Hyperthyroidism: 1 in 10 Cats Have It. Does yours?!

As cats age, there are several conditions that they commonly develop. Of these, hyperthyroidism is one of the most prevalent. In fact, studies suggest that as many as 1 in 10 senior cats will develop hyperthyroidism.
Cats (and dogs) have two thyroid glands. They are situated approximately midway up the neck on either side of the trachea (windpipe). While some dogs develop a condition of underactive thyroid glands (hypothyroidism), in cats, overactive thyroid glands are much more common. Although you usually wouldn’t be able to feel the thyroid glands, when they become overactive, you may be able to feel them as little bumps in the neck if you feel for them very carefully. This is one of the things your veterinarian will look for when examining your cat.


The exact cause of hyperthyroidism is not know but several possible factors have been suggested by researchers in the field. Interestingly, hyperthyroidism was unrecognized until just a few decades ago and this has led experts to suggest a link between hyperthyroidism and the chemicals found in the linings of pet food cans as well as other endocrine disruptors.
These ultimately lead to the development of benign tumors in the thyroid tissue resulting in overproduction of thyroid hormones (hyperthyroidism).


There are many vague symptoms that can be associated with hyperthyroidism. These include increased drinking and urination, weight loss despite food intake, and hyperactivity.
Many of these can be associated with other conditions so if you notice them, you should visit your vet so they can perform a full physical examination and the labwork necessary to make a diagnosis.


The diagnosis of hyperthyroidism is reached relatively simply through measuring the levels of thyroid hormones circulating your cat’s bloodstream. If these are increased above a certain level, it is consistent with a diagnosis of hyperthyroidism. Your vet will also assess other parameters and may recommend a urinalysis and x-rays. It is common for cats to be affected by multiple chronic diseases at the same time. Chronic Kidney Disease in particular is common in cats with hyperthyroidism and must be ruled out since treatment of hyperthyroidism can have a negative impact on kidney function.

Treatment Options

There are numerous treatment options for cats with hyperthyroidism. The ‘best’ option will depend on you, your cat and your financial means.


The gold standard of treatment involves injecting your cat with radioactive iodine. The radioactive iodine isotope, iodine-131 is used for this. It concentrates in the tumor in the thyroid gland and kills it leaving normal thyroid tissue behind. The downside of this approach is the cost associated, the limited treatment centers available and the need for your cat to remain hospitalized throughout treatment.


The abnormal thyroid tissue can also be removed surgically. This can ‘cure the condition but can also be expensive. In addition, many cats with hyperthyroidism are poor candidates for anesthesia and this often makes this treatment option less viable.


This is often the mainstay of treatment. It involves oral administration of a drug called methimazole once or twice daily. It can be very effective but requires lifelong administration and follow up. It also isn’t effective in 100% of cases and, as with any drug, there can be side effects.


Thyroid hormones require iodine to be produced by the body. This is obtained from the diet. There is now a commercially prepared, prescription diet (Hills Y/D) that is iodine restricted such that the body cannot produce thyroid hormones in excess. While this shows signs of being a promising treatment option, the long term impact of limiting iodine in this way is unknown.


Hyperthyroidism is a very common condition in older cats. Although treatable, it can often eventually contribute to a decreased quality of life. If you have questions about assessing quality of life, do not hesitate to reach out to us or to your regular veterinarian for support and guidance.