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What is hyperthyroidism?

Hyperthyroidism is a condition that arises from an excessive production and secretion of active thyroid hormones by an abnormally functioning thyroid gland. First recognised in the late 1970’s, the frequency of diagnosis has escalated dramatically - currently it is the most common endocrine (hormone system) disease in cats and also one of the more frequently diagnosed disorders in the veterinary field. The condition is estimated to affect 1 in 300 cats.

What causes hyperthyroidism?

Despite several studies, the exact cause of the disease remains unknown. Recent studies have found that there is no breed association with risk. Other studies have found cats fed mostly tinned cat food, especially fish or liver and giblet flavour, and cats using cat litter are at increased risk.

Soybeans have also been implicated as a potential culprit as it is used as a protein source in some commercial cat foods. Furthermore, the thyroid gland contains more selenium than any other tissue, which suggests this trace element may play an important role.

Which cats are affected?

The disease typically affects middle-aged to older cats, with an average age of onset at 12-13 years.  Rarely can it affect cats as young as 4 years of age. There is no breed or gender predilection.

What are the clinical signs of hyperthyroidism?

Thyroid hormones are involved in a wide variety of functions in the body, including the regulation of heat production, metabolism and interaction with the nervous system. Therefore almost any organ system can be affected and subsequently a wide variety of clinical signs are possible.

Typically the signs start out very subtle and slowly progress, causing most owners to not notice them for several months. Some owners may also put these signs down to the normal “ageing” process.

The most common clinical sign is weight loss, despite a voracious appetite. Affected cats may seem constantly hungry and “finicky eaters” stop being “finicky”, eating everything that is offered to them or that they can get to.

Affected cats often gobble down their own food before going on to eat all the other pets in the household’s food as well; they may even try to steal your food off your plate if given the chance. Remember that cats are a lot smaller than us humans – losing for example 500g is not significant weight loss for a human but for a cat it’s about 10-20% of their total body weight and definitely a cause for concern.

Affected cats also become hyperactive – they may seem restless, anxious and irritable, constantly moving or even pacing, sleeping for short periods only and wakening easily. When brought to the vet these cats typically won’t sit for the vet to examine them and become aggressive when attempts are made to restrain them.

Affected cats are also weak and tire very quickly – you may notice it can’t jump as well as it used to. They also can’t cope with stress – short car rides, bathing, boarding or vet visits may weaken them severely or even cause them to collapse.

The digestive system may also be affected, causing vomiting, soft, bulky, stinky stools and sometimes even diarrhoea.

How is hyperthyroidism diagnosed?

Hyperthyroidism can cause a wide variety of clinical signs, many of which can be caused by several other disease processes as well. Vets may perform several tests on your cat to rule out, amongst others, diabetes, kidney disease, heart disease, pancreatic disease and cancer. These tests may include blood tests, urine analysis, X-rays and even an electrocardiogram or ECG.

The diagnosis is confirmed by measuring the amount of thyroid hormone circulating in the cat’s blood.   Affected cats’ values may be up to 19 times more than the top normal value.

It is worth noting that up to 10% of affected cats may have borderline or even normal levels of thyroid hormone. These cats are either mildly affected or are suffering from another non related concurrent illness which is suppressing the thyroid levels. Therefore, the vet may decide that a single measurement is not enough to either rule in or - out thyroid disease and may recommend additional tests.

How is hyperthyroidism in cats treated?

Affected cats are generally started on anti-thyroid medication to stop the excessive secretion of thyroid hormones. The medication can be given between one and three times a day. The vet will typically start on a certain dose and repeat the thyroid hormone levels by doing a blood test after two weeks from commencing treatment.

Depending on the result, the dose may be increased or decreased and the blood tests repeated again after two weeks. This process will continue until the thyroid hormone levels return to the normal range. Once this is achieved, thyroid blood levels will need to be retested every 3 to 6 months or whenever otherwise indicated. 

The success of the therapy will mostly depend on you, the owner, as you will be the one tasked with administering the medication. It is important that you stick to the prescribed dosage and return to the vet for scheduled follow-ups or whenever your cat shows any signs of illness.

Cats are notoriously hard to pill! If you find it impossible to administer the medication, let the vet know as soon as possible – we will be happy to show you how to give the medication or help you come up with an alternative.

In some cases the vet may opt to surgically remove either one or both of the thyroid glands. Generally, the procedure cures the disease, but there is always a risk of it recurring – therefor the vet may want to monitor the progression of the disease every 6 to 12 months; if it recurs the cat will have to go onto medication again.

The vet may also recommend a diet change as there are therapeutic diets which have been specifically developed for this condition.

The last option is destruction of the thyroid gland via radioactive iodine, which is currently not readily available in South Africa.

What’s the connection between hyperthyroidism and kidney disease?

Hyperthyroid cats are predisposed to kidney disease due to the effects of the higher metabolism on the kidneys. Conversely, the higher metabolism also masks the signs of kidney failure. Consequently these cats often start showing signs of kidney failure once their thyroid condition is brought under control.

The vet may decide to monitor you cat’s kidney enzyme levels along with the thyroid levels in order to diagnose kidney failure as early as possible.

What’s the connection between hyperthyroidism and heart disease?

Hyperthyroid cats are prone to developing heart murmurs, abnormally fast heart rates and abnormal heart rhythms. The vet may notice these abnormalities when listening to the heart with a stethoscope.

The excessive amount of circulating thyroid hormone makes the heart work harder than normal, which is further worsened by having to keep up with the cat’s higher metabolic rate. Over time the heart muscle starts thickening and/or the heart chambers start dilating to increase their size in an attempt to cope with the increased workload.

Congestive heart failure sets in eventually when the heart cannot keep up anymore fluids starts building up in the lungs and/or the abdomen.

If the vet suspects heart disease, he or she may decide to investigate further with ECGs, ultrasonic heart scans and X-rays. Depending on the findings, the cat may have to go onto heart medication as well.

What is the prognosis for hyperthyroidism in my cat?

The prognosis will depend on the cat’s physical condition, age, and whether other diseases are present. When treated, the average survival rate is 2 years and the quality of life is acceptable.

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