ABSTRACT: This article reviews the physiology and concurrent conditions associated with hyperthyroidism and how these affect the veterinary nurse's role.

Hyperthyroidism is the most prevalent feline endocrine condition encountered in general veterinary practice, with one in 300 cats having the condition.1 The nurses role in caring for this patient centres upon strict monitoring and an understanding of the condition and its treatment, to establish the best practice of patient care.

The multi-systemic nature of the condition means that the feline hyperthyroid patient should be nursed with caution.

Thyroid gland physiology

The thyroid gland is of major importance as it plays a large role in the regulation of various body systems, including the central nervous system, where it induces sympathetic drive.2 Delay in treatment of these cases will lead to clinical signs which reflect an increase in the metabolic rate, including weight loss, polyuria, polydipsia and polyphagia. Tachycardia is also present, owing to the direct effect of thyroid hormones on the cardiac muscle.3

Hyperthyroidism can also cause behavioural changes and the owners may notice their pet becoming hyperactive, although these do not generally manifest until the patient is restrained when they can become restless and difficult to handle. The thyroid gland is often enlarged, which can lead to the clinician palpating a goitre in these patients.4

It is with these things in mind that the feline hyperthyroid patient should be nursed with caution as it is a multi- systemic condition.

The thyroid and parathyroid gland are supplied with blood from the cranial thyroid artery via the common carotid artery (Figure 1). The parathyroid gland secretes parathyroid hormone which regulates the level of calcium within the circulation. Therefore, if it is inadvertently removed during the thyroidectomy procedure, it can result in low circulating calcium (hypocalcaemia).

Figure 1 :The thyroid gland. Adapted from source15

The clinical signs of hypocalcaemia are neuromuscular dysfunction, neuronal excitability, twitching, restlessness, facial rubbing, dyspnoea and pyrexia.5

The close proximity of the thyroid and parathyroid glands explains why removal of the thyroid gland can be complicated.

Pathophysiology

The thyroid gland becomes enlarged as a consequence of either hyperplasia or carcinomatous change that leads to excessive secretion of thyroxine. Benign adenomatous hyperplasia/adenoma occurs in 98-99 per cent of cases,6 whilst the malignant thyroid adenocarcinoma is only prevalent in 1-2 per cent of cases.

Often, the patient may have concurrent disease of the renal or cardiovascular systems. These are generally evaluated in terms of clinical significance once the patient has achieved normal thyroid function – known as euthyroidism – which is achieved by administering medication to the patient for three to four weeks prior to surgery. This acts by interfering with the synthesis of thyroxine, which reduces the clinical signs of the disease and allows the metabolism to stabilise prior to surgery.7

Hyperthyroidism and renal disease

Renal disease can be masked by hyperthyroidism. In order to function, the renal system relies upon an adequate supply of blood through the renal artery.

 Hyperthyroidism gives rise to an increase in cardiac output and intra-renal vasodilation. This increases glomerular filtration rate (GFR) and renal blood flow, which also decreases creatinine levels leading to a reduction of muscle mass in the hyperthyroid patient.

After surgery, the metabolic rate returns to normal and the blood pressure decreases. The consequent sudden decrease in perfusion of the kidney can result in the unmasking of pre-existing, occult renal disease. This occurs in approximately 30 per cent of cats treated for hyperthyroidism.1

Accurate assessment and recording of body weight, together with body condition scoring are essential when monitoring patients during medical stabilisation. Urine specific gravity (USG) can also be monitored during treatment to give an evaluation of renal function.

Polton and Branscombe (2008) advise that the nursing care of a renal failure patient should incorporate these monitoring skills alongside blood sampling for urea and creatinine levels.8

Hyperthyroidism and cardiac disease

Hyperthyroidism causes the myocardium to elicit stronger contractions, thereby increasing the myocardial demand for oxygen, which results in tachycardia and, eventually, left ventricular hypertrophy.9 Increased metabolic rate can induce cardiac overload of the left ventricle as a result of an increase in blood volume passing through the heart. This results in an increase in chamber size and hypertrophy of the chamber wall.10

Conscious right lateral and dorso-ventral radiographic views may be required to assess the patient for cardiomegaly and the VN needs to be aware of the need for sensitive handling, especially if there is evidence of pulmonary oedema and/or dyspnoea.

Veterinary nurses also need to be aware that the thyroid hormone has a positive chronotropic effect that results in a sinus tachycardia.11 An electrocardiograph (ECG) recording is vital for these patients and allows the veterinary nurse to monitor for any rhythm or rate abnormalities.

