“Are you a cat person or a dog person?” I’m asked on a regular basis, as if my answer might reveal some medical truth concerning the objective superiority of one species over the other. As it happens, I share my home with one of each, offering sound excuse to shrug my shoulders and dodge the question entirely. Although the two are frequently painted as polar opposites, I’m not sure that medical science supports that conclusion. But then I remember the curious nature of their thyroid glands, and I wonder if, perhaps, I’m mistaken.
At the risk of oversimplifying, you can think of the thyroid gland as the body’s metabolic gas pedal. It normally produces hormones that help set the overall rate at which energy is burned by other organ systems. As you might expect, it can malfunction in one of two ways, producing either too much or too little. Human thyroids can find themselves afflicted in either direction. But dogs and cats, for whatever reason, have decided to go their separate ways on this one.
If a cat’s thyroid is going to cause problems, it is almost certain to speed up—a condition called hyperthyroidism—and nearly everything else speeds up with it. Affected cats burn calories too fast and try to compensate with ravenous appetites. But they can’t eat enough. Fat supplies melt away, and muscle is broken down to help feed this new demand for more energy. Apart from simply wasting away, hyperthyroid cats also have to contend with heart disease resulting from an increased heart rate and blood pressure. It’s life-threatening if untreated.
Dogs, on the other hand, will often find that their thyroid glands have grown lazy—a state called hypothyroidism—and the symptoms run opposite to what you’d expect in a hyperthyroid cat. Reduced calorie consumption causes them to pack on pounds, even though they’re eating far less than before they were sick. Families begin to notice a general lethargy, with maybe a few mood swings tossed in for good measure. And the altered metabolism tends to cause skin problems ranging from hair loss to infection. It’s not usually dangerous, but it does quite a job on the dog’s quality of life.
The good news is that these things aren’t terribly difficult to handle. Hypothyroid dogs have it particularly easy. Because they’ve found themselves short on thyroid hormone, we just give it back to them in the form of a supplement. There may be a bit of fiddling to get the dose where we want it, but, once we do, they’re right as rain.
Cats are a bit trickier because their overzealous thyroid glands need to be put back in check. Luckily, the thyroid is the only part of the body that uses iodine to function—it is a key ingredient in the production of thyroid hormone—and we can take advantage of that fact to bring it under control. Most commonly we use long-term medication to prevent it from using available iodine. In other cases, we might choose a brief course of radioactive iodine. It sounds alarming, but it’s finished in a couple of weeks and potentially curative. In other cases still, we make use of special iodine-restricted diets to limit thyroid function. The right choice depends on numerous factors that you’d want to discuss with your vet.
For reasons nobody knows, that’s the way it is. Cats become hyperthyroid, dogs become hypothyroid, and never the twain shall meet. Or almost never, at any rate. You’ll find rare exceptions here and there, but they only serve to prove the rule. I’m not sure if any of this is reason enough to declare a victor in the rivalry between dog people and cat people, but on the odd chance you have a preference for which endocrine disorder you’d rather find yourself treating, I suppose it could be useful in deciding your own affiliation.
Dr. Mike Fietz is a small-animal veterinarian at Georgetown Veterinary Hospital. He received his veterinary degree from Cornell University in 2003 and has lived in Charlottesville since.