Hyperthyroid cats

What is hyperthyroidism ?

The thyroid glands of the cat are 2 small glands just along the trachea in the neck. They produce thyroid hormones, which regulate the cat's metabolism.

One of the most common hormonal problems in aging cats is the overproduction of thyroid hormone. In most cases this is caused by a benign tumor (adenoma). In the contrary to dogs, malignant tumors of the thyroid gland rarely occur in cats. However, the overproduction of thyroid hormone must be treated, as it can have harmful effects on the cat.

The most common symptoms are increased appetite (begging or even stealing food) and weight loss. Because the cat is still eating well, she does not seem to be sick right away. Because of the hyperthyroidism, however, she needs much more energy, which the cat tries to compensate by eating more. If this is not enough, she will 'burn' her fat reserves and eventually also the muscles as an energy source.

Other signs that regularly occur are more frequent vomiting, more drinking and consequently more urination, more attention, more restless or even more aggressive behavior, a high heart rate and / or a poorly groomed coat.

None of these symptoms are exclusive to hyperthyroidism, however, and can also be caused by other diseases. Also, absence of these symptoms does not mean that the cat can not have hyperthyroidism. A minority of cats exhibit atypical symptoms, such as reduced appetite or slumber and suffer.

A visit to the vet is therefore necessary for the diagnosis of hyperthyroidism.


What are the treatment options for hyperthyroidism in the cat?


1. Thyroid-inhibiting medication (Felimazole, Thiapheline, Carbimazole, ...):

Thyroid inhibiting medication can offer a solution for many cats. In order to obtain optimal effect of the medication, you should give the medication 1 or 2 times a day, for the rest of their life. Some cats do not allow this well, or spew out the medication later, so this treatment will not work properly.

Moreover, there can also be severe or less severe side effects. This usually occurs within the first weeks to months of therapy. The most common side effects are severe itching, especially at the head, or gastrointestinal complaints (frequent vomiting, loss of appetite, diarrhea). After stopping the thyroid inhibitors these symptoms disappear fairly quickly. Worse side effects may affect red and white blood cells, or cause liver problems or bone marrow suppression.

How does the medication work?

Thyroid-inhibiting medication will not cure the thyroid gland that works too fast, but will ensure that hormone production remains under control. It is important to make this distinction because the condition can spread over time, which means that you have to check regularly to see if you are giving the correct dose. This can be done on the basis of a blood test. In addition, the thyroid can increase in size. In some cats it will become so large that it will press the trachea or esophagus and cause coughing or swallowing problems. In addition, the hyperthyroidism can evolve so far that it can no longer be controlled with medication.


2. Surgery

Surgery can be an efficient treatment, but also has disadvantages. Firstly, hyperthyroid cats are mostly cats of age, which increases the risk of anesthesia. In addition, hyperthyroidism is often associated with heart problems, which is less favorable for anesthesia.

When the two thyroid glands are removed, there is a chance that the parathyroid glands are also removed. These are necessary for controlling the calcium balance. Removal of these parathyroid glands leads to a deficiency of calcium in the blood. In that case, your cat may have to receive life-long calcium supplements, in very rare cases it is a life-threatening condition.

Finally, there is a chance that thyroid tissue remains behind after the operation, as a result of which the hyperthyroidism continues to exist. Microscopic small pieces can be left behind in the neck, and in about 10% of the patients thyroid tissue is also present in the chest, which can not be removed surgically.

Should you still consider surgery, it is highly recommended to have at least one diagnostic thyroid scan made. In this way, the exact location of the hyperactive thyroid tissue can be determined.


3. Iodine-lean diet (Hill's Prescription Y/D)

For the production of thyroid hormone, both normal and overactive thyroid glands require iodine, which is absorbed from the food. The principle of the iodine-poor diet is to limit the amount of iodine available in the diet to what is necessary to make a normal amount of thyroid hormone.

This approach will normalize the amount of thyroid hormone, but just as with thyroid-inhibiting medication, the cause of the problem is not addressed.

It is important to make this distinction, because the condition can spread over time and increase in size. In some cats it can become so large that it presses on the trachea or esophagus and cause coughing or swallowing problems.

