Your Cat’s Thyroid

My dear departed cat Salem, along with his failing kidneys in his later years, also developed hyperthyroidism when he was about 7 years old.  This is another common ailment of our furry friends when they get to middle age.

Before this illness developed he was a very overweight cat weighing more than 12 kilograms and I had him on weight loss food.  He was an indoor/outdoor cat and we lived in Canberra, Australia’s bush capital.

I knew that something was wrong when he all of a sudden lost almost half his body weight.  I took him to the vet and they diagnosed hyperthyroidism.

What is hyperthyroidism?

The thyroid gland in cats, like humans, is a butterfly-shaped organ residing in the neck that produces the thyroid hormone.  This in turn regulates metabolism.  When a cat becomes hyperthyroid – the gland is working too hard and starts to produce excessive amounts of the thyroid hormone (T4) in the bloodstream which creates a whole bunch of health issues in both cats (and humans).  In most cats a small benign tumour will form on one or both sides of the thyroid gland sending the gland into panic mode where it starts to over-produce T4.


The most common signs are dramatic weight loss accompanied by an increased appetite as well excessive thirst, increased urination, hyperactivity, vocalisation (sad meowing) over-grooming and unkempt appearance, panting, diarrhoea and sometimes shedding and vomiting.

Immediate Treatment

Initially my vet put my cat Salem on a dose of Methimazole for a year or so. Over time, this would have been an expensive solution as my cat was then only 7 and he would have required that medication for the remainder of his life.  We were also advised that this medication had the potential for side effects including loss of appetite, lethargy and possible blood cell abnormalities.  Although while he was on this medication, we did not notice any adverse effects with our cat Salem.

The medication will suppress production of the thyroid hormone but it will not slow the growth of the tumour and over time the dosage will likely need to be increased to manage the condition.

white oval medication pill beside blister pack
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More permanent treatment options

Because of his age and general health we decided to proceed with the more costly, but supposedly permanent solution, which was radioactive iodine therapy. Radioactive iodine becomes concentrated in the thyroid gland, where it irradiates and destroys the overactive thyroid tissue and no anaesthetic is required. This seemed like a good choice despite the AUD$1500 price tag initially as it still was cheaper than giving him the medication every day for the rest of his life.

In Australia at the time (2010) the treatment was via special radioactive capsules.  Salem was then put in isolation in a special licensed animal hospital for a week or so until we were able to collect him.  With this treatment you cannot take your pet home until the radioactivity from the isotope they are administered has destroyed the failing organ and the radiation has decayed to safe levels. They check this by testing the animal’s faeces and urine.

I believe now that there is a treatment where the cat is anaesthetised and given an injection of radioactive iodine and that this treatment is becoming more affordable for the average pet owner.

This treatment worked, or so we thought, for several years.  Then, the thyroid problems resurfaced and we had to give him capsules for the remainder of his life.  The vet advised me that the radioactive iodine treatment was 99% effective.  Unfortunately we were the 1 percent and my cat’s tumour grew back and we had to continue with the medication treatment option as we couldn’t justify another $1500 for a repeat of the radioactive iodine therapy.

Left untreated, your hyperthyroid cat will likely die of heart failure as the over production of the thyroid hormone continues to put stress on their hearts and all their other organs.  Untreated cats will sometimes have blood in their stools and some will stop eating altogether.

My cat Salem lived to the ripe old age of 17 years.  He eventually passed after developing kidney disease which became untreatable (see my other blog post).

Strangely, I too have thyroid disease (Hashimoto’s) and I take thyroid hormone tablets because my thyroid no longer produces the hormone (hypothyroid) the opposite problem.  I was such a kindred spirit to that cat and I believed I developed my auto-immune thyroid disease in sympathy with him.

adorable animal blur cat
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2 thoughts on “Your Cat’s Thyroid

  1. Iodine 131 can be very effective, but dosage calculations can be very problematic without nuclear medicine scans to check on the level of thyroid metaplasia

    1. We left that decision in the skilled hands of the nuclear technicians and/or our vets who administered the drug. Despite my background in radiation protection (policy) – I abrogated that responsibility to the skilled professionals who we paid thousands of dollars to cure our poor cat.

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