ABSTRACT: Hyperthyroidism is a hormonal condition, involving over-activity of the thyroid gland(s). It is the most commonly diagnosed hormonal condition in cats and tends to occur in middle to old age.

Veterinary nurses will be most commonly involved in spotting signs of the disease in their clinics or when conversing with clients on the telephone and in reception. They may also be involved in monitoring of medical or surgical treatment, or in assisting discussions between veterinary surgeons and clients as to which mode of treatment would be most appropriate for a particular patient This article will concentrate on these points, but also deal briefly with other aspects for the sake of completeness.

Introduction

Hyperthyroidism is a hormonal condition, involving over-activity of the thyroid gland(s). It is the most commonly diagnosed hormonal condition in cats and tends to occur in middle to old age, with 95 per cent of cases being over 10 years of age. Rare cases, however, have been detected in patients as young as one year old.

Conversely, hyperthyroidism is very rare in dogs and this article will, therefore, focus on the feline disease.

Normal thyroid physiology

The two thyroid glands are situated in the neck, on either side of the trachea. They take up dietary iodine from the blood stream and make a hormone called T3 via a pathway involving several steps. T3 is released into the circulation and, once in the tissues, it is metabolised to T4 (also called thyroxine) which is the biologically active thyroid hormone.

Thyroxine is important for many body processes and, essentially, drives metabolism. Production and release of T3 is stimulated by thyroid stimulating hormone (TSH), which is released from the anterior pituitary gland in the brain.

Release of TSH is itself stimulated by thyroid releasing hormone (TRH), which is released from another part of the brain, called the hypothalamus, in response to low levels of T3 and T4 in the body. Conversely, high levels of these circulating hormones will feed back negatively and suppress the release of both TRH and TSH. Figure 1 summarises the physiology of the thyroid gland.

Figure 1: Normal thyroid physiology

The thyroid glands produce another hormone called calcitonin, which is important in calcium regulation, but is of no relevance to hyperthyroidism.

What causes hyperthyroidism?

In the vast majority of cases (97%), the condition in cats is caused by a benign growth in one or both thyroid glands (Figure 2). Whilst these are benign and do not present a cancerous risk, the growth produces thyroxine at a very high rate, which leads to increased levels of thyroxine within the circulation. The cause of these thyroid growths (called adenomas) is not known.

Figure 2: Gross appearance of thyroid adenoma. (Image courtesy of A Hotston-Moore and A Harvey, University of Bristol)

The remaining two to three per cent of feline cases – and the majority of the rare canine cases – are caused by excessive production of thyroxine from a malignant growth (called adenocarcinoma) of the thyroid gland. These cases may also suffer from the effects of local invasion by the tumour and metastatic spread to lymph nodes, lungs and other organs.

 

Presentation

Thyroxine drives the body’s metabolism and so hyperthyroid cats effectively go into ‘overdrive’. Typically, they have an increased appetite, whilst at the same time losing weight (Figure 3).

Figure 3: Weight loss in a hyperthyroid cat

Drinking levels can increase and there may be vomiting and/or diarrhoea. Sometimes, owners may report that their cats show a personality change, such as increased activity (sometimes to the point of mania) or aggression. Skin lesions may develop and affected cats often groom themselves vigorously, resulting in lesions of self trauma.

On clinical examination, it is usually possible to palpate the enlarged thyroid gland in the neck. The heart rate will be increased (tachycardia) and there may be a murmur, gallop’ rhythm or arrhythmia audible with a stethoscope. If left unchecked, thyroxine may eventually have a detrimental effect on the heart, with thickening of the ventricle walls, know as hypertrophic cardiomyopathy (Figure 4). Some cases may not present until they show signs of cardiac failure such as weakness, dyspnoea owing to pleural effusion/pulmonary oedema, or ascites.

Figure 4: Hypertrophic cardiomyopathy seen on ultrasound

Most cats with hyperthyroidism will develop high blood pressure (hypertension) and this can lead to secondary problems in the CNS, eyes, heart or kidneys. Hypertension is often most notable in the eyes, where the development of acute blindness and pain owing to retinal detachment, or bleeding into the eye, may be observed (Figure 5).

