A thyroid problem has been diagnosed in your cat. In this letter we want to give you the necessary explanation about this condition and also discuss the different treatment methods with you.

The thyroid gland

The thyroid gland is a small organ that consists of 2 thyroid lobes. As you can see in the picture, the lobes are in the neck of the cat, to the right and to the left of the trachea. The thyroid gland produces thyroid hormones which have a stimulating effect on the metabolism. During the adolescence of the cat these hormones are also important for the growth and development of the cells. Above the thyroid gland there are 2 parathyroid glands, these are responsible, among other things, for the regulation of the calcium metabolism.

A cats thyroid gland
A cats thyroid gland

  1. Normal thyroid lobe
  2. Parathyroid gland
  3. Parathyroid gland
  4. Enlarged thyroid lobe

What is hyperthyroidism

Hyperthyroidism is a condition that usually occurs in older cats. In this condition there is an overly active thyroid gland, often 1 or both thyroid lobes are enlarged. In 98% of cases hyperthyroidism is caused by a benign thyroid tumor. In the remaining 2% there is a malignant thyroid tumor. When a cat has hyperthyroidism, too much thyroid hormone is produced, which causes a metabolism that works too fast. Due to this, the cat's energy combustion will increase and the energy requirement will also be higher. In many cases we see that cats start eating more to meet their energy needs. Despite the fact that they are going to eat more, it is still possible that they do not get enough energy and you can see that the cats can even lose weight. Losing weight despite a good appetite is a typical symptom we see in a cat with hyperthyreodism. Other symptoms may include: more drinking and urinating; restlessness; increased activity; increased heart rate; poorly groomed hair; diarrhea and / or vomiting. Every cat is unique and therefore the symptoms can also differ per cat. It does not mean that your cat must show all of the symptoms meantioned above to have hyperthyriodism.


A cat with hyperthyroidism should always be treated. Hyperthyroidism causes a heavy burden on organs such as the heart and kidneys. If no treatment is started, the cat will eventually die due to exhaustion and / or heart failure. Fortunately, there are several options for successful treatment:

  • Medication
  • Special food
  • Surgical removal of the thyroid gland
  • Treatment with radioactive iodine

The operation and treatment with radioactive iodine are seen as "permanent" treatments and are curative. Something is done about the cause of the problem. The medication and food are not "permanent" treatments and only take care of the symptoms but do not 'cure' the problem. The production of excess thyroid hormone is slowed down, but the cause (usually a benign thyroid tumor) is not treated. There has been a study showing that after 6 years 25% of thyroid tumors have become malignant. This can also happen if you treat with medication or thyroid diet.

All options are explained below. What is important to remember is that every cat and owner are different and so everyone will prefer a different treatment. There is no best solution. A number of factors that can be included in the choice of treatment are: the age of the cat; if pills can be given to the cat; if the cat goes outdoor and if there are other cats in the house; any other physical problems such as kidney failure; the costs. The vet will discuss the best treatment for your cat in consultation with you.


Treatment with medication means that the cat will have to be given tablets twice a day. These tablets block the production of thyroid hormones and must be given for life. If the cat can easily be given tablets to, this is a fine solution. The effect of the tablets is best if they remain intact. It is therefore better not to crush the tablets. There is also an ear ointment available for cats that really can not be given tablets to. The disadvantage of medication is that in some cats, side effects may arise. Most common side effects are: vomiting, diarrhea, poor appetite and skin problems. After the diagnosis of hyperthyroidism has been made, the medication is started. After 3 weeks we will schedule a check with the vet. During this check, attention is paid to the weight, the heartbeat is listened to and the blood test is repeated. Via the blood test we can see whether the level of thyroid hormone has dropped sufficiently and whether the medication dosage is good or not. We also look at the kidney levels. In the beginning, it may take some time to properly set up the medication and multiple checks may be needed to determine the correct dose of the medication. Once the cat is properly adjusted, we recommend that you schedule a check every six months to monitor the thyroid hormone and kidney levels in the blood. The reason that we also measure the kidney levels during the blood test is because hyperthyriodism can mask a kidney problem. The overactive thyroid gland works kind of like doping for the kidneys. As long as the level of thyroid hormone in the blood is high, we can’t make a correct statement about the condition of the kidneys. When the level of thyroid hormones in the blood is normal we can see how the kidneys are actually doing.

As soon as hyperthyroidism is diagnosed during a consultation with the vet we want to start treatment as soon as possible. The fastest way to do this is by starting with medication. Before starting a "permanent" treatment, we also start with medication. We first want to lower the level of thyroid hormones so that we can see what the actual condition of the kidneys is. Because we start the cat on medication, treatment is started immediately and you as the owner have the time to choose which treatment you prefer. The cat will get in a better condition in the meantime which lowers the risks for a possible operation.

