Hyperthyroidism – How will this affect my cat?

Overactive thyroid glands are a common and serious problem in our older cats. However, if diagnosed early, hyperthyroidism can often be treated successfully.

Signs of hyperthyroidism can be subtle when the disease is in its early stages, but the most common thing owners first notice is that their puss is eating well, or even seems unusually hungry, but is losing weight. Cats often become thirsty, and may seem irritable and restless. Mild to moderate diarrhoea and/or vomiting is also common. However, individual cats can have quite different signs – sometimes even overweight cats are found to have the condition. As the disease progresses and affects other body organs many other signs may develop.

This information will help you understand your cat’s condition as well as the possible treatments and their outcomes.

What is Hyperthyroidism?

This condition occurs when the thyroid gland is over active and secretes too much thyroid hormone into your cat’s blood. We can measure the amount of hormone present by taking a small blood sample from your pet and sending it off to a laboratory for analysis. A general blood screen is often performed at the same time to ensure other parts of the body are functioning normally.

Where is the Thyroid Gland?

The thyroid gland is a small double-lobed structure in the neck. In hyperthyroid cats it may be enlarged enough for the vet to feel.

What does the Thyroid Gland do?

One of the most important functions of the gland is control of metabolism. The extra thyroid hormone encourages the metabolism to “speed up”. The heart rate increases and your pet’s appetite may increase but it will lose weight. Its behaviour may become restless and nervous. Vomiting and diarrhoea can occur. Coat colour may change or the hair feels harsh. Some cats’ voices become hoarse.

Old cat

Are there long-term problems?

If the hyperthyroidism is not treated then a heart condition (thyrotoxic heart disease) can develop. The disease makes the heart beat faster than normal (over 200 beats per minute compared to the usual 120-150 beats per minute). The heart is basically a muscular pump, so if it works harder, the muscle builds up. This results in the heart wall thickening until it cannot work efficiently anymore. This can be reversed but your pet might need other medication for the heart until the thyroid is stable.

Hypertension (high blood pressure) is another potential complication of hyperthyroidism and can cause additional damage to several organs including the eyes, kidneys, heart and brain. If hypertension is diagnosed along with hyperthyroidism, drugs will be needed to control the blood pressure. Following successful treatment of the hyperthyroidism, the high blood pressure will sometimes resolve.

Kidney disease (chronic renal failure) does not occur as a direct effect of hyperthyroidism, but the two diseases often occur together because they are both common in older cats. Care is needed where both these conditions are present, as hyperthyroidism tends to increase the blood supply to the kidneys, which can improve their function. So, hyperthyroidism can ‘mask’ kidney disease.

Is this disease treatable?

The choice of treatment is made in consultation with your vet, taking into consideration your cat’s age, health status and how you feel they will tolerate the different regimes.

The over active thyroid gland can be treated by:

Daily Tablets: The current oral drug of choice is called Vidalta. Vidalta is very cost effective and only needs to be given once daily as the tablets are formulated to release the active, carbimazole, over twenty four hours. Vidalta comes in two tablet strengths, 10mg tablets and 15mg tablets. It is a very small tablet that can usually be given easily to your pet. The only down side to it, is that it has to be given as a whole pill – not crushed or broken into the cat’s food. Hormone levels are then monitored regularly to ensure that the cat is getting the correct dose.

Prescription y/d Food: Low iodine food reduces the thyroid hormone levels without the need for any medication. This option provides a non-invasive, easy way to manage this disease for many hyperthyroid cats, including those who have early stages of kidney disease. The food is not medicated, it has a reduced amount of dietary iodine. This means the diseased thyroid then does not have the necessary ingredient to overproduce thyroid hormones. Because iodine intake from other food sources (treats, another pet’s food, etc) can compromise the effectiveness of low-iodine nutrition, it’s critical that you keep your pet on a strict diet of y/d only.

Daily Transdermal cream: Hyper-T earspot cream, rubbed into the inside of  the ear once daily. This is an excellent choice for a cat that cannot be pilled or won’t exclusively eat the y/d food. It is however more costly.

Radioactive Iodine: A dose of radioactive iodine destroys the abnormal thyroid tissue, but does not damage the surrounding tissues or the parathyroid glands.
The main benefit of this treatment is the high chance of a complete and permanent cure. Cure rate is as high as 95% with this form of treatment and no ongoing therapy is then required. Blood work is monitored following treatment to ensure a return to normal thyroid status. If this option is selected the cat must stay in hospital for seven days, as during this time their wastes are radioactive. Radioactive iodine is not recommended for cats that have overactive thyroid and kidney problems.

What is the usual treatment programme?

The success of any of the above treatments will often depend on the co-operation of your cat!

The aim of daily medication or y/d food is to bring the circulating thyroid hormone levels down to within the normal limits.

When treatment starts, regular blood samples every 3-4 weeks need to be taken to assess how your cat is responding to the treatment and to make ensure that the correct dose rate of medication is being given to your cat. These blood tests also check kidney function to make sure any hidden kidney problems haven’t been ‘unmasked’ by treating the over-active thyroid gland.  Clinical signs should improve rapidly e.g. weight gain, normal appetite and good coat condition.

Once your pet is stabilised within the normal levels, monitoring of the blood levels is then done every 6-12 months, unless there are any return of clinical signs.

If a cat has hyperthyroidism and kidney disease, this is usually managed with a lower dose of pills or cream, to keep the cat’s thyroid slightly overactive. This keeps better blood flow to the kidneys while still keeping the thyroid reasonably well-controlled: a bit of a balancing act!




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