Figure 5: Blood in the anterior chamber of the eye (hyphaema)

 Intracranial bleeding can lead to seizures and a variety of other neurological signs; and effects on the kidneys will be seen as decreased renal function, evident as increased serum urea and creatinine concentrations.

It is important to realise that most cats will not show all of these signs and it is far more common to see combinations which vary from one patient to the next. Occasionally, a cat may even present with the total opposite – becoming inappetent, depressed, lethargic and withdrawn. These cases are referred to as having ‘apathetic hyperthyroidism’. Nowadays, its is generally considered wise to screen all sick cats over the age of eight years for hyperthyroidism, even if classical signs are not present; indeed, the occasional surprising diagnosis is made.

Confirming the diagnosis

The signs described by the owner and the findings of a clinical examination are usually fairly typical, allowing a presumptive diagnosis. However, these signs can mimic other diseases and it is essential that, no matter how obvious the presence of hyperthyroidism, the diagnosis is confirmed by blood tests, in order to avoid erroneously treating non¬hyperthyroid patients.

In the majority of cases, measurement of a single total T4 level will show it to be raised above the reference range and allow confirmation of o
nes suspicions. However, in some cases – referred to as ‘occult hyperthyroid’ – total T4 levels may not be raised, but instead be in the high normal range. This is because total T4 can be suppressed by non- thyroidal illness.

In the case of hyperthyroidism, high levels of total T4 which lie just above the normal range can be suppressed such that they appear to be normal when they are not truly so. This is overcome by measuring free T4, which is less affected by non-thyroidal illness. The finding of a total T4 level at the higher end of the normal range, in conjunction with a high free T4, is diagnostic of occult hyperthyroidism.

Renal function

It is important to perform routine haematology and biochemistry when diagnosing suspected hyperthyroid cases and, in particular, renal function must be assessed. High levels of thyroxine increase blood pressure and this may help to perfuse failing kidneys, such that levels of urea and creatinine are not as raised as one would expect or, sometimes, are normal.

When hyperthyroidism is treated and blood pressure falls, the renal insufficiency may be unmasked and levels of urea and creatinine may rise dramatically; occasionally, terminal renal failure can be induced. However, as well as maintaining perfusion, high blood pressure can also lead to renal dysfunction through pressure damage to the renal tissue and, in these cases, treatment of hyperthyroidism may help preserve renal function.

This is a complex area that is not fully understood and further discussion is beyond the scope of this article. However, veterinary nurses should be aware that their veterinary surgeon colleagues will need to consider renal function when diagnosing and treating hyperthyroidism.

Treatment

There are three modes of treatment commonly used in the UK:

1.   medical

2.   surgical

3.   radioactive iodine therapy.

1.   Medical management

Medical management involves the administration of a drug which blocks the production of thyroid hormone within the gland, resulting in a reduction in the amount of thyroxine in the circulation. Currently, the only licensed drugs in the UK for this purpose are methimazole (Felimazole, Dechra) and sustained release carbimazole (Vidalta, Intervet/Schering-Plough). Tablets are usually well tolerated, although some cats may develop skin problems (especially facial eczema) or vomiting.

Usually, the biggest hurdle is whether the cats will be amenable to long-term administration of tablets or not. In younger cats, this option may also lead to greater expense as more years of treatment will be required than for older cats. Conversely, in older cats or those with concurrent renal dysfunction, this treatment option may be beneficial as surgery can be avoided and the thyroxine level can be reversed on stopping treatment should adverse effects develop with renal function.

When a cat first starts on anti-thyroid medication, its metabolic rate is very fast and it may metabolise the drug very quickly. Once it stabilises, metabolic rate slows and the drug may be maintained for longer in the body. Therefore, cats often need a higher dose to bring them under control than that needed to maintain them under control, and close monitoring of total T4 levels during the first two months of treatment is important; firstly to ensure that the disease is controlled and, secondly, to ensure that the dose does not subsequently need to be reduced.

