A calico cat came in for her semi-annual check up. By all reports she was doing well. She was playful and eating like a pig. But, I when I went to pet her, I realized that she had lost a lot of weight. The scale showed a 5-pound weight loss in just 6 months! Calli had always been, well, let's be blunt here, kinda fat.

So, when she suddenly showed up at a good body weight, I knew something was wrong. Her owners were pleased that Calli had finally lost weight, but upon further questioning we realized that she was losing weight even though she was being given more food. And, she had been vomiting more frequently. Her owners thought this was just hairballs, but why all of a sudden?

We decided to run a full panel of blood and urine tests to look for chronic diseases that might not make Calli feel too sick but might cause significant weight loss.

I was worried about things like kidney disease, hyperthyroidism and the big C word - cancer.

Fortunately, everything looked good on Calli's bloodwork except for her thyroid level. It was sky high. So, Calli had a hyperactive thyroid, a very common condition in older cats.

Hyperthyroidism is caused by a benign tumor of the thyroid gland. The thyroid regulates the body's metabolic rate. Overproduction of thyroid hormones by this benign tumor increases metabolic rate, resulting in weight loss, but it also can cause an increase in blood pressure or cause a deadly thickening of the heart muscle.

We set Calli up for a day of testing to see if her blood pressure and heart were still OK. Fortunately, they were, so all we had to do now was to start to bring her thyroid level back into the normal range.

The first step was to start Calli on a daily medication called methimazole. This drug is made for cats in the form of a chewable tablet. Unfortunately, Calli didn't like the pill, no matter how hard we tried to disguise it.

But, not to worry; I called one of our local compounding pharmacies and had the methimazole made into a gel that the owners could simply rub on the inside of Calli's ear twice a day. Using this method to give methimazole isn't FDA studied and approved, but it does seem to work for most cats and it is certainly better than not medicating at all.

After three weeks of applying the gel twice a day we rechecked Calli's weight and blood work. She had gained half a pound and seemed to be feeling fine, but she was starting to resist the twice a day handling required to apply the gel. Calli liked to be stroked and handled, but only on her own terms. I knew that, as soon as possible, we needed a permanent solution to Calli's overactive thyroid.

Her bloodwork revealed that her thyroid level was back into the normal range and her kidney values looked good. Rechecking her kidney values was important because the higher blood pressures associated with hyperthyroidism can make poorly functioning kidneys appear to be working adequately.

When the thyroid level is suddenly brought back down, sometimes an underlying, old age kidney problem might show up.

But, this wasn't the case with Calli, probably because we caught her problem before her blood pressure was affected. This meant that Calli was a good candidate for radioactive iodine therapy to permanently destroy the benign tumor in her thyroid gland.

Only a few hospitals in South Carolina are set up to do the radioactive iodine treatments because the drug is so radioactive that significant monitoring by government agencies is required.

Fortunately the Animal Medical Center in Mount Pleasant has the facility and licensing to use radioactive iodine. We got Calli scheduled for her treatment. She went in on a Monday when she received her radioactive iodine intravenously.

Because the thyroid gland uses iodine to make thyroid hormone, the overactive thyroid-producing tumor takes up most of the radioactive iodine and the tumor cells are killed.

Animal Medical Center took care of Calli in a radiation-proof room for a few days until radiation levels in her urine (where most of the radioactive iodine is excreted) were low enough for her to go home. Her owners still had to wear gloves when they scooped her litter box for a few more days, but things were back to normal in no time.

Calli no longer has to take medication and her thyroid levels have stayed in the normal range. She has regained a little weight, but her owners are working hard to keep her from getting fat again.

Calli had a happy ending because her problem was caught early. Calli is a good reminder that if your cat is over 10 years old, seeing your veterinarian every six months can make the difference between simple solutions and severe complications.

(Dr. Katherine A. Saenger, DVM, owns Bees Ferry Veterinary Hospital and Boarding Kennel (www.beesferry.com) and is a former board member of the Charleston Animal Society (formerly John Ancrum SPCA.) If you have questions about pet behavior or health, or suggestions for column topics, e-mail her at vetsaenger@bellsouth.net, and watch the column in coming weeks for her responses.)