an owner will see in an affected cat
are weight loss, increased appetite, drinking and
urinating more, diarrhoea and vomiting. Not all cats will
necessarily show any of these signs, however, and a definitive
diagnosis is important. Diagnosis is usually with a simple
blood test, but occasionally more thorough testing may be
Hyperthyroidism is the most treatable disease of older cats, and
most cats have an excellent response to therapy. The three main
treatment options are medication, surgery and radioactive
1.Oral medications: Most cats will start treatment with
tablets (usually twice daily), and some cats will continue these
for life. Whilst on tablets, cats should be checked regularly to
ensure that they are on the correct dose.
Very rarely, a cat may have an adverse reaction to these tablets,
which is reversible when the medication is stopped.
2. Radioactive Iodine treatment: Radioactive iodine (I-131)
is a simple, effective, safe and permanent treatment for cats with
hyperthyroidism. An appropriate dose of radioactive iodine is
injected (or given orally), and destroys the abnormal tissue in
the thyroid gland. Your cat must remain in a referral hospital for
about a week after the dose is given and you should minimise
contact with your cat for a few weeks after coming home (as there
is often still some minimal radioactivity). No further medication
is required after the treatment is performed.
A recheck 2 weeks after your cat goes home is recommended to check
body weight, heart rate and general condition, then a blood test
to check thyroid levels is performed 6 weeks after treatment.
This is a permanent treatment, which involves the surgical removal
of the thyroid gland. If both thyroid glands need removal, then
it is recommended to perform this operation in two stages, 6 weeks
apart. Again, rechecks at 2 and 6 weeks are recommended. If only
one gland is involved, there is a chance the other thyroid gland
may require removal in the future.
There are many factors involved in choosing the right treatment
option in each case, and close consultation with your vet is vital
for the best decision for you and your cat.
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