Once stable, continued monitoring every three to six months is advised. Commonly, thyroid adenomas become more active with time and doses of drugs may need to be increased in line with this in order to maintain control. However, before increasing doses, it is important to ensure that the increasing T4 level is not the result of the cat not receiving the tablets, or spitting them out behind the sofa!

2.   Surgical management

The affected thyroid gland can be removed surgically (Figure 6), but it is important to stabilise the patient with tablets for at least two weeks prior to the operation, in order to reduce the anaesthetic risk. Removing the affected gland negates the need for tablets but, unfortunately, around 85 per cent of cases show recurrence in the second gland at a later date. For this reason, sometimes, both glands will be removed.

Figure 6: Intraoperative view of thyroidectomy. (Image courtesy of A Hotston-Moore and A Harvey. University of Bristol]

The biggest potential problem with surgery is that the thyroid glands lie very close in the neck to the parathyroid glands. In fact, one parathyroid gland on each side actually lies within the thyroid gland. Inadvertant removal of the parathyroid glands with the thyroid glands – or even temporary damage to them through inflammation and bruising – can lead to their dysfunction, resulting in calcium levels falling dangerously low.

Such patients are observed to develop muscular twitching and weakness, usually three or four days after surgery. The risk of hypocalcaemia is increased when both thyroid glands are removed in one operation. It is, therefore, advisable to carry out such operations on a Monday and hospitalise the cat through the rest of the week to monitor its calcium levels, so that any problems occur during the working week.

Hypocalcaemia can be treated successfully with intravenous/oral calcium and oral vitamin D (which increases calcium absorption from the intestine). However, this may be a problem for the cat that is undergoing surgery owing to its refusal to take anti-thyroid tablets in the first place.

Unfortunately, even when both thyroid glands are removed, thyroxine production may be taken on by small aggregates of thyroid tissue found elsewhere in the neck. Hyperthyroidism may therefore still recur, even in a cat that has undergone a bilateral thyroidectomy.

Once again, renal function must be considered before and after surgery.

3. Radioactive iodine

This involves the cat receiving an injection of radioactive iodine, which is taken up specifically by the thyroid glands and destroys them, whilst leaving the rest of the body unharmed. It is very successful and over 95 per cent of cases are cured with one injection, the remainder usually responding to a second injection. Unfortunately, after therapy the cat emits radiation and has to be hospitalised in a specialist facility until it is safe to return home – usually after two to four weeks.

The number of centres in the UK offering radioactive iodine therapy is limited. Nonetheless, radioactive iodine is an extremely effective method of treatment and should be considered as the treatment of choice for younger patients.

Iatrogenic hypothyroidism

Iatrogenic hypothyroidism occurs when treatment is too aggressive and thyroid levels fall too low. Cats treated medically can easily be ‘corrected’ from hypothyroidism by a dose reduction. Clinically significant surgical or radioactive iodine cases can be supplemented with thyroxine, which is usually only required temporarily as ectopic thyroid tissue should hypertrophy and start to produce enough thyroxine after time.

Conclusion

Hyperthyroidism is a very common disease in older cats. F
amiliarity with its clinical signs is important for veterinary nurses running health clinics for senior pets. Whilst most cases can be treated without significant complication, an understanding of the treatment options is also necessary in order to manage those patients which are more complicated. 

Author

Mark Maltman

BVSc CertSAM Cert VC MRCVS

Mark Maltman qualified in 1997 from the University of Bristol and has practised for 13 years in Horsham. West Sussex. He has gained the RCVS Certificates in Small Animal Medicine 12001) and Veterinary Cardiology (2004). He is now a partner at Maltman Cosham Veterinary Clinic in Horsham.

To cite this article use either

DOI: 10.1111/j.2045-0648.2010.00023.x or Veterinary Nursing Journal Vol 26 pp 79-83

• VOL 26 • March 2011 • Veterinary Nursing Journal