Feline Hyperthyroidism

Hyperthyroidism is the condition when the thyroid gland overproduces active thyroid hormone. It is one of the most common hormone imbalances seen in older cats (the average age of diagnosis is 13 years old).


What is feline hyperthyroidism?

Hyperthyroidism in cats is usually caused by a benign growth in the thyroid gland that causes it to overproduce T4 (thyroxine). It is important to realize that these tumors are almost always benign. Less than 3 – 5 % of hyperthyroid cats have a cancerous thyroid growth.

Overview of how the body regulates thyroid hormones 

  • T3 is the active form of the hormone responsible for metabolism regulation.
  • T4 is the precursor of T3.
  • The T4 that is released into the body is circulates as bound T4 and free T4.
  • Bound T4 is carried by blood proteins and is not available for tissue absorption.
  • Free T4 is absorbed by the tissues and is converted into T3.


Hyperthyroidism is a disease with a classical set of symptoms. The major complaint noted from owners with cats diagnosed with hyperthyroidism is weight loss despite an excellent appetite. Hyperthyroid cats may also show signs of increased thirst and water drinking. In non-hyperthyroid cats, the lobes of the thyroid gland cannot be felt with your fingers. In the hyperthyroid cat, at least one lobe is usually prominent and may be detected by the veterinarian during a physical exam. Chronic, intermittent vomiting or diarrhea may also be noted by owners with cats with un-diagnosed hyperthyroidism, however this is a vague clinical sign not exclusive to hyperthyroidism.



If hyperthyroidism is suspected, diagnosis is done by blood testing. It will be recommended that we run a full wellness panel, plus a regular T4 level to start. The wellness panel will run a full chemistry screen (which will check your cat’s liver, kidney and pancreas function and scans for signs of diabetes) it also looks at the blood cells for signs of infection, disease, anemia and hydration status. The T4 test is the initial thyroid specific test that will be performed. Other thyroid tests may be recommended on an as needed basis to continue with the diagnosis and determining the best course for treatment.

Regular or bound T4 (thyroxine) level testing

Since a cat with hyperthyroidism is over producing levels of T4, elevated T4 levels on bloodwork can indicate hyperthyroidism. If T4 levels are significantly elevated, the diagnosis of hyperthyroidism can be made. Some cats will have borderline results (T4 levels only slightly increased), in these cases further testing is required to definitively diagnose hyperthyroidism.

Free T4 (FT4) by equilibrium dialysis testing

As previously mentioned T4 exists in two forms: bound T4 and free T4. Only free T4 enters the cells and is converted to T3. Therefore, the concentration of free T4 corresponds to thyroid hormone activity at the cellular level. Free T4 level testing is more sensitive for detecting hyperthyroidism and it is also less likely to be affected by other illness interference (which can happen with regular T4 testing). An elevated free T4 accompanied with an elevated or borderline regular T4 is diagnostic of hyperthyroidism.

T3 testing (triiodothyronine) level

T3 is the active thyroid hormone, but unfortunately due to normal body biochemical mechanisms, the levels of T3 fluctuate a lot in the body. T3 levels are usually elevated in hyperthyroid cats, but this test is not routinely used to diagnose hyperthyroidism. It may be done as part of a larger a thyroid panel, so that it can interpreted along with other thyroid specific tests when diagnosing your cat.


Thyroid Scintigraphy (nuclear medicine thyroid scan)

Thyroid scintigraphy can provide valuable information regarding your cat’s thyroid anatomy and physiology. This type of thyroid imaging is so sensitive that it can diagnose hyperthyroidism long before blood T4 levels become elevated. Cats are administered a short-acting radioactive isotope that concentrates in thyroid tissue, then a scanning procedure is performed to image their thyroid gland. Even the smallest of thyroid tumors can be seen on the imaging. The downside to this testing is that it is not readily available and can only be preformed at a limited group of radiotherapy facilities.


There are a few different treatment options available to treat feline hyperthyroidism.

