Hyperthyroidism in Cats



Zack was long and lean, his black tiger stripes sharply outlined, muscles rippling as he prowled around the exam room. A Domestic Shorthair cat (DSH), he claimed two humans as his own and they were devoted to him. Zack was magnificent in his prime, weighing in at 14 1/2 pounds. As a kitten he liked to chew on electric cords. His owners claimed he got zapped and was “slow” for months but assured me he eventually returned to normal. Since “normal” included enjoying being vacuumed, I always had the sense that Zack liked to live on the edge. He had tons of personality! When Zack was 10 years old he was brought in because he was visiting the litterbox with unusual frequency. In addition to frequent urination he had lost weight, down to 11 pounds. The rest of his examination was not unusual and I suggested we check some lab work, running both blood and urine tests. The initial tests came back very high in glucose (sugar) in both the blood and urine, suggesting Zack might have diabetes. We ran another test, fructosamine, which in a cat tells us how high his blood sugar has been for the last 2-3 weeks. Zack’s test was very high, confirming Diabetes mellitus (sugar diabetes). He was started on daily injections of insulin and responded beautifully. He ate more slowly, urinated only a few times a day, and re-gained his lost weight.


Zack was maintained on this therapy for four years. As he entered his senior years you would never know that he had an affliction. His owners had incorporated his insulin injections into their daily routine and Zack thrived, bossing them pretty badly. When he was 14 years old Zack came in for his annual exam, but things weren’t quite right. He was eating heartily but had dropped in weight again. Was it the diabetes or was he just “getting old”? His muscles had thinned with the atrophy of age and he moved more slowly, perhaps a bit of arthritis? He was a little less black, a little more grey in his tabby markings. Zack was also a whole lot more grumpy about submitting to a blood test!


This time the tests came back positive for Hyperthyroidism. An over-active thyroid gland was producing way too much thyroid hormone (T4). People talk a lot about hypothyroidism, the opposite condition when not enough hormone is produced and the body becomes sluggish, often obese. Hyperthyroidism in cats is the exact opposite. When the cat has too much thyroid hormone circulating in its blood stream the body’s metabolic rate increases. Excess thyroid hormone stimulates the heart to beat faster and may cause enlargement of the heart or even lead to heart failure. High blood pressure (hypertension) can damage the eyes, kidneys, heart and brain. The underlying cause for hyperthyroidism in a cat is a tumor, commonly a thyroid adenoma which is overwhelmingly benign and unlikely to spread anywhere else in the body.


If the tumor in the throat is very large it can sometimes be removed surgically with great success. Because of the disease effects on the heart and kidneys in these older cats the risk of surgery and potential damage to the parathyroid gland that sits side-by-side must be thoroughly considered before surgery is recommended.


Most cats will start a course of methimazole, an oral medication that reduces specifically the production of thyroid hormone. Very effective, it controls the disease but cannot eliminate it. Each cat is monitored via blood tests to maintain their T4 at optimal levels. If the cat has kidney function problems, the T4 will be kept higher because the increased blood pressure is beneficial to perfuse the ageing kidneys. It’s a delicate balance. Once a cat is regulated on medication and the T4 level desirable, an owner may also consider referral to a center for thyroid irradiation. The thyroid gland needs iodine to function. It is easy to attach a radioactive particle to iodine, which is selectively picked up by the thyroid and then destroys the abnormal gland, leaving all other body tissues alone! Woohoo! A cat treated with irradiation will be cured of the tumor and hyperthyroidism! The down side of radiation therapy, admittedly a big one, is that the procedure is expensive, well over $1,000.


The upside for cats is that the oral medication is very effective. Most cats are well controlled with this medication and live out a normal lifespan. Super-Cat Zack is entering his 16th year and is as feisty as ever!


Christine B. McFadden, DVM


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