Hyperthyroidism: the Nitty-Gritty
I’ll say it again, a truism of veterinary medicine: cats are not small dogs.
You almost never hear of a dog having an OVERactive thryroid gland.
In cats, it is one of the most common diseases once they hit double digits in age.
I have 2 such cases right now, so I wanted to discuss the condition as well as the problems it can cause.
Hyperthyroidism occurs because of a tumour in the thyroid gland. It is not what we tend to think of as a cancer, because it does not invade into the rest of the body and cause illness just by being there. But tumour cells are, by definition, cells growing out of control. And, being out of control, they do not listen to normal feedback that says “OK, we have enough of what you are making, you can stop now…” So tumourous thyroid gland cells just keep going at full speed, making thyroid hormone and more thyroid hormone.
We diagnose it by measuring the level of thyroid hormone in the blood: sometimes the level is 3-4 times normal.
Thyroid hormone sets the metabolic thermostat. When there is lots and lots of thyroid hormone, the thermostat is cranked all the way up. The body is burning the metabolic candle at both ends.
In Spice, a 15 year old Siamese, this affected her liver. Cats are prone to a condition called Fatty Liver Syndrome, where fat is mobilized too rapidly to be processed by the liver. The fat accumulates in and damages the liver cells, producing a form of liver failure. So why is the fat mobilized? Most commonly, it is because the cat is not eating, whether from being ill or from getting out and lost. In the case of hyperthyroidism, however, they may still be eating but the demand for energy is so ramped up that the body makes use of fat energy too.
In Kala, an 18 year old Himalayan, thyroid disease appears to be affecting his heart. By increasing the body’s demand for energy and nutrients, hyperthyroidism places a terrific strain on the cardiovascular system. A regular heart rate for a cat getting an exam (and usually suffering “white coat” syndrome, after all, who likes going to the doctor’s?) is 200-220 beats per minute. I get worried at a heart rate over 240 bpm. Kala’s was at 268. Keeping the heart super-fast all the time is bad for it. The heart muscle gets its own blood supply delivered in the rest between the beats. If it is full-speed all the time, the rest between the beats does not add up to much. The heart muscle itself, so busy providing nutrients and energy to the rest of the body, starts to starve for its own nutrients and energy.
Now, heart muscle can get sick of its own accord – cats are prone to a condition called HCM (Hypertrophic Cardiomyopathy) where the heart muscle gets thick and stiff and full of scar tissue. We will not know if he has this too until after getting his thyroid condition under control – how much heart trouble is left when the blood sample says the thyroid is better?
Coming next – Hyperthyroidism: What To Do About It