Feline Hyperthyroidism

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hyperthyroid cat
Symptoms of feline hyperthyroidism
Diagnosis of feline hyperthyroidism
Treatment of feline hyperthyroidism


Feline Hyperthyroidism is a relatively common disease of older cats. These cats tend to lose weight despite having a good appetite. This is a treatable condition.

hyperthyroid cat

The following symptoms are usually seen in cats with hyperthyroidism:

  • weight loss
  • good, possibly even ravenous appetite
  • rapid heart rate
  • sometimes vomiting or diarrhea
  • high blood pressure
  • dilated pupils (This symptom can be seen in cats who have had untreated hyperthyroidism for some time. The increase in blood pressure can cause retinal detachment which can cause blindness.)

Many cats with hyperthyroidism will have a thyroid gland that can be felt by your veterinarian. However, it can often be difficult to feel this gland. If your vet suspects feline hyperthyroidism they will do a blood test called a T4 level. They will likely also do a geriatric blood screen including kidney and liver enzymes. Cats with hyperthyroidism can sometimes have other health issues. Also, other diseases that affect older cats such as diabetes or kidney disease can look similar to hyperthyroidism.

hyperthyroidism treatment
Added Sep, 2011:There is now a new treatment for hyperthyroidism that doesn't involve pills!

There are three types of treatment that veterinarians currently use for cats with feline hyperthyroidism, radioiodide therapy or a medication called methimazole (Tapazole and Felimazole), and using a food called Hill's y/d.

Radioiodide therapy

This is probably the better treatment for most cats. However, it can be expensive. In radioiodide therapy the abnormal thyroid tissue is destroyed with radiation. It is a safe treatment for the cat and not painful. However, because there is radiation involved, the cat usually needs to be hospitalized in a special facility for about a week.

Most cats who have had radioiodide therapy do not need any further treatment for hyperthyroidism. However, occasionally a cat will need a small dose of medication to maintain a normal thyroid level

Medical therapy

Most people in my practice opt for medical therapy simply because it is less expensive. A medication called methimazole (tapazole or felimazole) is given once or twice daily. This is a very small pill that is given. Your cat will need this medication for the rest of his or her life.

About 2-4 weeks after starting the medication your vet will do another T4 blood level to determine if we are giving enough (or too much) medication. Once the T4 level is where we want it to be your vet will want to do the T4 test either every 3, 6 or 12 months. It is important to follow your vet's advice on doing this testing because cats who have abnormal T4 levels can have serious health problems as a result.

Your vet may also want to do some additional tests on your cat's kidneys as the thyroid level starts to stabilize. Some cats will develop kidney disease as the thyroid level comes back to normal. Your vet may also want to check your cat's blood pressure and check his or her heart periodically. Cats with hyperthyroidism often have heart disease or high blood pressure. If this goes untreated, the cat can develop a condition called retinal detachment which can cause permanent blindness. It is also very uncomfortable for a cat to have high blood pressure. It is theorized that some geriatric cats who howl at night time do so because they have a headache that is caused by high blood pressure!

Hill's y/d

Introduced in the fall of 2011, Hill's y/d may be a revolutionary treatment for hyperthyroidism. This treatment is discussed here.

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(Dr. Marie does not answer questions via the comments section, though!)

Dr. MarieDr. Marie is a veterinarian who practices in a busy animal hospital in Ottawa, Ontario. She created Ask A Vet Question as a resource for good, accurate veterinary advice online. Dr. Marie treats dogs, cats, hamsters, guinea pigs, and rats. She has been a vet since 1999.

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