Hyperthyroidism In Cats
Cause And Treatment

Ron Hines DVM PhD 4/29/06

Overview Of The Disease:
Hyperthyroidism is the most common hormonal disease of cats that veterinarians see. It most often occurs in older cats, those 7-12 years of age. Male and female cats, neutered and unneutered are all susceptible to the disease. The thyroid gland consists of two lobes located at the base of the neck and in hyperthyroidism; usually both lobes of the gland are affected. These glands produce the hormone, thyroxine, which governs rate of metabolism. The more hormone that is produced, the higher the metabolic rate and the more calories the cat burns as energy. Excess thyroxine affects every organ in the cat’s body. Muscles, heart, stomach and intestines, urinary and nervous system are all affected. The disease will also change your cat’s behavior. Complications resulting from hyperthyroidism include heart enlargement, increased blood pressure, kidney damage and intestinal problems. The most common cause of hyperthyroidism is the presence of hormone-producing, hyper-functional, nodules within the gland. Much less frequently, a true tumor of the thyroid gland is the cause of the disease.

Signs Of The Disease:
The most common reason for which hyperthyroid cats are brought to me is weight loss. These cats all remind me of the pink panther. They are lean in the extreme and often have stopped grooming. Owners report that the cat is hyperactive and has a ravenous appetite despite remaining thin. They also tend to drink and urinate excessively. Other common complaints are vomiting, diarrhea and personality changes. When I examine these cats I often detect a heart murmur, rapid heart rate, and scarred shrunken kidneys. Often the owners report that the cat has lived on a diet of fish-flavored cat foods` or human canned tuna. In some cases, I can feel the enlarged gland under the neck.

Diagnosing Thyroid Disease:
Luckily, thyroid disease can be diagnosed by a simple, inexpensive blood test. Because kidney, liver and certain cancers can have similar signs to hypothyroidism, I do a complete blood chemistry analysis of all organs on cats that I suspect may have the disease. The values within this panel that detect the disease are T-4, Free T-4 and T-3. These tests measure the amount of thyroid hormones present in the blood. Normal cats have values of 0.8-4.0 micrograms/deciliter, 10-50 picograms/leter and 40-150 nanograms/decileter respectively. Hyperthyroid cats have levels of T-4 as high as three times these amounts. In addition many hypothyroid cats have elevated kidney function tests and liver enzymes. I also x-ray the cat’s heart for evidence of enlargement.

Treatment:
Hyperthyroidism is treated with medications, surgery to remove portions of the gland, or radioactive iodine therapy. Cats in which the disease has been present a long time may not respond as well as early cases. The most effective form of treatment is destruction of abnormal thyroid tissue with injection of radio-iodine, a radioactive element. When successful, enough dormant cells are left in the thyroid to supply the proper amount of thyroxine while the cells that secreted abnormally high levels of thyroxine are destroyed. Less than 5% of these cats will need thyroid hormone replacement after the procedure. The drawbacks of this procedure is its high cost and the fact that only select veterinary centers have the equipment necessary to perform the procedure.
The next most effective treatment is surgical removal of the diseased portions of the thyroid gland. During surgery, it is a difficult decision as to how much thyroid to remove. Too much and the cat will need thyroid hormone replacement for the rest of it’s life. Too little and the disease will reoccur. One must be careful in doing this surgery not to damage the adjoining parathyroid glands or laryngeal nerves. Many veterinarians premedicate their patients for a few weeks with methimazole prior to surgery to lessen surgical risk
The third option is to control the disease is with a drug that inhibits thyroid hormone production. This drug is methimazole (Tapazole). I give most cats 2.5mg twice a day for two weeks at which time I do another blood test. I increase the dose to 5-10mg twice a day if too much thyroid hormone is still present and recheck their thyroxine levels two weeks later. Once I have determined the right amount of medication to give, I check blood thyroxine levels every three months. I am not enthusiastic about the results of this drug. Too many owners report that their cat stopped eating entirely on the medication. Vomiting and lethargy on Tapazole are also common. It is very difficult to get cats to take this bitter pill. Another major problem I have in treating these cats medically is the emergence of hidden kidney disease during treatment. Although hyperthyroidism is a very serious condition, failing kidneys is even more serious. Unfortunately, many of my clients are left unable to afford referral for radio-iodine treatment and yet unsatisfied with methimazole administration. Cost problems of this nature are a good reason to purchase pet health insurance.

Hyperthyroidism in cats is often the cause of elevated blood pressure or hypertension. (systolic blood pressure exceeds 160 mm Hg). When this occurs, smaller blood vessels may burst under increased pressure, particularly vessels within the retina of the eye. This can lead to retinal detachment and sudden blindness. To reduce the blood pressure anti-hypertensive drugs should be used. A number of agents called calcium channel blockers are available for treatment of hypertension although many of these are not specifically licensed for cats. Examples of two drugs commonly used are benazepril ( Fortekor®, Lotensin, 0.125-0.25 mg/lb once daily ) and amlodipine ( Istin®, NORVASC, 0.09 +/- 0.015 mg/lb once daily).

Prevention:
Statistically it has been found that cats fed a diet of fish-based cat food develop more hyperthyroidism and other glandular ailments. This may be due to rancid polyunsaturated fats found in poorer animal-quality fish products. The feeding of cat foods in pull-top cans seems to also be associated with this disease in some inexplicable way. A always suggest that cats on fish diets be weaned off of them and , when that is not possible, that they be given daily supplemental doses of vitamin E and B-1 (thiamine).

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