When you choose to give this diet, it is very important that it is the only food that the cat eats. A snack in between or a candy as a treat is out of the question! Cats that come outside are often also more difficult to treat with the diet, because outside they may eat other things. Moreover, this diet can not be combined with other diet foods, such as a kidney-sparing diet. In a family with several cats it is not recommended to give the other cats of the family (without thyroid problems) this diet.


4. Treatment with radioactive iodine (radioiodine therapy)

As mentioned, the thyroid gland needs iodine to function. Normally most iodine is removed from the diet, but the thyroid gland can also pick up iodine that is injected (under the skin or via an intravenous catheter). Moreover, the thyroid makes no distinction between ordinary iodine or radioactive iodine. The thyroid glands of cats with hyperthyroidism also absorb this iodine, even to a higher degree than normal thyroid tissue. When the hyperthyroid cat receives radioactive iodine (in this case: 131I), it accumulates in the overactive thyroid gland. Here the radioactive iodine will lose its radioactivity by releasing radiation. Part of the radiation (the so-called beta particles) will be released locally, causing the affected thyroid cells to be destroyed. In this way, only the overactive thyroid tissue is treated, and the normal tissue is spared. The parathyroid glands that are outside the thyroid gland remain unharmed.

Another part of the radiation, the gamma rays, leaves the cat, and is the radiation that we as the vet or owner are most exposed to.

The part of the radioactive iodine that is not absorbed by the thyroid gland will be removed from the body via urine, stool and saliva.

The big advantage of this treatment is that no drastic action is required (such as surgery), that only a short sedation is required (for the diagnostic scan) and that for the majority of patients it is a one-time treatment (instead of daily medication). In addition, the extra or ectopic tissue (which can be located in the chest) is also treated.

The disadvantage of the treatment is that your cat has to be hospitalized for several days because of the radioactivity, and that there are also measures to follow after returning from the radioactivity (see below).


What should be done for radioiodine treatment?


The diagnosis of hyperthyroidism is made by your vet on the basis of a blood test. Some patients already have a longer history of hyperthyroidism and have already had multiple blood tests. To make the appointment as smooth as possible, these examinations and the patient file of the cat must be forwarded to us.

Which tests are needed?

  • A complete hematological and biochemical blood test with total thyroxine (TT4).
  • Preferably also a urine test, especially in cats with a suspicion of kidney problems.
  • If there is a murmur of the heart or an irregular heart rhythm is established, it is recommended to do an ultrasound of the heart. This can happen at your vets practice or with us. Heart problems are often associated with hyperthyroid. Often it is a       temporary problem, in other cases it is necessary to give (temporary) medication to support the heart.
  • An ultrasound scan of the abdomen is recommended in some patients to detect underlying problems or simultaneous other diseases. Whether or not an ultrasound scan is needed, we can partially assess it on the basis of the blood tests and the clinical examination of the cat. An ultrasound scan of the abdomen can be done at your vets practice or in our clinic.

How is it going for my cat?

In general, there are 3 different scenarios before your cat receives the radioiodine treatment.

  • No medication: You choose to have the iodine treatment carried out immediately. This is possible, for example, if the cat does not want to take the thyroid-inhibiting medication or does not tolerate it. In that case, we ask for a comprehensive blood test (see above) that is at most 6 weeks old. If it is older, a new study must be carried out. A 'pre-treatment period' with thyroid inhibitors is therefore not an obligation for radio-iodine therapy, but can be strongly recommended in some patients. This will be recommended on an individual basis.


  • After a short period of time with thyroid inhibitors: in some patients it may be advisable to give thyroid inhibitors for a short test period, for example to be able to check the kidney function with a normal amount of thyroid hormone, or to have the cat become stronger for radioiodine treatment. Practically, the trial period goes something like this:
    • The diagnosis of hyperthyroid is made by your veterinarian (extensive blood test).
    • Period of ± 3 weeks with thyroid-inhibiting medication.
    • Blood test while the cat is still receiving medication: control of total T4 and kidney values (urea and creatinine), possibly also a urine test in the 3rd week.
    • An appointment can be made for radioiodine treatment.
    • The thyroid inhibitors are stopped 10 days before the treatment.
    • If the first blood test (ie the blood test for the period with medication) is not older than 6 weeks at the time of the radioiodine treatment, there is no need for additional research. In the other case, you will have another blood test 5 days before the radioiodine treatment (hematology, biochemistry, TT4).
    • After a longer period with thyroid inhibitors: some cats get thyroid-inhibiting medication or the iodine-poor diet for a longer period of time (months or even years). If you decide to go to for the radioiodine treatment, we ask you to request the complete file from the cat at your vet and to deliver it to us.


