Wednesday, March 16, 2016

Feline Hyperthyroidism

Feline hyperthyroidism graphic has a white cat and says, "Feline Hyperthyroidism: Symptoms, Diagnosis, & Treatment."
Feline hyperthyroidism is a common endocrine disorder that occurs when a cat's thyroid produces excess thyroid hormones, mainly T3 and T4.  It usually presents in middle-aged or older cats, and there are no differences among gender or breed in terms of prevalence of the disease.

According to Cornell University, 98% of hyperthyroidism is caused by an adenoma, a non-cancerous tumor on the thyroid gland.  In only two percent of cases, hyperthyroidism is caused by a carcinoma, a cancerous tumor on the thyroid gland.

Because thyroid hormones affect many organs in the body, secondary conditions may occur as a result of feline hyperthyroidism.  For instance, excess thyroid hormones can cause the heart to beat faster.  Over time, cats with hyperthyroidism can develop an enlargement and thickening of the heart's left ventricle.  If left untreated, these changes can affect the normal functioning of the heart and may even lead to heart failure.  Fortunately, once the hyperthyroidism has been treated, the changes to the heart will often improve or resolve completely.

Additionally, cats with hyperthyroidism may also develop hypertension (high blood pressure).  Hypertension can cause damage to the kidneys, eyes, heart, and brain.  If a cat has hypertension in addition to an overactive thyroid, medication may be prescribed to manage the blood pressure.  After the hyperthyroidism has been treated, blood pressure issues will often resolve, and medication to treat hypertension may no longer be necessary.


Because thyroid hormones affect the functioning of many of the organ systems within the body, symptoms of feline hyperthyroidism may vary.  A cat may experience only one or two of the symptoms of hyperthyroidism.  On the other hand, a cat may experience many or all of the symptoms.  Symptoms of feline hyperthyroidism include:
  • Weight loss.
  • Increased appetite.
  • Nervousness or anxiety.
  • Increase in activity level or restlessness.
  • Increased thirst.
  • Increased urination.
  • Poor coat condition.
  • Diarrhea.
  • Muscle weakness.
  • Increased respiration rate.
  • Difficulty breathing.
  • Increased heart rate.
Occasionally, cats with an overactive thyroid may experience decreased appetite or depression as well.  

Signs of the disease may be subtle at first but usually become more prominent as the disease progresses.  


A diagnosis of hyperthyroidism is typically made when thyroid hormone levels in a cat's blood are high.  However, occasionally cats exhibiting symptoms of hyperthyroidism have normal hormone levels.  In these cases, a test called a T3 suppression test may be performed.  

If a diagnosis cannot be made from blood tests, a nuclear medicine scan may be ordered.  This test is done at certain specialty veterinary clinics.  A small dose of a radioactive compound is given to the cat.  The compound travels through the bloodstream to the thyroid.  Overactive thyroids accumulate more of the compound than do normally functioning thyroids.  After the test is complete, the kitty must remain in the hospital for a few days until the compound has cleared its body.  

Other blood tests and a urinalysis are also usually done to check the functioning of the other organs in the body as well.  


There are three options for treating feline hyperthyroidism:
  • Medication.
  • Surgery.
  • Radioactive iodine therapy.
Each treatment option has its own advantages and disadvantages, and a cat's specific circumstances will often dictate which treatment is best for him or her.

Medication: Methimazole (Tapizole) is the medication used to treat feline hyperthyroidism.  It works by reducing the production and release of thyroid hormone from the thyroid gland.  The medication must be given one to three times a day and typically takes several weeks to reduce the hormone levels in the blood.  

Methimazole may have side effects including depression, lack of appetite (anorexia), vomiting, and fever.  These side effects may resolve without having to stop the treatment.  A more serious side effect is anemia.  If this side effect occurs, it typically happens within the first three months of treatment.  If anemia occurs, the medication is usually stopped and another treatment option will need to be considered.

Methimazole is relatively inexpensive and readily available.  Some owners find pilling their cat difficult or even impossible.  In these cases, owners can get the medication compounded into liquid, oral paste, chewable tablets, medi-melts, chewable treats, or transdermal gel.  While getting the medication compounded is more expensive, it's a great option for those who cannot pill their kitties.

Methimazole must be given for life.  If the medication is stopped and another treatment option isn't pursued, a cat's thyroid hormone levels will return to being too high.  Routine blood tests will need to be performed periodically to check a cat's thyroid levels to make sure the medication is effective and to check for potential side effects.

Surgical Removal: Surgical thyroidectomy, removal of the thyroid glands, is a relatively straightforward procedure with good success rates.  The advantage of surgically removing the thyroid glands is that it produces a permanent cure in most cats.  

As with any surgery, though, there are risks to a thyroidectomy.  Damage to the parathyroid glands may occur, for instance.  The parathyroid glands are clse to or within the thyroid glands and are critical to maintaining proper blood calcium levels.  Damage to the parathyroid glands may mean that a cat needs to take regular calcium or vitamin D supplements to maintain proper blood calcium levels.  

