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Hyperthyroidism is a common disease in older cats. It is due to excess production of thyroid hormone by the thyroid gland. This excess thyroid hormone causes an increase in the cat's metabolism. This usually causes the affected cat to lose weight despite an increased appetite. It also forces the internal organs to work excessively hard which usually causes at least some damage to those organs, especially the heart and kidneys.

If hyperthyroidism is caught soon enough and treated, the prognosis is good. This is one of the diseases that we screen for at least yearly in middle-aged and senior cats.

North County Cat Hospital


Disease in Cats

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In the early stages of hyperthyroidism, there may be no obvious signs. As the disease progresses, the signs of hyperthyroidism in cats can include one or more of the following:

  • Increased appetite

  • Weight loss

  • Increased thirst

  • Vomiting

  • Diarrhea

  • Rapid breathing

  • Difficulty breathing

  • Increased activity

  • Increased vocalizing

  • Confusion or acting senile

  • Changes in behavior, including aggression

There are a lot of other potential signs and there is no one sign that is always present in hyperthyroid cats.


The diagnosis is usually made with simple screening blood tests. If these tests are borderline, we sometimes have to run additional tests.

In the early stage of hyperthyroidism, cats may seem completely normal.

(By the way, we recommend keeping cats indoors.)

Treatment Options:

There are four treatments available for hyperthyroid cats in the United States that are commonly discussed: methimazole, surgery, Hills y/d, and radioactive iodine (I131).


This is a medication that is given twice daily. It can be administered as a pill, treat, liquid, or topical cream.

The topical methimazole is made by a compounding pharmacy and is applied to the skin of the ear. Since this medication is made to be absorbed through the skin, there is risk that humans or other pets might be exposed to it. We recommend against the topical form of medicating if there are pregnant women or small children in contact with the cat, or if the cats in the household groom each others ears.

Methimazole is well tolerated in most cats but can have a number of serious side effects in a few cats. Some of the more common side effects include:

Things that can be monitored at home:

  • Decreased appetite. The appetite will normally go down as the thyroid level goes down, so it can sometimes be difficult to tell if the methimazole is upsetting the stomach.

  • Vomiting

  • Lethargy

  • Itchiness

Things that are monitored with blood tests.

  • Liver disease

  • Bone marrow suppression


Surgical removal of the thyroid gland is relatively expensive and has a number of potential complications. We never recommend it.

Hill's y/d Diet

HIll's released the prescription diet y/d in 2011. It is a low protein (34.1% dry matter protein) very low iodine diet that can normalize thyroid levels in some cats. If used, it is critical that the affected cat not receive any other food or treats and that healthy cats not eat the y/d. We have concerns about the long term effects of feeding this food and only use it if other options aren't feasible. 

Radioactive Iodine

Although using a radioactive substance to treat a cat sounds scary, this is actually a relatively safe treatment with very few side effects. The two main downsides are expense and the fact that the cat needs to be hospitalized at a specialty facility for about a week after the treatment.

After the radioactive iodine (I131) treatment is performed, the cat will usually have a lower than normal thyroid level for a period of time. This is problematic if the cat also has underlying kidney disease.

Hyperthyroidism and Kidney Disease:

Cats with both hyperthyroidism and chronic kidney disease pose a special problem.

Kidney disease manifests as a decrease in kidney function. This decreased function can have a negative impact on the cat's quality of life.

Hyperthyroidism forces the kidneys to work harder. This causes further damage to the kidneys but at the same time improves their function while the hyperthyroid state persists. 

When we treat a cat with hyperthyroidism, we can "unmask" kidney disease. In other words, the function of the kidneys can decrease when we treat the hyperthyroidism - not because of damage done to the kidneys from the treatment but because the kidneys are no longer receiving the excess thyroid hormone. They are no longer being forced to work overtime.


Cats who are hyperthyroid and have kidney disease are sometimes tricky to treat.

After the Diagnosis:

If we have diagnosed your cat with hyperthyroidism these are the steps that we usually recommend. If you use a different veterinarian, you will be better off following their directions.

Start Treatment with Methimazole:

Start treating with the recommended dose of methimazole - given every twelve hours.

Monitor Closely at Home:

Monitor at home for side effects including lethargy, vomiting, poor appetite, itchy skin, and any abnormal behavior. If you have any question at all about side effects, call us. If you think that you might be seeing serious side effects from the medication, stop giving it until you talk to us.


Return in three to four weeks to reweigh and for a blood panel and possibly a urinalysis. We will be looking for:

  • Effectiveness

  • Side effects 

  • Unmasking of kidney disease

Adjust the Dose:

Adjust the dose of methimazole based on the above criteria and continue with the new dose.

Repeat Every Three to Four Weeks:

Return every three to four weeks for six months to repeat the tests. Serious side effects are usually seen in three months but it can sometimes take longer than this. (Because of expense, some people decide to recheck the panel every three to four weeks for three months instead of six.)

Consider Treatment With I131:

If the kidney blood values and urinalyses are normal when the thyroid is also at a normal level, then the cat is likely a candidate for I131 and we can discuss having that done.

Proceed with I131 or continue the established dose of methimazole and return every four to six months for an exam, blood panel, and urinalysis.

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