Hyperthyroidism in Cats: Everything You Need to Know

Hyperthyroidism is caused by pathological changes in the thyroid gland (thyroid), which leads to elevated levels of the thyroid hormone T4, which controls the metabolism.

In the vast majority of cases, the underlying cause of hyperthyroidism in cats is a benign thyroid tumor. In about 1-2% of cases, the tumor is malignant. Cats over the age of 8 are usually affected, but the disease can also occur in younger individuals.

Hyperthyroidism is the most common metabolic disease in older cats, and domestic cats are at slightly greater risk of developing the disease than purebred cats.

The disease worsens over time if left untreated. With proper treatment, the symptoms can often be managed and the cat can live a normal life.


The metabolic hormone T4 is an important hormone, which affects many of the body’s functions and organs. An excess of the metabolic hormone can cause the following symptoms:

  • Weight loss despite unchanged or increased appetite
  • Increased thirst and urination
  • Changed fur quality
  • Changed behavior (often restlessness / increased activity level, but sometimes it can also be seen that the cat likes to walk away and become less social)
  • Vomiting and diarrhea


The veterinarian may suspect hyperthyroidism based on the cat’s medical history and symptoms at the clinical examination. The diagnosis is made through a simple blood test, where the level of the metabolic hormone is analyzed.

To rule out other causes of the disease, other blood tests are usually performed. In individual patients, the level of metabolic hormone may be very close to normal, and in these cases further studies may be needed.

It is not uncommon for the heart to be affected by the increased amount of hormones. If the clinical examination shows that the heart is affected, it may be relevant with further examinations with, among other things, ultrasound.


There are currently four effective treatment methods:


There is a special veterinary feed with a very low content of iodine. Because iodine is one of the components of the metabolic hormone, the reduced supply means that the body can no longer produce such large amounts of the hormone. This method of treatment requires that the cat does not eat anything other than this food.


Thyroid hormone levels can be kept in check with medical treatment. In that case, individual adjustment of the dose is required, but with the right dosage, the treatment is relatively safe and the cat can eat what it wants. The medicine may cause some side effects in a small proportion of patients, usually in the form of vomiting, decreased appetite, listlessness or itching. The medicine is given in tablet form, 1-2 times / day and the treatment is lifelong.


The diseased part of the thyroid gland can be surgically removed. In some cases, a new lump appears in the remaining thyroid tissue, so that the symptoms return. There is also a risk that the cat will have too low production of thyroid hormone, which in that case may need to be treated medically. ¨

Radioactive iodine

Radioactive iodine can be injected subcutaneously, after which it accumulates in the thyroid gland and destroys the diseased tissue. The method is usually successful, but requires admission to an animal hospital that has approval to perform the procedure.

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