Additionally, damage to the nerves near the thyroid glands may occur during surgery.  Nerve damage may result in abnormal pupil size in the eyes and droopy eyelids.  Voice box damage, resulting in a change in a cat's voice, may also occur.  

Some cats who have a thyroidectomy develop hypothyroidism (abnormally low thyroid levels) and must take thyroid pills.  On the other hand, a few cats who have the surgery remain hyperthyroid.  In these cases, there are thyroid cells in abnormal places in the cat's body, such as in the chest, where surgical removal is difficult.  Thyroid tissue in other parts of the body is known as ectopic thyroid.  If an owner and veterinarian determine surgery is the best treatment option for a cat with hyperthyroidism, a nuclear medicine scan may be performed before surgery to check for ectopic thyroid.  If ectopic thyroid is found in the kitty's body, surgery probably isn't the best treatment option.  Additionally, surgery may not be the best treatment option for cats with other conditions such as kidney disease or heart disease.

Radioactive Iodine Therapy: Radioactive iodine therapy is only performed at specialty veterinary clinics.  It is usually very effective and rarely causes cats to develop hypothyroidism.  In radioactive iodine therapy, radioactive iodine is administered to the kitty intravenously.  The radioactive iodine destroys abnormal thyroid cells while leaving normal thyroid cells in tact.  It does not affect any of the cat's other organs.

Radioactive iodine therapy is curative in approximately 95% of hyperthyroid cases and has no side effects.  The procedure does not require anesthesia.  However, it can only be performed at clinics that have a license to utilize radioisotopes.  Radioactive iodine therapy has no significant risks for cats.  However, humans who have close contact with the kitty must take precautionary measures.  A cat must remain in the hospital until his or her radiation level has dropped within acceptable limits.  This means that a kitty receiving radioactive iodine therapy may need to be hospitalized for one to three weeks.

For the few cats who remain hyperthyroid after radioactive iodine therapy, the treatment can be repeated.  In rare cases, cats may become hypothyroid after this type of therapy.  In these cases, thyroid hormone replacement may be necessary.

If you notice any of the symptoms of hyperthyroidism in your cat, please take him or her to the vet for an exam, blood tests, and a urinalysis.  If your kitty has this common endocrine disorder, you and your vet can determine which treatment option would be best for your companion.


Cornell University Hospital for Animals: Hyperthyroidism in Cats.

Washington State University College of Veterinary Medicine: Hyperthyroidism in the Cat.

Cornell University College of Veterinary Medicine: Hyperthyroidism in Cats.


  1. This is much more common than people think. My Golden Retriever and I even took the same type of medicine for our hyperthyroidism!

  2. This is the most thorough coverage of information about this common disease that I have ever seen. Thank you so much. Hugs, Janet

  3. Wow, what an informative post! There is a cats-only clinic near home that performs the RIT; I've always filed that away in the back of my head...just in case.

  4. Excellent post. The kitty before me had thyroid problems. M forgets now is she was hyper or hypo, but the problem was easily solved when we put her on the proper medication. It's important to get it diagnosed early tho if you can.

  5. It does seem to be awfully common! I'm just glad Binga doesn't have it.

  6. Excellent post! Great information to have. Pinning!

  7. Thanks for this great information about this condition! Fortunately none of my cats has ever had thyroid problems, but it's always good to watch out for the symptoms.

  8. It seems to be very common. Though my cat's never had such symptoms and diagnosis of course. Though thank you a lot for sharing this really useful article.

  9. This is so very important. We have had a few who suffered this, and a couple who were hypothyroid. Thank you.

  10. That was really a great bunch of useful info gang, thanks for the terrific post!

  11. We haven't had to deal with this- and hope not to! Thanks for posting such a very thorough explanation.

  12. Excellent post,I am printing this one out. I
    have 3 cats on thyroid meds and didn't know all this.

  13. I always learn something new when I come to your blog! My cat is due for a vet visit and I think he is drinking a little more than usual. I just made a note to ask her about that.

  14. I always learn something new when I come to your blog! My cat is due for a vet visit and I think he is drinking a little more than usual. I just made a note to ask her about that.

  15. I always learn something new when I come to your blog! My cat is due for a vet visit and I think he is drinking a little more than usual. I just made a note to ask her about that.

  16. Very good information here. Thank you for the great post!

  17. Thanks fur the good info.Years and years ago we had a kitty with this trouble...

  18. What a great, informative post! I used to work at a cat clinic, and hyperthyroidism was by far one of the most popular diseases we saw. Thank you for sharing this! Purrs!

  19. Methimazole has some very serious side effects that you didn't mention, it almost killed Socks.

    You also failed to mention that Hyperthyroidism can be controlled with a Y/D diet. That is what Socks is now on and his blood work has been very good. I just wish more brands offered a Y/D food, as Hills Science Diet is the only Y/D food available.

  20. Great posty. Fank you fur all yous luv and support.

    Luv ya'


  21. What an incredible job writing and researching this subject. Thank you for sharing with all your cat loving readers. Purrs from Deb and the Zee/Zoey gang

  22. This is such an informative post, and this is such important stuff to know. Thanks for making us aware.