A recent (less than 6 weeks old) blood test is needed while the cat is still receiving medication or eating diet foods, with hematology, biochemistry and TT4. On the basis of the file of the cat it will be decided whether the cat can stop 10 days with the medication / diet food, or an appropriate period will be recommended.

To make an appointment, you must provide us with the file and the blood tests of the cat in advance. No appointment can be made without this information. Use the contact form for this. 

Ask your vet to check if the file is complete.

How does radioiodine treatment work ?


  • The cat is expected at the veterinary clinic on Monday morning (8.30 to 12 am). The actual treatment takes place in the afternoon. You do not have to stay for this.
  • First of all, a diagnostic scan of the thyroid glands is made: for this the cat is injected with a radioactive substance (pertechnetate). This substance will be absorbed by the thyroid gland and 20 - 30 minutes later a scan can be made. If possible, the scan is performed without sedation or anesthesia, however some patients require an anesthesia.

The scan will give us an idea about the activity of the thyroid gland(s) and the possible presence of extra thyroid tissue, which can occur in some animals.

Scinti katExample of a scan of an overactive thyroid.










 Scinti kat 2Example of a cat with extra (ectopic) tissue that extends into the chest cavity.

  • After the diagnostic scan, the actual therapy follows: for this the cat is injected with the radioactive iodine (131I). After this injection, the catheter is removed, and the radio iodine can start its work.
  • Patients remain hospitalized until the Friday after the treatment (5 days): the radioiodin that is released from the thyroid gland or the excess radioiodine is mainly excreted via the urine, saliva and faeces. In order to absorb the largest amount of radioactive waste, the cats also stays with us for 5 days. The cats can be picked up on Friday between 9 and 12 am.

Bringing or collecting the cat outside the usual hours (Monday morning and Friday morning) can be exceptional and only after explicit agreements on this. An extra hospitalization cost will be charged.

What to do with the cat after the treatment.

After being home you need to follow some guidelines for the first two weeks:

  • Keep contact with the cat to a minimum: They can’t sleep on the bed, they can’t be on your lap for to long,… . A short stroke or cuddle is no problem at all but you have to wash your hands afterwards. 
  • There must be NO contact between the cat and pregnant women or young children. Keep the litterbox out of their reach.
  • Avoid contact with urine, defecation and saliva. Wear disposable gloves when cleaning the litterbox and wash your hands afterwards. 
  • Collect and store the contents of the litterbox of the first 2 weeks after being home and save these for 3 months (shed, garage, …). After these 3 months it may be given with the household waste. There is no radioactive risk remaining.

How do we know if the treatment succeeded?

This is done by using follow-up blood tests, which can be done by your own veterinarian.

We recommend a follow-up blood test on the 1st, 3rd and 6th month after therapy. Especially the thyroid value (Total T4) and the kidney values (urea and creatinine) are important.

You will often see improvement(s), such as healthier fur or a weight increase.

Afterwards we recommend an annual follow-up blood test. You can ask your veterinarian for a routine blood test for old cats combined with the thyroid value.

We strongly suggest to let us know the results of each blood test after the treatment (or you can ask your veterinarian to keep us posted). This data can be used to improve the therapy.


Departure to the veterinary Faculty.

What should I bring with me?

  • Cats are often picky eaters, that’s why we ask you to bring some food from home. 
  • Possible medication (different than the thyroid inhibitors) that your cat needs.
  • Some owners like to bring a blanket or basket with the scent of home. You should know we can’t give these items back because of the radioactive contamination. 
  • All information that you receive from your veterinarian (patients sheet, blood tests). We need this information in advance - see also 'checklist / contact form'.
  • If the cat has already undergone a scan in Utrecht or somewhere else, we ask you to bring the results or mail it to us.
  • Important: When you arrive at the clinic it’s important that your cat has fasted since Sunday evening (take the food away from 22pm, water is allowed).

Route description and contact details 

The route decription and contact details can be found here.