Hyperthyroidism In Cats

Hyperthyroidism is caused by an overproduction of thyroid hormone by the thyroid glands. Hyperthyroidism is a very common disease primarily seen in middle-aged  and older cats, with an average age of onset at 13 years. There are two thyroid gland located in the neck and one or both of the glands can enlarge and produce excessive thyroid hormone. In most cases, enlargement of thyroid glands is caused by a tumor called an adenoma, which is non-cancerous. In rare cases, hyperthyroidism can be caused by a malignant tumor known as a thyroid adenocarcinoma.

Thyroid hormone affects nearly all the organs of the body. Hyperthyroidism can sometimes cause secondary problems Thyroid hormone stimulates the heart to beat at a faster heart rate and with a stronger contraction. Over time this can lead to the development of an enlarged and thickened left ventricle of the heart. If this is not treated, the changes will compromise the normal function of the heart and can even result in heart failure. Heart disease may cause fluid to build up in or around the lungs, causing difficulty breathing. Once the underlying hyperthyroidism has been controlled, the cardiac changes will often improve or may resolve completely.

Hyperthyroidism can also cause hypertension, or high blood pressure, which can cause damage to several organs, including the eyes, kidneys, heart and brain. sometimes drugs are needed to help control the hypertension although treating the hyperthyroidism will often resolve the high blood pressure.

The signs of hyperthyroidism can be subtle initially but become more severe as the disease progresses. The most common clinical signs are weight loss, increased appetite and increased thirst and urination. Hyperthyroidism can also cause vomiting, diarrhea and hyperactivity. The hair coat may appear matted or greasy. Hyperthyroid cats can develop aggressive or “cranky” behaviour, their heart rate can be increased and a heart murmur may be found. If not controlled, the signs can progress to weakness, depression and difficulty breathing.

Hyperthyroidism is diagnosed with a combination of a physical examination as well as blood testing. Sometimes enlargement of the thyroid glands can be palpated in the neck area. Heart rate and the presence or absence of a heart murmur will be checked. Blood pressure may also be tested. If thyroid disease is suspected, blood testing will be required to confirm the diagnosis. Because hyperthyroidism can predispose a cat to other conditions, it is important to evaluate general health; especially in the heart and kidneys. The blood testing will look at a number of organs plus electrolyte balance and urine testing will check for any other problems such as a bladder infection.

There are four main options for treating hyperthyroidism in cats; medication, surgery, radioactive iodine therapy or diet therapy. Each has it advantages and disadvantages.

Anti-thyroid medications act by reducing the production and release of thyroid hormone from the thyroid gland. These medications do not cure the problem but rather control the disease while they are being given. If the medication is stopped, the thyroid gland will continue to produce more thyroid hormone than is needed. The advantages of anti-thyroid medication is that the drugs are readily available and relatively inexpensive. Some cats may experience side effects, including vomiting, loss of appetite, anemia and lethargy and cannot continue to take the medication. Lifelong treatment involving twice daily medication is required. Routine blood testing should be done periodically during treatment; to evaluate the effectiveness of therapy, to monitor kidney function and to look for potential side effects.

Surgical treatment, which involves the removal of the thyroid gland (thyroidectomy) is a relatively straightforward surgical procedure that has a good success rate. The advantage of surgery is that it is likely to produce long-term or permanent cure in most cats and therefore eliminates the need for long-term medication. The surgery requires general anesthesia which may have added risks for older cats if heart or kidney disease is present. There is a concern of causing inadvertent damage to the parathyroid glands which lie close to or within the thyroid glands and are crucial to maintaining. stable blood calcium levels. Uncommonly, cats may have thyroid tissue present that is outside of the thyroid gland. In these cases, thyroidectomy will not be as effective.

When available, radioactive-iodine therapy is often the treatment of choice for cats with hyperthyroidism. During the treatment, radioactive iodine (I-131) is administered as an injection and is quickly absorbed into the blood stream. The iodine is taken up only by the thyroid gland. The radiation destroys the abnormal thyroid tissue but does not damage the surrounding tissues. The majority of cats have normal hormone levels within 1 to 2 weeks of treatment. The advantages of radioactive iodine therapy are that the procedure is curative, has no serious side effects and does not require general anesthesia. Because a radioactive isotope is used, there are regulations as to the handling of the material. The radioactivity carries no significant risk to the cat but protective measures are required fo people who come into close contact with the cat. A treated cat has to remain hospitalized until the radiation has fallen to within acceptable limits, usually approximately 3 days after treatment. Radioactive iodine is curative in about 95% of all hyperthyroid cases. For the few cats where hyperthyroidism persists, the treatment can be repeated. Rarely, a permanent reduction in thyroid hormone levels, hypothyroidism, can occur. In these cases, thyroid hormone supplementation may be required.

The last option for treating hyperthyroidism is with a special diet called y/d made by Hills. This diet limits the amount of dietary iodine which is required for the thyroid gland to produce thyroid hormone. Studies have been performed to determine that feeding a low-iodine food effectively reduces serum total thyroid hormone in hyperthyroid cats without negatively affecting other measures of health. It is important to note that iodine intake from other food sources such as treats, another pet’s food or prey animals can compromise the effectiveness of low-iodine nutrition. It is critical that a hyperthyroid patient is fed only the prescription diet. In multi-cat households, this can be a challenge. The y/d food is not meant to be given to non-hyperthyroid cats. Any cats that have access to the outdoors may hunt mice and birds which provide a source of iodine. Some cats do not transition to a new food easily and may refuse the prescription diet.


Older cats with hyperthyroidism often also have kidney disease. Treatment of these cats is a delicate balancing act. Hyperthyroidism can actually improve kidney function by increasing blood flow to the kidneys. Some cats with kidney disease will show a worsening of kidney function after treatment for hyperthyroidism. Routine blood tests should be done periodically with any of the treatment options.

The prognosis for cats with treated hyperthyroidism is generally good. Cats with pre-existing renal disease have a poorer prognosis. Without treatment, cats with hyperthyroidism will become very ill. They have a general restlessness and nervousness, may develop vomiting and diarrhea and may stop eating altogether. The effect of the excess thyroid hormone on the heart means the heart must work harder and often beats very rapidly. Over time serious heart problems may develop that can be fatal.


If you have any questions about thyroid disease and its treatment in cats, please contact us.