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Feline Hyperthyroidism: Symptoms & Treatments

February 17, 2010 | By FelinePine | Category: Health & Wellness

By Dr. Michele Gaspar DVM, DABVP (feline)

Maybe one of your older cats has seemed to have a “new lease of life.” He or she may be more active – even acting like a kitten – eating well, “chattier” than normal and perhaps losing some of that extra weight. Chances are that your kitty hasn’t found the feline “Fountain of Youth.” You may be living with a cat who has hyperthyroidism, the most common endocrine (glandular) disease of cats over seven years of age in the United States. The thyroid gland is shaped like a bow tie and lies on the trachea (windpipe). Thyroid hormone (thyroxine) helps to regulate many bodily functions, including, the metabolic rate, fat metabolism, the excitability of the heart, gastrointestinal tract motility and muscle function. When thyroxine is in excess, several things can occur: Fat stores decline (and the patient becomes thin), the heart rate increases, the appetite increases and the motility of the gastrointestinal tract speeds up. Without treatment, the typical hyperthyroid cat is a very active, thin kitty with an excellent appetite, a very fast heart rate and often diarrhea or vomiting. Many cat guardians mistakenly think that cats with these signs aren’t ill (in fact, many are thrilled that their once-sedate older friend is now acting like a youngster) and don’t take them to the veterinarian for an examination and labwork. That’s a mistake, as uncontrolled hyperthyroidism can have serious consequences, including high blood pressure, stroke and heart failure.

Veterinarians who are presented with a kitty who is suspected to have hyperthyroidism will take a detailed history and conduct a thorough physical examination. Most cats with hyperthyroidism will have enlargement of one – or both – thyroid glands. As the veterinarian carefully palpates alongside the trachea with his or her thumb and first finger, the enlarged gland will often “pop” as the fingers gently pass over it. Cats also can have thyroid tissue anywhere from the base of the tongue to the base of the heart. Not all cats with enlarged thyroid glands are hyperthyroid. For the overwhelming majority of hyperthyroid cats, the thyroid gland enlargements are benign; only a small percentage of cats have malignant thyroid tumors.

Hyperthyroidism is diagnosed on the basis of the clinical signs, a physical examination and labwork. Cats with hyperthyroidism most often have an elevated thyroid hormone (TT4) level and may also have elevations of liver enzymes. If your kitty has another disease (including kidney disease or gastrointestinal disease, for example), the TT4 may not be elevated and can be at the high end of the normal range. Additional lab tests and a radioisotope study may be necessary to definitively diagnose hyperthyroidism.


There are three treatments for hyperthyroidism:

1. Drug therapy (methimazole) allows us to control the cat’s hyperthyroidism, but does not provide a cure for the disease. Hyperthyroid cats generally begin methimazole twice daily.

2. For cats who cannot be pilled, the veterinarian can prescribe a methimazole cream that is rubbed on the inside of the kitty’s ears. This transdermal formulation allows patients to be treated properly and avoids the cat guardian having to chase the kitty twice daily for administration of tablets. Transdermal methimazole is available through compounding pharmacies and is generally not readily available through local pharmacies, as the tablets are.

Treatment with either methimazole tablets or the transdermal formulation is life-long and cannot be stopped. Often, cats require higher doses of the medication over time.

Methimazole therapy can cause serious side effects, including elevation of liver enzymes, low white blood cell counts, low platelet counts (these are the cells that help the blood to clot), itchiness of the face that can lead to self-trauma, and gastrointestinal problems, including vomiting and loss of appetite. If these signs occur, the medication has to be discontinued and other treatments pursued. Generally the side effects occur in the first 90 days of methimazole therapy.

3. Cats can be cured of hyperthyroidism through surgery or radioactive iodine therapy. Surgical removal of the affected part(s) of the thyroid is possible, although many hyperthyroid cats are not excellent surgical candidates, due to their high heart rates. Thyroidectomy (removal of the affected thyroid gland) should only be done by an experienced surgeon, as there are potential serious complications with the procedure, including the inadvertent removal of the parathyroid glands, which control calcium metabolism.

The “gold standard” of treatment for hyperthyroidism is I-131 therapy. This curative treatment involves the administration of one sub-cutaneous (under-the-skin) injection. Because this is a radioactive compound, the kitty is required to remain in the hospital for a specified period of time (generally 5-7 days). The cat is then released to his or her guardian with some requirements (no sleeping on the bed with his or her human for a specific period of time, avoidance of very young children, etc.) Because I-131 therapy requires certain requirements for the facilities where it is offered, it is generally found in veterinary referral centers, veterinary medical teaching hospitals affiliated with veterinary schools and specialized hospitals that only offer this therapy.

Cat guardians who opt for surgery or I-131 therapy for their cats will typically have to administer methimazole for a period of time (generally several months) prior to each of these therapies. The reason for this is that it is important for the veterinarian to assess how well the kitty’s kidneys handle the decrease in thyroid hormone. When a cat is hyperthyroid, there is increased blood flow to all organs, including the kidneys. This increased blood flow can, ironically, benefit the kidneys, and in the case of cats with underlying kidney disease, can result in falsely normal values for the BUN and creatinine (two blood tests that are commonly assessed to check on kidney health). When the excess thyroid hormone is treated, the “hidden” kidney disease can be “unmasked.”

For this reason, cats on methimazole are generally checked every three weeks for the first three months of treatment with labwork consisting of a complete blood count (CBC), urinalysis, chemistry profile, platelet count, TT4 and blood pressure check. After the initial three month period, the kitty is check periodically (generally every 4-6 months, depending on his or her general health).

The prognosis for cats with hyperthyroidism who are treated appropriately is generally very good and most respond well with appropriate weight gain, reduced heart rates and a normal appetite.


What causes hyperthyroidism?
Over the years, a number of possible causes for this disease have been postulated. At present, there is increased interest in the possible link between flame retardants used in furniture and carpeting and hyperthyroidism in cats. Another possible link is the white film coating present on many pop-top cans, including those used for cat foods.

As veterinarians and cat guardians are becoming more proactive with laboratory testing at wellness visits, hyperthyroidism is being diagnosed earlier in many cats. In fact, it’s possible for cats with normal weights (even obese ones) to have early hyperthyroidism. Prompt treatment of hyperthyroidism results in a kitty who doesn’t have to live with a racing heart and gastrointestinal upset and enjoys every single day.
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