Hyperthyroidism is a common problem in older cats. Also known as
hyperthyroid disease, it is caused by the enlarging of the thyroid gland and the increased production of
thyroid hormones. Usually, this is caused by a benign tumor on one or both of the glands. Though these
tumors can be cancerous, the chance of a tumor being malignant is very low: 2-5%.
Signs and Symptoms
Cats very rarely show all the symptoms of hyperthyroidism, but one or two of the symptoms may be severe enough
to warrant a trip to the vet. Cats with hyperthyroidism often have an increased appetite but are losing weight.
Some may become irritable or nervous. It is not uncommon for cats with hyperthyroidism to look as though they
haven’t groomed themselves in a while. Vomiting, diarrhea, and weakness are also common, but any cat showing
these symptoms for more than 24 hours should see a Vet. Never allow your cat to vomit for more than 24 hours as
this can cause severe dehydration and be a sign of an even more serious problem.
Diagnosis and Treatment
Your vet will perform a physical exam, palpitating the neck to feel for the enlarged gland or glands. They will
check heart rate and blood pressure as well and perform a CBC, a type of blood test that will check for a
variety of things including the thyroid levels. Hyperthyroidism can mimic other diseases such as chronic renal
failure (CRF) so it is important that this test be performed. The CBC will also show other system functions
which are important when choosing a method of treatment.
There are three common treatments for cats with hyperthyroidism: Medication, surgery, and radiation. Each of
them have pros and cons, but hyperthyroidism is not a life threatening illness, so caregivers have some time to
make the decision on which treatment is best.
Medication is the most common and financially easiest option. The most commonly prescribed
drug is methimazole which is often prescribed once or twice a day for the remainder of your cats life. They
should receive regular blood checks and exams to monitor hormone levels and check that your cat is not
suffering from medication related side effects. If your cat has kidney or heart problems, this is the only
treatment option for them, it is relatively inexpensive, and it is non-invasive. Side effects can include
vomiting, fever, anemia, and lethary. More serious side effects include a rash, liver damage and bone marrow
suppression though these are rare.
Surgery is often effective but is best performed by a specialist in the field. It is often the choice when only
one gland is involved and scans are usually performed before hand to check the extent of the diseased tissue
and to locate any thyroid tissue growing elsewhere that may exclude surgery as an option. It is absolutely
necessary that a exam and work up be done to exclude hypertrophic cardiomyopathy- a condition commonly seen
with hyperthyroidism. Cats must also remain on thyroid medication for 15 days prior to the surgery so that
kidney function can be retested as hyperthyroidism can often mask renal failure. Surgery eliminates the need
for medication, however it comes with risks. It is possible to damage the parathyroid gland, can cause
hypothyroidism, is certainly more expensive, and comes with all the inherent risks of placing an older cat
under general anesthetic.
Radioiodine treatment is a relatively new treatment choice, though it appears to be
gaining in popularity where available and where owners can afford to choose it. It involves a single, under
the skin (or subcutaneous) injection of radioactive iodine (R131). This radioactive iodine then seeks out
and destroys all the diseased tissue without harming normal tissue. The cat must remain in the hospital for
up to two weeks, depending on state law, until its radioactive levels are acceptable. Owners are able to
visit, but will only be able to view their pet through a lead window. However, for this small inconvenience
and separation, pets and owners are often provided with a permanent cure and no serious side effects. Cats
with hypertrophic cardiomyopathy, kidney problems, diabetes or other serious conditions are not eligible for
this form of treatment. It is also nearly as expensive as surgery and can also cause hypothyroidism.
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