Methimazole treatment and monitoring 


The most common medication prescribed to treat feline hyperthyroidism is called methimazole. This medication blocks the production of T4 and T3. Once the medication has taken full effect in the system T4 levels will begin to lower as the body stops overproducing it. This medication is available in different preparations to ease administration to your cat. It is commercially available in a tablet form, but can also be compounded into a liquid, a flavored chewable tablet or a transdermal gel. Medication is usually given once or twice daily depending on your cat’s response to the medication and the level of control needed.

When treatment is started on your cat, T4 levels will need to be checked in about 2 weeks to make sure the dosing is correct. If the methimazole dose is adjusted, T4 levels will again need to be checked in 2 weeks. Once your cat is stabilized and blood work is showing good control of T4 levels you will need to continue with the prescribed dose of methimazole for the rest of your cat’s life. Blood work will need to be done periodically for the rest of their life as well. T4 levels will need to be done every 3-12 months, as recommended by the veterinarian to ensure T4 levels remain controlled and dosing does not need changing. A full wellness panel may also be recommended once a year to monitor your cat’s overall health.

Dietary treatment and management with Hill’s thyroid care y/d diet


Hill’s Prescription Diet has developed a food to manage hyperthyroidism. This food is called thyroid care y/d diet. The food is not medicated, but is an iodine-deficient food. It works to treat and manage hyperthyroidism by not providing your cat with the necessary dietary iodine to overproduce thyroid hormones. In order to work this food must be fed exclusively to your cat; no treats, other cat food or table food can be given to your cat (this can be difficult in multi-cat households as this diet is not for cats who are not hyperthyroid). Other medications and supplements (especially colored or flavored ones, or ones containing fish or shellfish ingredients) may also need to be re-evaluated, as they may be a source of dietary iodine. To treat hyperthyroidism with this dietary therapy, any current thyroid medication must be weaned off while slowly transitioning your cat to this diet.

As with methimazole treatment, blood work is needed when starting your cat on this diet. T4 levels should also be checked at 4, 8 and 12 weeks after transition to this diet is completed. If the T4 levels are controlled, then this therapy might be a good fit for your cat. Long term blood testing is also needed once stabilized. Blood work (either a T4 level or a full wellness panel) will be needed every 3-12 months depending on recommendations of the veterinarian. Follow up physical exams and blood pressure monitoring a couple times a year may also be needed while on this diet.

Radioiodine therapy

This option is considered the gold standard when it comes to treating hyperthyroidism. This procedure cures hyperthyroidism in 90-99% of cats, meaning after the procedure is completed, no further treatment or management is needed. This treatment is done at specialty centers that are equipped with the proper facilities for handling radioactive iodine (I-131).  First, they must make sure your cat is a good candidate for this treatment, this screening process will include a physical exam, blood work, x-rays and in some cases a thyroid scintigraphy scan. Once your cat is deemed an appropriate candidate for this procedure they are given a dose of I-131 under the skin and are quarantined at the facility for 3-7 days, as they wait for the radiation levels to decline. When injected with I-131 the abnormal thyroid tissue takes it up and is then destroyed by the radiation, decreasing the amount of hormone released. Excess I-131 is then excreted by the kidneys into the urine (hence the quarantine time). Once the T4 levels drop below the normal range, the remaining normal thyroid cells are stimulated to become active again and they take over normal production.

Surgical treatment

Surgical removal of the thyroid gland is possible in hyperthyroid cats since these thyroid tumors are usually benign and well-encapsulated. Surgery will be performed at a specialty center. A referral/screening process is in place to ensure your cat is at a low anesthetic risk and is overall a good candidate for this procedure. This screening process will include a physical exam, blood work, x-rays and in some cases a thyroid scintigraphy scan. This surgical procedure generally has a good success rate and is curative as long as the entire tumor has been removed (if the entire tumor is not removed, hyperthyroidism will usually return within 6-24 months). Successful surgery eliminates the need for long-term treatment and monitoring. A known risk with this procedure is the potential to damage the parathyroid glands, which are located behind the thyroid glands. The parathyroid glands are responsible for maintaining blood-calcium levels, and if damaged low blood calcium can result.