An overactive thyroid gland, or hyperthyroidism, is a very common condition in older cats. It is, in fact, the most common hormone disorder in cats, and one of the most important causes of weight loss.
What causes it?
In cats, like all mammals, the thyroid glands in the neck make a hormone called thyroxine. This hormone controls the animal’s basic metabolic rate - how fast they convert food into energy. In cats with hyperthyroidism, part of the gland malfunctions, and becomes overactive - it continually produces thyroxine and doesn’t respond to the signals from the cat’s pituitary gland telling it to stop. As a result, the cat’s thyroxine levels go higher and higher, and their metabolic rate climbs uncontrollably. The condition is much more common in older cats - it is very rare below the age of 6 years, and most common after the age of 12.
What are the symptoms?
The most common symptom is weight loss, combined with ravenous appetite - because affected cats are burning food faster than they can eat it. Other signs and symptoms include increased thirst, sometimes vomiting and diarrhoea, increased breathing rate and heart rate. Because the cat has all this extra energy, they generally become hyperactive - sometimes they’re really playful or kittenish, but often they become hyperaggressive. Other possible signs include abnormal heartbeats (a “gallop rhythm”), thickened claws, an unkempt or scraggly coat, and a swelling on the neck (a “thyroid nodule”) caused by the enlarged gland.
How is it diagnosed?
Well, when your cat comes in for a checkup, you’ll notice we usually listen to their heart rate. If a cat’s heart rate is over 180-200 beats per minute at rest, it’s very suspicious! However, a definitive diagnosis is very easy - we take a small blood sample and test it for the thyroxine levels. If these are significantly elevated, it means the cat has hyperthyroidism.
Occasionally, cats present with another illness as well, and in these cases the diagnosis can be more complicated. Any serious disease or illness will lower the natural production of thyroid hormones, so a cat with hyperthyroidism at the same time as another condition may have apparently normal thyroxine levels - this is called “sick euthyroid syndrome”. If we suspect this, we’ll usually try to treat the other illness, then repeat the blood test.
What can be done about it?
Fortunately, hyperthyroidism is very treatable nowadays.
There are four treatment options available:
Medication - there are a variety of medications available that chemically reduce the production of thyroxine. These are available as tablets, and usually have to be given once or twice a day. Periodically, your cat will need a blood test to make sure the dose of medication is correct; and the tablets must be continued for life. Most cats do really well on the tablets; however, a very small number develop side effects, and require a different dose, or a different form of treatment.
Special diets - the thyroid glands need iodine to manufacture thyroxine; as a result, specialist prescription-diets that are almost totally iodine-free are available. These are less effective than medication, but much easier to give! However, the cat mustn’t eat any other food at all (no snacks, treats or mice!), or the diet won’t work.
Surgery - although it sounds scary, the affected thyroid gland can be removed, and the cat returns to normal. However, cats with hyperthyroidism don’t cope with anaesthetics well, so they generally have to be stabilised with medication before surgery can be attempted. Although there are some risks, the operation provides a complete cure of the condition, and cats usually make a full recovery. The condition can however affect the other thyroid gland meaning it is sometimes necessary to remove both glands.
Radiation Therapy - the cat goes to a specialist veterinary hospital, and undergoes a course of nuclear medicine that destroys the abnormal thyroid tissue. They then have to stay in isolation until they are no longer radioactive before they come home. Again, if successful this cures the condition, although it is expensive and not many hospitals offer it.
So, what happens if I don’t get it treated?
Eventually, one of two things will happen - they will either starve to death, or die of heart failure and/or blood clots as their heart is forced to beat faster and faster until it can’t keep up. Neither is pleasant, so treatment is very strongly advised.