Hyperthyroidism in Cats

The thyroid is a small gland located in the neck of mammals that is responsible for the production of thyroid hormones.  Thyroid hormones act like an overseer on a work site and when it is not working properly, the workers (or in this case, organs) either stop working as hard as they need to or they work so hard they eventually break down.

In dogs, the most common dysfunction of the thyroid glands is hypothyroid or a thyroid that is not encouraging the other organs to work as hard as they should be.  Besides rare cases of hypothyroidism that are caused by problems in the pituitary and hypothalamus in the brain, hypothyroidism in dogs is almost always an autoimmune problem.

Cats, being hard workers in their own way, are predisposed to having a thyroid gland that overworks or is a “hyperthyroid.”  This means that their thyroid hormone circulates in greater than needed amounts and signals the other organs to work even harder.

Humans have an autoimmune form of hyperthyroidism called Graves Disease.  Hyperthyroidism in cats, however, is not a signal of immune function dis-regulation.  Instead biopsies of overactive thyroids in cats show that hyperthyroidism in cats is caused by hyperplasia, or simple cellular overgrowth, of the gland.

Almost all cats who develop hyperthyroidism are middle aged or older which significantly hampers most owner’s ability to detect the early symptoms.  Having an overactive thyroid is a bit like being on a stimulant.  At first your cat may seem to have a little bit more energy and may feel better than he has in years.  If this happened in young cats, we would probably have owners knocking on our exam room doors with stories of their wild and hyper cats jumping off the walls and causing problems all over the house.  Instead, hyperthyroidism hits cats right as they are experiencing a natural age related decline in activity.

Therefore, most owners perceive their hyperthyroid cat as ”doing great,” and still being “really playful.”  A slight weight loss may be a detectible during early stages of thyroid disease, however, not every cat will experience significant weight loss at this stage.  Unfortunately, this stage is the ideal time to intervene and get a cat onto medication to help reduce the negative effects of too much circulating thyroid hormone. 

The next things that happen as a result of too much circulating thyroid hormone is that the organs start to become overtaxed.  Heart rate will increase.  Blood pressure will increase.  The strain on the kidneys becomes greater.   The stomach and intestines will become hypermobile or will start working faster.  People with hyperthyroidism report changes in mood and a host of psychiatric conditions up to and including psychosis.  Humans also report headaches and other pains.  While we cannot prove psychological changes happen in cats, it is likely they have their own versions of these psychological and pain associated issues.

To owners this stage of disease may manifest in symptoms like weight loss, irritability, sleeping more (probably from pain or exhaustion), exacerbation of underlying heart disease, vomiting, diarrhea, dull hair coat, changes in appetite, increased thirst, and changes in litter box use.  During your cat’s annual exam the vet may also notice things like a new or worsening heart murmur, a “gallop” heart rhythm, or even physical enlargement of the thyroid gland itself.  Treatment is crucial at this stage to prevent organ failure later.

Just like a human body cannot continue taking an ever increasing dose of stimulants without major damage occurring in the body, left untreated hyperthyroidism eventually will lead to kidney failure, neuropathy and heart failure.  The majority of cats who have ongoing untreated thyroid disease now need to be treated not only for thyroid disease but also for heart of kidney disease as well.

Like almost all diseases, early detection is the best way to limit the damage.  At home the easiest way to monitor your cat’s health is to observe his behavior and food intake as well as monitoring his weight.  At the vet, we usually get our first suspicions of hyperthyroidism by listening to you and by comparing your cat’s current weight, heart rhythm and sound, and general health status against your cat’s past physical exams.  We may also find hyperthyroidism as a result of health assessment blood work.  The good news is that most cases of hyperthyroidism can be diagnosed by collecting a blood sample and sending it to a diagnostic lab.  A very small percentage of early stage cats will have borderline results and need a second blood sample for an additional test.

Hyperthyroidism can develop at any time during a cat’s older years so a negative test one year does not mean you cat will never develop the disease.  It is important to continue monitoring for this disease because hyperthyroidism in cats is not only relatively easy to treat if caught early, left untreated it will eventually kill your cat.  There are several ways to treat hyperthyroidism.  For a single cat homes where the owner can prohibit access to ALL other foods, there is a special diet for hyperthyroid cats, which can control thyroid hormone production.  Other people administer a medication to their cat twice daily.   For owners who cannot give their cats pills, the medicine can be made into a liquid or even topical transdermal formulation.  The other way to treat hyperthyroidism, radioactive iodine administration, one of the ways they treat thyroid disease in humans.  Although treatment with radioactive iodine initially costs more and require a few months of extra litter box maintenance, it requires the least long-term work for both owner and cat.  The closest referral hospital currently performing radioactive iodine therapy is the Metropolitan Veterinary Hospital in Copley.  Regardless of what method of treatment works best for you and your cat, early treatment is one of the best ways of extending your cat’s happy and healthy lifespan.

Angela Albers, DVM

Stow Kent Animal Hospital

Portage Animal Clinic

Additional Resources:

http://vet.osu.edu/vmc/feline-hyperthyroidism

http://www.veterinarypartner.com/Content.plx?P=a&s=0&C=0&A=516