Other considerations discussed by Martinez (2008) consist of a thorough pre-anaesthetic investigation, including the monitoring of plasma catecholamine levels that are associated with stress. These will increase heart rate and blood pressure.12

Hyperthyroidism and anaesthetic nursing

Hyperthyroid patients are considered high-risk patients, as the sudden release of excess thyroxine – known as a ‘thyroid storm’ – gives rise to a list of clinical signs, such as tachycardia, hyperthermia, tachypnoea and neurological abnormalities. Thyroid storms can occur as a result of palpation of the thyroid gland during thyroid surgery.13

Nursing support will focus on cooling the patients body temperature. If the patient is already receiving medication for pre¬existing heart failure, fluid therapy is indicated to support the circulatory system. Blood pressure is simultaneously monitored on account of the tachycardia.

The concept of the ‘thyroid storm’ is not discussed within standard nursing texts and this gap in nursing knowledge could alter the outcome of surgery if the veterinary nurse is unable to recognise – and, therefore, deal with – the complication.

The raised metabolic rate of these patients results in an increase in oxygen demand and resultant sensitivity to hypoxaemia. Peri-operatively the patient may experience a degree of tracheal blockage from the mechanical pressure of the enlarged thyroid gland, which will increase the oxygen demand and also the respiratory effort.

Oxygen, glucose and carbon dioxide production are increased within this patient.2-14 The muscles used for ventilation become weak, resulting in a reduced vital capacity and lung function. The increased carbon dioxide production, together with decreased tidal volume, can result in hypercarbia.

Another factor that must be considered during anaesthesia of these patients is the fluid therapy rate. Discussion of this is beyond the scope of this article but is a critical element of the monitoring process because of the potential for blood pressure to fluctuate.

Role of the veterinary nurse

The veterinary nurse’s role in the care of the feline thyroidectomy case is varied and demanding. Each patient should be classified according to their individual health and surgical status, and grouped into the following surgical categories:

1.   unilateral stable patient

2.   unilateral unstable patient

3.   bilateral stable patient

4.   bilateral unstable patient.

All patients should have their heart rate, respiratory rate and temperature monitored as routine. Each surgical category will require different considerations for nursing care (Table 1).

Patients in category 1 will be medicated and will require the removal of one of the thyroid glands. This is considered the lowest risk category because the patient will be euthyroid, which would have unmasked any concurrent conditions. Also, the removal of only one thyroid lobe reduces the risk of hypocalcaemia developing. 

Patients in category 2 are a slightly higher risk in that they will not be euthyroid, but will only be having one gland removed. They will still have an increased metabolic rate and hyperactivity may still be present; handling, therefore, may be difficult.

Patients in category 3 will be euthyroid and will also require blood pressure monitoring, fluid therapy and close observation for hypoparathyroidism. The thyroid storm risk is heightened because of the removal of both glands.

Patients in category 4 present the highest risk because concurrent disease may be masked and both glands are being removed. Monitoring for thyroid storm, hypocalcaemia, hypoxia, laryngeal oedema, hypothermia and potential blood loss – resulting from haemorrhage at the highly vascular operative site – are essential.

Nursing care

Concurrent disease presents a problem in hyperthyroid patients and for their nursing care because there is a reliance on the ability of the veterinary nurse to interpret and pass on vital information to the surgeon – detecting the presence of pulse deficits, cardiac arrhythmias or tachycardia, for example.

Poor renal perfusion will become evident when the patient is euthyroid, because renal perfusion will be reduced owing to metabolic normalisation unmasking the real status of kidney function. Being able to take good-quality radiographs of the thorax and use an ECG is vital, as the patients may be dyspnoeic and suffering from undetected cardiac abnormalities.

The veterinary nurse needs to be aware of concurrent conditions when handling these patients too. The renal system must be monitored in terms of blood pressure, perfusion and urinary output. Fluid therapy will support the perfusion of the kidneys during the operation.

Conclusion

There is a need, especially in veterinary nursing texts, to highlight the importance of the nursing care required for patients affected by hyperthyroidism – including the need to monitor, note and relay parameters whilst the patient is hospitalised for surgery.

In addition, the author recommends that VNs should be given a good level of knowledge of the physiology of the thyroid gland in the early stages of their training.

Nursing care of patients with hyperthyroidism can be very demanding but the well-informed VN can make a considerable difference to the management of these cases. SS

Author

Laura Daniels RVN MBVNA

Laura has been in practice for five-and-a-half years, working in a busy Tier-3 hospital, based in Cheshire. Since qualifying in January 2009, she has developed a keen interest in orthopaedic nursing care and is working to build her knowledge and experience in this field.

To cite this article use either

DOI: 10.1111/|.2045-0648.2012.00222.x or Veterinary Nursing Journal Vol 27 pp 374-377

References

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Veterinary Nursing Journal • VOL 27 • October 2012 •