What is it?
Hyperthyroidism is a condition where the thyroid gland (in the neck) becomes overactive. As a result, it produces excessive amounts of thyroid hormones (also known as thyroxine), which act to speed up the cat’s metabolism.
What causes it?
In most cases, it is due to a (benign) tumour in the thyroid gland. This tumour makes thyroxine, but does not respond to the signals from the brain telling it to stop when it has made enough, so thyroxine levels continually rise. Thyroxine controls the cat's metabolic rate, so an excess of thyroxine increases their basal metabolic rate, causing psychological and physiological hyperactivity.
What cats are at risk?
Hyperthyroidism is primarily a disease of older cats and is most common over the age of 12 years. It is very rare under 6 years of age.
What are the symptoms?
The typical symptoms include weight loss (despite ravenous hunger); increased thirst; a high heart rate; and hyperactive, or “kittenish” behaviour (this may also display as abnormal aggression). Other signs may include vomiting and diarrhoea, thickened claws, and an unkempt coat. In many cases, there will also be a detectable swelling in the neck (due to the enlarged gland) - a thyroid nodule.
How is it diagnosed?
A simple blood test will diagnose abnormally high levels of thyroxine in most cats. Occasionally, the levels appear falsely normal because of another illness (sick euthyroid syndrome), and the hyperthyroidism cannot be diagnosed until the other disease (e.g. kidney failure) has been treated.
How can it be treated or managed?
There are 4 treatment options. The simplest is with diet - a special low iodine diet is given. The thyroid needs iodine to make thyroxine, so by limiting iodine, you limit production. This only works if the cat eats only the special food, without any treats, snacks or live prey. More commonly, we would use certain medications (carbimazole or its derivatives) - daily or twice daily tablets are very effective at controlling the condition in most cats. The condition can be managed medically, but for a "cure", the cat will need to undergo surgery - once the cat has been stabilised with diet or medication, the vet will remove the overactive thyroid gland. If this isn't an option, there is also a more modern approach, using Radioactive Iodine Treatment - the cat goes to a veterinary referral hospital where they can be given nuclear medicine treatment to destroy the hyperactive thyroid gland.
Can it be prevented?
Sadly, there is no way to prevent feline hyperthyroidism.