Information paper - Thyroid gland in cats that works too fast

What is a thyroid problem?

The thyroid gland consists of 2 lobes, which lie on either side of the windpipe in the throat area. The main task of the thyroid is to produce thyroid hormone, also called T4. This hormone regulates the speed of metabolism and thus the rate of consumption of nutrients and energy.

In cats we usually see an overproduction of thyroid hormone, this condition is called hyperthyroidism. The thyroid gland then works too fast and causes a faster metabolism. Hyperthyroidism occurs mainly in older cats, 8 years and older. It is the most common metabolic disorder in cats.

What is the cause?

Overproduction of thyroid hormone is caused by a benign tumour or hyperplasia in 98% of cases, only in 2% by a malignant tumour. 

The underproduction of the hormone is very rare in cats, but we see it more often in dogs. In about 70% of cases both lobes of the thyroid gland are hyperactive.

What are the symptoms of hyperthyroidism?

  • Losing weight, despite a large appetite
  • Drinking and urinating a lot
  • Vomiting (often from eating too quickly) and diarrhoea
  • Behavioural changes: hyperactivity, aggression, meowing a lot, sleeping in other places, often lying on cold surfaces
  • Faster heartbeat, sometimes development of a heart murmur

In some cases, other symptoms occur, which are more consistent with apathetic hyperthyroidism. In these cases the appetite is reduced, the cat becomes sluggish and shows muscle weakness.

What is the consequence of hyperthyroidism?

Because the metabolism increases in speed, all organs in the body have to work faster. As a result, ‘wear and tear’ complaints may arise in other organs.

  • The faster beating of the heart leads to an overload of the heart. The heart muscle can become thickened as a result (hypertrophic cardiomyopathy).
  • Increased blood pressure may occur. This rise in blood pressure can cause complications in the form of eye problems, kidney failure and/or brain problems.
  • The kidneys are overburdened. The kidneys are already sensitive to wear and tear in older cats, and the extra load increases the risk of developing kidney failure.
  • The liver is also burdened by the increased metabolism, we often see an increase in liver enzymes in the blood. Often the liver normalises again after the start of treatment.

How do we make the diagnosis?

By means of a blood test the total amount of thyroid hormone (total T4) can be determined. We recommend also determining the kidney and liver values. If there are indications of reduced kidney function, we also check the urine.

Sometimes an enlarged thyroid gland can be felt. A blood test is then necessary to confirm the diagnosis, as there are also thyroid tumours that do not produce extra hormones.

How do we treat hyperthyroidism?

There are 3 different therapy options:

  1. Medicinal treatment
    The overproduction of thyroid hormone can be inhibited by medication (Felimazole©). The pet must take these twice a day for the rest of its life. The dosage of the medication is determined over the course of several weeks, depending on the measured T4 value in the blood and any complications present.

    The disadvantage of the drug treatment is that it does not offer a permanent cure. Furthermore, cats may show side effects from the drug. The main side effects are anorexia, skin problems, bone marrow depression and the progression of renal insufficiency.

    To check for these side effects we recommend a blood test one month after starting treatment. As a follow-up to treatment, we recommend a check-up twice a year. During this check-up, we perform an extensive physical examination, check the parameters in the blood and take a blood pressure measurement.

    We also regularly use the medication to stabilise cats for the next therapy options:


  2. Surgical treatment
    The hyperactive lobe of the thyroid gland can be surgically removed. The advantage is that no more hormones can be produced by the tumour. The disadvantage is that it is a major operation. Furthermore, in 70% of cases, both thyroid lobes overproduce.

    Another disadvantage is that there is another organ very close to both thyroid lobes: the parathyroid gland. The parathyroid gland is a few millimetres in size and has an important function in calcium management. During an operation, the parathyroid gland can be damaged or removed, causing the parathyroid gland’s hormone production to cease. This is certainly a risk in the case of bilateral removal. After the operation, the calcium level in the blood must therefore be monitored. If there is a calcium deficiency, this must be remedied by medication, sometimes for life.

    It can also happen that thyroid tissue is present in the chest cavity, also called ectopic thyroid tissue. The ectopic thyroid gland can also be hyperactive. It is therefore necessary to know where hyperactive tissue is present before surgery is performed. The only way to visualise this is by means of a scan (scintigraphy) at a specialist clinic.


  3. Treatment with radioactive iodine isotopes
    Radioactive iodine is administered via an infusion. It is only absorbed by the hyperactive thyroid tissue and destroys it; the healthy thyroid tissue is not destroyed. It gives a good result in about 95% of sick cats. The advantage is that after this procedure the pet no longer needs medication. In addition, no anaesthesia is necessary and there is no risk of damage to the parathyroid glands.

    The disadvantage of this procedure is that due to the use of a radioactive substance, the animal must be hospitalized for a week in a specially isolated room. This treatment is performed in the Netherlands at the animal clinic De Lingehoeve in Lienden.

Which treatment do we choose?

The choice depends on a number of factors, such as age, whether a cat takes medication easily, the general physical condition of the cat, the kidney function and the cost of the treatments. We will go through the options with you to choose the best possible therapy.

If you have any further questions regarding this information letter, please do not hesitate to contact us.

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