Feline Hyperthyroidism

Hyperthyroidism (over-active thyroid glands) is a very common problem that we see (generally) in older cats (over 7 years old). It is caused by the progressive overproduction of thyroid hormone by the thyroid glands; of which the cat usually has two, situated either side of the windpipe in the neck. In hyperthyroidism, the metabolic rate increases dramatically and this usually results in dramatic sudden weight loss, despite having an increased appetite. However, some cats have a depressed appetite with hyperthyroidism.

Pathology

 In the majority of cases, the cause is a benign growth in one, or both, of the thyroid glands. The abnormal gland becomes enlarged and produces more hormone than normal. The cause of this change is presently unknown. These cats usually respond very well to treatment if the condition is recognised early and appropriate treatment given. In rare cases (about 2%), the enlargement of the gland(s) can be due to a malignant tumour. This condition is much more difficult to treat successfully and the outlook in these cases is guarded.

Clinical Signs

 Affected cats develop a variety of symptoms, which can be very subtle at first, but become much more obvious as the disease progresses. The “classical” signs include recent weight loss despite an increased appetite, increased thirst, grumpiness and restlessness. Affected cats usually have a very high heart rate (which can cause high blood pressure) and usually develop an unkempt coat. Some cats will develop vomiting and mild diarrhoea. As the disease advances, cats can become weak, lethargic and stop eating. Thyroid hormone controls the metabolic rate, so it can cause some serious complications if left untreated. It can cause the heart to work harder, resulting in high blood pressure and heart enlargement. This can eventually result in major problems such as heart failure, retinal bleeding, “stroke” and kidney failure.   

Diagnosis 

If hyperthyroidism is suspected, a thorough clinical examination and some blood tests will be needed to confirm the diagnosis. Sometimes, one or both enlarged thyroids can be felt in the neck; but sometimes this is not present as the affected thyroid can “hide” behind the windpipe or sink very low in the neck. Rarely, there can be thyroid tissue within the chest of some individuals where it cannot be felt. A blood test looking at thyroxine (T4) levels is usually the gold standard for diagnosis. Kidney and liver function is always checked with the same blood sample and a blood count is performed. Occasionally, a clinical examination may strongly suggest hyperthyroidism, but a blood sample reveals a normal thyroxine level; there are a number of reasons that this may happen and we may advise a repeat test in 3-4 weeks.

Treatment

 There are 3 main options for treating hyperthyroidism:

 1. Medical Management (Pills):  anti-thyroid drugs are available in tablet form and these act to reduce the production and release of thyroxine from the thyroid gland. They do NOT cure the condition; merely control it in the short- or long-term. The drug of choice is called Felimazole and usually thyroid levels fall to normal within 3 weeks of starting treatment. To maintain control this drug needs to be given for life, sometimes with increasing dosages. Lifelong administration of tablets to cats can be major problem for some owners and cats. Regular blood tests will be needed to monitor effectiveness and kidney function.  

2. Surgical Thyroidectomy:  surgical removal of the affected thyroid tissue (“thyroidectomy”) gives the chance of a permanent cure and this operation is commonly done. Sometimes, hyperthyroidism can recur if previously normal thyroid tissue elsewhere becomes abnormal. To reduce anaesthetic and surgical complications, it is normal to try to stabilise the patient using medication given for 3-4 weeks prior to surgery. Any associated high blood pressure or heart issues should also be treated prior to surgery. The cat has two thyroid glands; one each side of the airway. Associated with each gland is a smaller gland, called the parathyroid gland, and they control blood calcium levels in the body; this is extremely important. These glands are very small, being approximately 2-3mm across. One of the risks of surgery is inadvertent damage to the parathyroid glands, although every care is taken to minimise the risk in any case. There are a number of scenarios here, but the main points are as follows:

•  It is impossible to tell whether just one of both thyroid glands are abnormal, until they have been inspected surgically
  
• At surgery, if only one thyroid gland is enlarged, then this is removed; care is taken to preserve the associated parathyroid. However, even if this gland is damaged, there is always the one the other side so the risk of causing problems with blood calcium levels is minimal
  
• If both thyroid glands are abnormal, it is still usual to only remove the worst one at this operation; this is to minimise the risk of damaging BOTH parathyroids at the same time, which could cause problems. We would then blood test your cat about 6-8 weeks later to check the thyroid level. If hyperthyroidism persists, we would consider a further operation, or may opt to use medical control at this stage. This delay is in order to let the “first” parathyroid to recover
  
• All cats having a thyroid removed are hospitalised post-operatively; if your cats has only had one thyroid removed, this is usually 24 hours. However, if we have performed a second thyroidectomy, the operation is normally done on a Monday, you cat will be hospitalised and a blood calcium test will be taken on the Thursday to check your pets status
  
• A routine blood test to check thyroid levels is taken 6-8 weeks post-operatively; this is done in all cases
  
• In most cats, any damage to the parathyroid glands is only temporary and any treatment will only be needed for a few days or weeks
  

 3. Radioactive Iodine Therapy Radioactive iodine (I 131):  can be used as a safe and effective treatment for hyperthyroidism. Like surgical treatment, it can offer a total cure with no ongoing treatment needed. It is administered by injection under the skin; the radioactive iodine is then taken up by all the abnormal thyroid tissue and the radioactive iodine then destroys progressively destroys it. It is a very localised, selective radiotherapy. The advantages of this treatment is that it has no side effects, is curative, treats all affected thyroid tissue at once and does not require an anaesthetic. The disadvantages are that it can only be performed at specialist centres with the appropriate facilities (eg. Bristol University), it requires that your cat is hospitalised in a special radioactive facility for up to 6 weeks and it is relatively expensive (usually £1500 approximately). 

Other Points

• In rare cases, if your cat has had both thyroids surgically removed, or has had radioactive iodine treatment, a permanent reduction in thyroid levels may occur…. a condition known as hypothyroidism. The symptoms include lethargy, obesity and poor haircoat; if this occurs then thyroid hormone supplementation may be needed (as tablets). 
  
• In rare cases, the tumour in the thyroid gland causing the problem can be a malignancy, known as a thyroid adenocarcinoma. These are more difficult to treat and the outlook is always more guarded. Some can be very successfully treated with high dose radioactive iodine treatment, whereas external beam radiotherapy may be more appropriate for others. The only way to diagnose these tumours is with a surgical biopsy so they are usually detected when your cat is having the first thyroid surgically removed.    
  

 RFW 2007