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Hyperthyroidism in Cats


There are many ailments that can affect middle-aged and senior kitties, such as Hyperthyroidism. Some of these health problems can have a very gradual onset, meaning that you may not notice changes to your cat right away. Semi-annual examinations are important for the health of your cat. This is because your vet can provide safe and effective preventive care while looking for signs of disease. 

Routine weight checks and physical examinations can help your veterinarian look for signs of hyperthyroidism, which is a common disease that affects middle-aged and senior cats. The thyroid glands are located on either side of the trachea and can be palpated just under the neck. One or both glands can become enlarged due to this disease, making them very easy to palpate. 

regular vet checks

What is hyperthyroidism in cats?

The thyroid gland is responsible for the release of thyroxine, a hormone that helps to regulate many metabolic functions in your cat’s body. When too much thyroxine is released, it can speed up your cat’s metabolism and have many adverse effects. Hyperthyroidism can cause cats to lose weight despite a good appetite. In fact, this weight loss can cause some cats to have a ravenous appetite! Other cats may experience an increase in thirst and urination, and they also tend to vocalize more at nighttime. 

Is hyperthyroidism in cats painful?

Hypothyroidism can be painful for your cat due to the various complications it can cause. 

For example, cats with hyperthyroidism can develop heart disease. The heart muscle tends to thicken as a result of thyrotoxic cardiomyopathy, and this will cause an increase in your cat’s blood pressure. Cats with high blood pressure or hypertension can seem restless and may have trouble breathing in severe cases. 

Hypertension can also cause vision problems due to an increased risk of retinal detachment. This will result in blindness and can have an acute onset. 

What to do if you suspect your cat may have hyperthyroidism? 

If you suspect that your cat may have some of these clinical signs, it is best to make an appointment with your veterinarian as soon as possible. Testing is straightforward and involves collecting a small blood sample to check thyroxine levels. 

When levels are very high, this is consistent with a diagnosis of hyperthyroidism. If levels are in the higher end of normal, additional blood work may be necessary to completely rule it out. This may involve looking at different types of thyroid hormone for a specialized blood test, or your vet may tell you that in-house testing needs to be repeated in a few weeks. 

Additional diagnostics are important to rule out other causes of weight loss in cats. Diabetes mellitus and kidney disease are just some of these illnesses. In some cases, diagnosing and treating hyperthyroidism may unmask underlying kidney disease, and so it is important to recheck kidney values several weeks after initiating therapy for hyperthyroidism. 

Treatment for cats with Hyperthyroidism 

Hyperthyroidism can be treated in a few different ways. 


Surgical removal of the affected thyroid gland is one such treatment, and if only one gland is affected, a second surgery may be necessary in future if the opposite gland becomes diseased. Surgery is a more invasive form of treatment, and specialized testing is often needed to rule out the presence of ectopic thyroid tissue elsewhere in the body. 

Radioactive iodine treatment

While age isn’t necessarily a disease, very senior kitties or cats with heart disease may not be ideal candidates for surgery due to the need for general anesthesia. Instead, radioactive iodine can be used. This type of treatment involves the injection of a medication that destroys abnormal thyroid tissue but won’t harm other internal organs. Anesthesia is not necessary for this form, but because the medication is radioactive, your cat will need to stay in the hospital for at least one to two weeks while the medication is excreted from his body. 

Thyroid hormone supplements 

Specialty hospitals are necessary for thyroid surgery and radioactive iodine treatments. If you are unable to get to one of these hospitals, another option is to supplement thyroid hormone orally. A medication called methimazole is utilized to suppress excess levels of thyroid hormone. It is typically given twice a day, but dose adjustments are sometimes necessary. It is important to check thyroid levels every three to six months to ensure that the dose given is appropriate. Side effects of methimazole include vomiting, diarrhea, fever, and liver problems. 

Prescription diet 

If your cat is difficult to medicate, there is a prescription diet that is made by Hills called Prescription Diet y/d. It contains extremely low levels of iodine, which is supplemented by foods. Iodine is converted to thyroxine via the thyroid glands, so low levels of iodine mean that there will be lower levels of thyroxine in your cat’s bloodstream. For this food to work, your cat cannot have any other foods or treats. This makes things tricky for kitties in multiple cat households. 

If hyperthyroidism is detected early and treated, your cat will eventually be back to their healthy self  

Whatever treatment option you choose, remember that it can take several weeks of treatment for your cat to return to normal.

Happy cat

It is important to monitor for possible side effects, and routine monitoring is necessary to ensure that hyperthyroidism doesn’t return. If caught in the early stages and treated appropriately, cats with hyperthyroidism usually have a good prognosis. Make sure to ask your vet about routine lab work for your middle-aged or senior kitty so that their hyperthyroidism doesn’t go undetected. 

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