Parkside Animal Hospital
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FELINE HYPERTHYROIDISM
by Kaye Wigginton, D.V.M.
published in the Northeast Times This disease is one of the most common endocrine (hormone) disorders that affect cats. Generally, the affected cat is middle-aged to older. The median age has been reported to be between 12 and 13 years old. Some cats as young as 4 years old have been diagnosed (this is very uncommon). Screening of seemingly healthy cats usually begins around 8 years old as part of their geriatric workup, or as recommended by your veterinarian. The incidence of this disease has dramatically increased in the last two decades. The reason for this could be diet related, environmental, or immunologic.
Thyroid Gland – What is it?
The thyroid gland is small and is made up of two lobes; one lobe sits on each side of the trachea (windpipe) in the neck. The gland produces two hormones: thyroxine (T4) and triiodothyronine (T3). These hormones are responsible for regulating the body’s metabolic rate and therefore affect every system in the body.
What is hyperthyroidism?
This occurs when the thyroid gland becomes overactive and produces excessive amounts of the thyroid hormones. The most common cause is due to a benign (non-cancerous) tumor called an adenoma. In about 1-2% of the cases, the tumor is malignant (cancerous).
What are some of the clinical signs of hyperthyroidism?
There are a variety of signs associated with this disease. The signs that you, the owner, may notice are: weight loss, increased appetite and food consumption, increased water intake, increased urination, increased activity, behavior changes, increased vocalization, unkempt hair coat (may appear greasy or matted), hair loss, vomiting, diarrhea, weakness, panting, difficulty breathing, loss of appetite, decreased activity. Your cat could have one, some or all of these signs during the course of this disease. Some other signs that your veterinarian may find on physical examination are increased heart rate, heart murmur and high blood pressure (hypertension). Hyperthyroid cats who are not treated can develop hypertrophic cardiomyopathy (a condition in which the heart muscle becomes excessively thick) and this will lead to heart failure and death. Also, if they have hypertension, it can cause sudden blindness due to detachment of the retinas.
How is hyperthyroidism diagnosed?
It is important to know that the clinical signs listed above can be associated with many other diseases and disorders besides hyperthyroidism. This is why a complete assessment by your veterinarian is so important. Your veterinarian will begin with a full physical exam including, but not limited to, palpation of the neck to check for enlarged glands (even if these can’t be felt, it does not rule out the disease), checking body weight, body temperature, heart rate, and possibly blood pressure and eye pressure assessments as well. If hyperthyroidism is suspected, then a CBC (complete blood count), chemistry panel, urinalysis and thyroid hormone testing will be ordered. Most hyperthyroid cats will have an elevated T4 level. A small percentage of affected cats will have normal T4 levels and if your veterinarian still suspects your cat has the disease, he or she may recommend other tests to confirm or rule out the diagnosis. The CBC, chemistry panel and urinalysis will let your veterinarian know if there are other diseases present and they also can help determine which treatment option is best for your cat. If concurrent heart disease is suspected, then chest radiographs (x-rays), EKG, and an ultrasound study of the heart may also be recommended.
How is hyperthyroidism treated?
In this section, we will limit our discussion to the actual treatment of the hyperthyroidism. Keep in mind that if there are any concurrent problems, such as heart disease, hypertension, kidney and liver disease that they may also need to be treated.
Currently, there are three options for treatment.
candidate. This can be curative unless all the abnormal tissue is not removed. Serious complications could include damage to the parathyroid gland (an extremely important gland located in close proximity to the thyroid gland and whose sole function is to maintain calcium levels within a very narrow range so that the muscular and nervous systems can function normally) or the nerves located in the area. This is not an option if thyroid tissue is also in the chest cavity (called ectopic thyroid). Follow up calcium levels are required to determine if the parathyroid gland was damaged or inadvertently removed during the surgery.
that is given daily. This is not a cure and the tumor (adenoma) will continue to grow. The medication may need to be given once to three times daily for life. This may be difficult for some patients and their owners. There may be side effects that the cat may not be able to tolerate and periodic blood work is required to determine the T4 values so that dosage can be adjusted as needed. Over time the cost of this drug and the follow up blood work will be close to the one time radioactive iodine treatment. This option is considered the preferred one in cats who have other complicating conditions such as kidney failure. This is a great drug to be given short term to stabilize a cat prior to the radioactive iodine treatment or surgery.
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