What is Hyperthyroidism?
Hyperthyroidism in cats arises from overactivity of the thyroid gland. This is usually due to non-cancerous ‘growth’ of the gland. We don’t yet know what triggers this growth to occur. The thyroid gland is responsible for controlling metabolism, therefore excessive activity of this gland causes symptoms associated with an increased metabolic rate. You may have heard people talk about weight gain and lethargy due to an underactive thyroid, it is the opposite for hyperthyroidism. The most common signs observed in hyperthyroid cats include weight loss despite a good or ravenous appetite, hyperactivity displayed as restlessness, vocalisation or aggression, increased thirst and therefore increased urination, unkempt coat due to poor grooming, vomiting and diarrhoea.
Diagnosing Hyperthyroidism in Cats
Astute readers may at this point be thinking, “that sounds similar to signs of kidney disease or diabetes” and they would be correct. It is virtually impossible to differentiate hyperthyroidism from chronic kidney disease, the other very common disease of older cats, based on clinical signs alone. Diabetes however usually occurs in younger animals and many diabetic cats are weak rather than restless. Diagnosis of hyperthyroidism is by blood and urine tests (which will also differentiate between hyperthyroidism, chronic kidney disease and diabetes) after a careful clinical exam by your veterinarian. The blood test will detect elevated levels of thyroid hormone in the blood in the case of hyperthyroidism, although levels may be normal in very early cases or if there is another serious illness.
Why is it Important?
Although the clinical signs of hyperthyroidism in cats arguably don’t sound that bad, hyperthyroid cats should be treated since it is the signs you don’t see that are affecting your cat’s quality of life. That overactive metabolism is causing all sorts of havoc internally; the heart is racing and therefore has to work harder, leading to heart enlargement; blood pressure may be elevated to dangerous levels causing retinal detachment and subsequent blindness; the body is burning up energy, which is worsened by having to produce more red blood cells to carry more oxygen to ‘overactive’ tissues; there may be osteoporosis leading to weak bones. With the body running in overdrive, eventually the energy reserves are used up (hence the weight loss) and there is no energy to put into bodily functions like maintenance of the immune system, thereby increasing susceptibility to infections. Many older cats may already have bad dental disease, increasing this risk. Although I would never put a cat to sleep based on a routine diagnosis of hyperthyroidism if the owners refused to treat, I do think treatment is warranted to maintain quality of life.
Treatment of Hyperthyroidism
There are three categories of treatment for hyperthyroidism in cats: medical management, surgery or radioiodine therapy. The former only manages the disease while the latter two options are curative. The major caveat for treatment is whether or not there is concurrent chronic kidney disease – one of the benefits of hyperthyroidism is that it improves blood circulation through the kidneys which may compensate for diseased kidneys. Therefore treating the thyroid disease may lead to overt kidney failure or significant worsening of pre-existing kidney disease. It is a delicate balance in cats that have both conditions to balance the thyroid treatment with the kidney disease to minimise the adverse effects of both, and in many cases it may not be worth treating the thyroid disease in this situation. I always recommend treating newly diagnosed hyperthyroid cats medically for one or two months and closely monitoring kidney function, and only if the kidneys are stable after this time would I proceed to a curative treatment.
Antithyroid medications are most frequently given as tablets and this can be very challenging in some cats, especially as they may require tablets multiple times a day for the rest of their lives. Side effects are uncommon but include vomiting, inappetence and lethargy. More serious side effects are rare. An alternative option is a cream that is applied to the inside of the ear and the medication is absorbed through the skin. This is much easier to give and has fewer side effects but is unfortunately also more expensive. It is worth mentioning that you should wear gloves when using this since it is designed to be absorbed through the skin! It is important to closely observe treatment in the first few months by watching clinical signs and by blood and urine tests to monitor the levels of thyroid hormone and to check the kidney status. Therefore the first few months of treatment can be expensive. However, once stable, blood tests can often be done once or twice a year to check that the thyroid hormone levels continue to be normal.
Surgery is curative but is quite technically challenging and will likely be referred to a specialist surgeon. Therefore this can be expensive – however it only needs to be done once There are a number of risks with surgery and with the associated anaesthetic (remember heart disease is common with hyperthyroidism) that need to be discussed in detail with the surgeon before the procedure.
This is, in my opinion, the best treatment option for hyperthyroid cats, particularly if diagnosed ‘young’ (e.g. 10-12 years old) since the alternative is daily medication for life. This will usually require referral as most clinics don’t have the facilities to perform this procedure. The medication is given as a tablet and specifically targets the overactive thyroid tissue and destroys it. The procedure is very safe and effective but usually requires a stay in hospital of a week or so. Like surgery, this procedure is expensive but is only done once, so the overall cost needs to be compared to the cost of ongoing medication and monitoring including blood tests.
Prognosis for both management and cure of hyperthyroidism in cats is excellent as long as there is no concurrent kidney disease. Symptoms are reduced very rapidly and the body starts to return to normal, including the heart in early cases. I firmly believe treatment of hyperthyroidism makes a huge difference for our senior cats, and for owners too by reducing the vomiting, diarrhoea, yowling, aggression and sometimes inappropriate urination. Although older cats can live with hyperthyroidism, treatment significantly improves their quality of life. It’s often hard for a vet to say all of this in a short 15 minute consultation so hopefully this answers some of the questions you might have if you are living with a feline family member with hyperthyroidism.
Disclaimer: The opinions expressed here are my own and I am only providing generic information based on my knowledge and experiences. This information is not a substitute for a consultation between you and your veterinarian.