An overactive thyroid (hyperthyroidism) is a common condition of older cats, typically causing obvious weight loss and behavioural changes. A range of treatments are available, including surgery to remove the affected part of the thyroid gland. Your vet may recommend this option, as it offers the prospect of a permanent cure for this condition.

But surgery is not without risks. As this condition occurs in older cats, some of whom have other medical problems, it may not be an appropriate option for your cat. To guide you, we explain the health implications of an overactive thyroid, as well as the risks and benefits of surgery as a treatment for an overactive thyroid.

What is an overactive thyroid?

An overactive thyroid occurs when the thyroid gland produces an excessive amount of thyroid hormone. The thyroid gland is usually located at the base of the neck or just inside the chest. It is made up of two parts, which sit either side of the windpipe.

The vast majority of cases result from a non-cancerous growth within one or, more frequently, both parts of the thyroid gland. While this is a very common condition (about 10% of cats over the age of 8 may suffer from it), the reasons for the change in the thyroid gland remain poorly understood. Thankfully, aggressive thyroid cancers account for fewer than 5% of cases.

What are the signs of a hyperactive thyroid in cats?

Thyroid hormone has a wide range of effects on the body. It particularly impacts the heart, kidneys and bladder, gut, skin, muscles and behaviour. Because of this, cats with an overactive thyroid may display a wide range of clinical signs. Some of these overlap with other common conditions of the older cat. The classic clinical signs of an overactive thyroid in cats are marked weight loss, often with an increased appetite, and a noticeable change in behaviour. This might include being more vocal, hyperactive and sometimes more aggressive. Less obvious common signs are an abnormally fast heart rate and ‘goitre’ – the emergence of the abnormally enlarged part of thyroid gland which can be seen or felt under the skin.

An overactive thyroid can cause, or contribute to, several medical problems such as abnormal enlargement of the heart (cardiomyopathy) and high blood pressure. It may also lead to gastrointestinal disorders, such as inflammatory bowel disease and bladder infections. These problems can be serious, but usually improve with treatment of the overactive thyroid. 

Some cats also have kidney disease that develops independently of an overactive thyroid, but whose effects are usually masked by the condition. Kidney disease is a progressive, life-limiting condition. Unfortunately, some of these cases will get worse when the overactive thyroid is treated. 

How is an overactive thyroid diagnosed?

Diagnosis of an overactive thyroid often starts with a blood test. This is to measure red and white blood cell numbers, assess the function of the major organs (liver and kidneys), and measure the level of thyroid hormone in circulation. If the thyroid level is in the lower half of the normal range, or below it, an overactive thyroid can be ruled out. Anything above the top end of the normal range confirms the diagnosis. Some cases, particularly those in the early stages of this condition, are trickier to diagnose with a single blood test. 

These cats have clinical signs suggestive of an overactive thyroid and their thyroid level is high but still within the top end of the normal range. In these cases, your vet may recommend additional blood tests to diagnose the condition. Equally, if there is evidence of kidney changes on the initial blood test, your vet will advise testing your cat’s urine. They may also recommend additional blood tests to further assess kidney function. This will determine the most appropriate treatment options for your cat.

How can an overactive thyroid be treated?

There are four treatment options for an overactive thyroid, each with its own distinct advantages and disadvantages; anti-thyroid medication, an iodine-restricted diet, radioactive iodine therapy or surgery to remove the affected part of the thyroid gland (thyroidectomy). All are effective at treating the condition but in different ways. Medication and dietary management only control the condition for as long as they are given. Surgery or radioactive iodine therapy offer the prospect of a long-term cure.

Most cats with an overactive thyroid will initially be started on anti-thyroid medication for a couple of months to reduce the excessive thyroid hormone. This stabilises them prior to other forms of treatment, particularly surgery or radioactive iodine treatment. It also allows a better assessment of any other medical conditions the overactive thyroid may have been masking or making worse.