Hill's Y/D diet

The Y/D diet of Hill's is a iodine restricted diet. Iodine is required for the production of thyroid hormone. If we give iodine-restricted nutrition, the production of thyroid hormone will be inhibited. After 6-8 weeks, the effect of the food is optimal. The advantage is that it is easy to administer, after all it’s just food. The disadvantage is that the cat really can’t eat anything else so he must like this food. For cats who go outside it’s not convenient, the chance that they will eat something when they’re outdoors is great. As soon as they eat something else, they will consume extra iodine, which ensures the production of extra thyroid hormone. This will cause the complaints to come back. Another disadvantage is that, unlike with the medication, you can’t change the dosage of the diet. If the level of thyroid hormone is still too high after a few weeks of medication, we can increase the dose of medication. If, after a few weeks of iodine-restricted diet, the level of thyroid hormone is still too high, we can’t change anything. There is no point in giving more food. The diet can only lower the level of thyroid hormone to a certain amount, this is different for each cat. It is therefore possible that after a few weeks of feeding we perform the blood test and it turns out that the level of thyroid hormone is not low enough, then another treatment has to be chosen. If on the other hand the level drops enough it doesn’t necessarily mean that this will go well for life. As mentioned earlier, a treatment with food will do nothing to the cause of the problem. The thyroid gland can continue to grow as time goes by, so that the level of thyroid hormone in the blood will rise again.


In surgery, the thyroid gland will be removed by us. By removing the thyroid, the symptoms will disappear. There are a number of things that need to be taken into account. We remove both thyroid lobes but leave the parathyroid glands. Sometimes we see a temporary reduction of calcium after the operation due to reduced blood circulation in the parathyroid glands. In that case, we need to give extra calcium and vitamin D. In a very few cases it can happen that the parathyroid glands are permanently damaged by the operation. In that case, lifelong extra calcium and vitamin D will have to be given. There may also be ectopic thyroid tissue in the body that can’t be surgically removed. Ectopic thyroid tissue is thyroid tissue that can be found elsewhere in the body, for example in the chest cavity. This tissue can also be overactive and cause hyperthyroidism. It would be best to have a scan (scintigraphy) performed prior to the surgery to see if there is ectopic thyroid tissue in the body. There is only one clinic in the Netherlands where this scintigraphy can be performed and it has to be done under anesthesia. If there is ectopic thyroid tissue this can become overactive after a while (if it’s not overactive already) and the cat will develop the same symptoms as before the operation. Lastly, it is an operation, so there is always a narcotic risk.

Radioactive iodine

About the treatment with radioactive iodine we have an extensive explanation for you from Gent University. This is not a treatment that can be done in our clinic. The treatment is possible in one clinic in the Netherlands and one clinic in Belgium. In short, the treatment means that your cat will be admitted for 5 days. Radioactive iodine is administered directly into the bloodstream via a catheter. In this way, the possible ectopic thyroid tissue is also treated (which is not possible during an operation). The radioactive iodine will destroy the abnormal tumoral cells that cause hyperthyroidism. The radioactivity is the reason that your cat will have to stay in the clinic for 5 days. After returning home, there are also some precautionary measures that need to be taken.

What is not in the information of Gent University but important to mention is that, as with any other treatment, there is never a 100% guarantee that it will work. In about 2% of cases one injection is not sufficient, a second injection is often effective. Yet in 1.5% of cases, the treatment does not work, which means that the level of thyroid hormone remains high. It can also happen that a cat develops hypothyroidism after treatment. This is the opposite of hyperthyroidism and means that the thyroid works too slowly. This can happen to 3-8% of the cats. If this happens, the cat will still need tablets for this. Although in that case it seems like the treatment has been for nothing because the cat still has to be given tablets, the tablets for this treatment are not medicine but it’s a hormone. The chance of side effects of this hormone are considerably smaller than the chance of side effects of the medication against hyperthyriodism. The hormone also exists in syrup form.


As mentioned earlier, there are a number of factors that can be included in the choice of treatment. Cats can become very old nowadays, 20 years of age is no exception anymore. If your cat is still relatively young once the disease hyperthyroidism is diagnosed it is definitely worth to consider the "permanent" treatments; surgery or treatment with radioactive iodine. Although the costs are high initially, this is often cheaper in the long term than buying medication for years and having blood tests done for the cat. With a non-permanent treatment of medication or food, the benign thyroid tumor can also continue to grow, which means that the medication will have to be increased or the diet no longer works. As time passes, the benign thyroid tumor may even become malignant.

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