top of page

Overactive thyroid

General info

Hyperthyroidism or an overactive thyroid is the most common hormonal disease in the older cat. When a cat has an overactive thyroid gland, this is usually the result of a benign tumor in one or both thyroid glands. Rarely, hyperthyroidism is caused by a malignant tumor. Typical symptoms are weight loss, drinking and urinating a lot, a very good appetite, hyperactivity and gastrointestinal complaints. However, not all symptoms are equally pronounced in every cat. Precisely because this is such a common condition, it is often recognized at an early stage.


Orange Tabby Cat

The diagnosis is made on the basis of one or more symptoms of hyperthyroidism and a blood test in which the thyroid hormone, thyroxine (T4), has risen. Only in exceptional cases the diagnosis is more challenging and a specialist examination should be carried out.


Once diagnosed, it is important to choose the ideal treatment option for your cat. There are several options available, each with its pros and cons:

  • Low iodine diet (Hill's™ prescription diet™ y/d™ feline): the thyroid gland will produce fewer hormones since the raw material for this, iodine, is limited in the diet. However, it is very important that your cat only eats this food (no snacks, table scraps,…).

  • Thyroid inhibitors: This is a very effective treatment, which is often used. The medication is generally well tolerated by cats. A common transient side effect is the occurrence of gastrointestinal complaints. In addition, a number of rare but serious side effects can lead to treatment discontinuation, including itching of the head, bone marrow suppression, autoimmune disorders and liver problems. The medication comes in the form of tablets and syrup. An ointment is also available in some countries.

Scinti hyperthyroid cat.png

Scintigraphic scan of 4 cats with hyperthyroidism (neck and part of the chest). The thyroid glands color strongly in black.

Prior to treatment with radioactive iodine, a scintigraphic scan is performed to determine the dose of radioactive iodine.

  • Radioactive iodine: The treatment is very efficient and is definitive in 95% of cases. Radioactive iodine not only suppresses the disease, but also tackles the present tumor (unlike low iodine diet and thyroid inhibitors). In most cases, no medication should be given after treatment. In rare cases, a second treatment is required or the thyroid gland functions too slowly after this treatment. However, precautions should be taken for several weeks after radioactive iodine treatment due to the presence of residual radioactivity. Even if the thyroid cancer in your cat is malignant, radioactive iodine is a possible treatment option. More information about this option can be found at In Belgium, this treatment can only take place at the Faculty of Veterinary Medicine in Ghent. You must always submit the complete medical file in advance. If your cat is eligible, an appointment will be scheduled.

  • Surgery : The thyroid(s) can also be surgically removed. This may be an option if your cat does not want to absorb or tolerates the medication/food well, and radioactive iodine is not an option. This can also be a good option for large growths with cyst formation. However, if both thyroid glands are removed, lifelong supplementation with thyroid hormones is necessary. If the parathyroid glands cannot be spared, lifelong supplementation with vitamin D and temporary calcium is also necessary. In exceptional cases, overactive thyroid tissue can again be noticed in other places such the chest cavity (ectopic thyroid tissue).

Additional conditions

It is always necessary to treat your cat for an overactive thyroid. A fast-acting thyroid takes a big toll on your cat over time. Some other conditions that can occur together are important to mention:

  • Kidney problems: The function of the kidneys and thyroid go hand in hand. A fast thyroid can potentially cover up kidney problems. This can cause a manifestation of the kidney disease when the thyroid problem is treated. For this reason, it is always important to monitor kidney function together with thyroid function.

  • Heart problems: A fast-acting thyroid can lead to a thickened heart muscle. This can have serious consequences if the hyperthyroidism is left untreated. The influence of hyperthyroidism on the heart can be completely reversible with proper treatment. The heart may need to be temporarily supported with medication. You can have this evaluated by a cardiology specialist.


The outlook is good if your cat has a benign tumor of one or both thyroid glands. Good options are also available for cats with a malignant tumor. An optimal treatment plan can be drawn up for each cat. Do not hesitate to contact us about this!

bottom of page