Anti-Thyroid Medication

Medication blocks the production of thyroid hormone. It is available as tablets, an oral liquid, or a gel that is applied to the skin inside your cat’s ears. Because medication does not permanently cure the condition, regular blood tests are needed to check and adjust the dose to prevent over or under correction of the thyroid level. Common side effects are reversible and include a loss of appetite, vomiting and irritant skin reactions. Medication is relatively cheap, but the total cost mounts up with the length of treatment.

Iodine-Restricted Diet

For cats that do not tolerate medication an iodine-restricted diet might be considered. This special food reduces the production of thyroid hormone and has few known side effects. To be effective, it needs to be fed exclusively (which means no treats or other food) and some cats will not take to it, so it is not appropriate for all cases. It is more expensive than normal food, but still an accessible option in most cases. As with medication, the total cost depends on how long it is given for.

Radioactive Iodine Therapy

Radioactive iodine therapy is the “gold standard” treatment. It is safe, offers the prospect of a permanent cure and avoids many of the complications associated with surgery. The vast majority of cats return to normal thyroid function within 3 to 6 months; although, about 5% of cats will require a second round of treatment. 

A radioactive form of iodine is administered over several days to reverse the abnormal changes within the thyroid gland. It does require a multi-day stay in a specialist hospital centre, as well as strict and careful precautions during the treatment period due to its radioactive nature (although this poses little risk to your cat). This option is therefore not always appropriate for cats affected by any other medical problems. It also won’t be effective for those who get stressed being away from home. It is also the most expensive treatment option. The most common complication of radioactive iodine therapy is overcorrection of the thyroid level, which is easily treated. 


Surgery to remove the affected part of the thyroid gland is known as a thyroidectomy. Your own vet may be able to perform this and it usually provides an effective long-term cure for an overactive thyroid. The procedure takes about 45 minutes and involves a general anaesthetic and clipping of your cat’s fur at the base of the neck. An incision is made through the skin and the muscles overlying the thyroid gland. The affected part of the gland is identified and delicately removed. Extra care is taken to tie off any blood vessel, avoid important nerves that run close by and preserve the parathyroid glands. This sits next to the thyroid gland and control the normal calcium level in the blood. 

Complications of surgery

Overall, complications with surgery are uncommon. The most concerning is hypocalcemia, an abnormally low normal blood calcium level caused by disruption of the parathyroid glands during surgery. This is more likely if both sides of the thyroid are operated on. As blood calcium is essential for normal muscle contraction, this complication can cause restlessness, muscle pain, twitching of the face, ears and other muscles. It can also result in stiffness and convulsions. These signs are usually seen within a few days of surgery, but can take up to 10 days to develop. Most cases respond well to calcium supplementation, although this may require hospital treatment for a few days. Your vet may prescribe calcium supplementation following surgery to prevent this complication. 

An underactive thyroid is another potential complication. This is more likely when surgery has been done on both sides of the thyroid gland and may require supplementation for a period after surgery. 

Finally, a small number of cats show no improvement following surgery or experience a recurrence of the overactive thyroid some time later. This is due to changes in the remaining part of the thyroid gland or because of thyroid tissue elsewhere in the body. This may require a second procedure to correct. Your vet may recommend regular blood tests to check for any recurrence.

Following surgery, your cat will need to be rested for 10 to 14 days to prevent complications. They should normally be kept in during this time, so you can check for any problems.

Is surgery an option for my cat?

Ultimately, this depends on the individual case. Your vet will be best placed to guide you on the most appropriate option for your cat and answer any remaining questions. An overactive thyroid is a common condition of older cats, which may exist alongside other serious medical problems, such as kidney and heart disease. Although surgical treatment is an effective and generally safe procedure, these other conditions may increase the risks. As such. it may rule out surgery for your cat. 

There are different risks with surgery compared to medication, dietary management or radioactive iodine therapy; but there are also distinct benefits. Surgery is also less expensive than radioactive iodine therapy and over the long term. It is also more cost-effective than either anti-thyroid medication or an iodine-restricted diet. Finally, it is more convenient. There are no referrals and no need to give regular medication or stick to a strict diet.

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