Hyperthyroidism in cats: what s new for VNs and owners

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1 Vet Times The website for the veterinary profession Hyperthyroidism in cats: what s new for VNs and owners Author : Claire Bloor Categories : RVNs Date : December 1, 2013 Claire Bloor MA Ed, BSc(Hons),VN, RVN, PGCE, QTLS, CertVN(Dent) discusses diagnostic techniques and treatments VNs should be aware of Summary HYPERTHYROIDISM, or thyrotoxicosis, is the most commonly diagnosed endocrinopathy in domestic cats. Barry (2013) and Mooney (2002) identified it was one of the most frequently diagnosed disorders in small animal practice. This was later supported by Gunn-Moore (2005), who suggested it affected approximately one in 300 cats in the UK at that time. Caney (2012) discussed a study suggesting 12 per cent of the UK cat population aged nine years or older are diagnosed with hyperthyroidism annually. This prevalence, and the potential for effective management of diagnosed cats, means all VNs should endeavour to remain up-to-date regarding current treatment modalities; this will benefit both their patients and concerned owners. This article aims to provide an overview of the disease before discussing current, recommended diagnostic techniques and treatments VNs should be mindful of in clinical practice. Key words treatment, follow-up, diagnosis, medical management, nursing management HYPERTHYROIDISM is a multi-systemic disease caused by excessive levels of circulating thyroxine (T4) and tri-iodothyronine (T3) hormones. Barry (2013) explained 90 per cent of cases of excessive production and secretion of T4 and T3 1 / 9

2 were due to benign nodular hyperplasia, adenomatous hyperplasia or adenoma of the thyroid glands bilaterally affected glands are thought to be present in around 70 to 80 per cent of all cases (Neiger, 2007; Caney, 2012; Barry, 2013). Only a small percentage of cases, between one and three per cent, are due to malignant thyroid carcinomas (Neiger, 2007; Barry, 2013). Hyperthyroidism caused by these differing underlying conditions is typically a senior cat disease, with the average age at diagnosis reported by various authors being 10 to 13 years (Neiger, 2007; Church, 2009; Schoeman, 2011; Caney, 2012; Barry, 2013), although some authors report potential ages at diagnosis as young as eight months up to 24 years (Scherk, 2009; Manchado, 2010). Thyroid hormones exert an effect on many body systems and are involved in the regulation of various core body functions, including regulating heat production; metabolism of macronutrients including carbohydrates, lipids and proteins; and maintenance of basal metabolic rate (Mooney, 2010). Clinical signs associated with hyperthyroidism can (according to Neiger, 2007; Church, 2009; Scherk, 2009; Manchado, 2010; Schoeman, 2011; Caney, 2012; and Barry, 2013) include: weight loss; polyphagia/anorexia; polydipsia; vomiting; diarrhoea/bulky, foul-smelling faeces; muscle weakness; lethargy/hyperactivity; tachypnoea/breathlessness; goitre; tachycardia; heart murmurs; systemic hypertension; 2 / 9

3 hypertensive ocular changes; and unkempt coat. Some of these clinical signs are seemingly contradictory, but it is important for the VN to treat every patient as an individual and appreciate not all cats will be at the same stage of the disease process. Neiger (2007) described a version of the disease called apathetic hyperthyroidism that occurs in approximately 10 per cent of cats, which results in clinical signs that are the opposite of what the VN may classically expect from hyperthyroidism; lethargy, weakness, weight loss and anorexia. This is also discussed by Scherk (2009), Manchado (2010) and Schoeman (2011). The VN must also be aware older cats may have concurrent disease processes the veterinary surgeon must consider and discuss with the owners when formulating a treatment plan for hyperthyroid cats. Daminet (2006) outlined the intricate relationship between thyroid and renal function, while Stepien and Brown (2006) described their management of a cat with concurrent cardiac, renal and thyroid disease, highlighting the extremely complex and inter-related natures of these disease processes. Graves (2009) later discussed the relationship between systemic hypertension and hyperthyroidism, stating studies regarding this relationship have produced variable results, highlighting the fact it is difficult in some cases for the veterinary surgeon to diagnose hypertension due to the pathophysiological complexities of hyperthyroidism and the circulatory system. Diagnostics The VN will play a large role in assisting the veterinary surgeon to reach a diagnosis of hyperthyroidism in feline patients, as well as supporting clients in the management of their cats following diagnosis. Caney (2012) discussed how prompt diagnosis can be achieved by employing the following in practice: Twice-yearly history taking and physical examinations of all cats 11 years or older, increasing to checks every three to six months in patients above 15 years. The establishment of senior health care clinics is an extremely good idea in every practice. Here, the vet and VN may identify early clues of hyperthyroidism from the owners verbally and from the cats physically, such as appetite changes and evidence of weight loss. Blood pressure measurement should be a feature of senior cat clinics, annually in cats seven and older and twice yearly in cats aged 15 years or older. The vet should also check the cat s eyes for any potential damage due to hypertension. From the age of seven, all cats should have their urine specific gravity (USG) checked annually and, from 15 years old, this should increase to twice yearly checks as with blood pressure. Caney (2012) suggested a reading of or less is abnormal and should be investigated further, a 3 / 9

4 reading of is considered normal, and values between and should be closely monitored. If abnormal results are detected, Caney (2012) believed further investigations are warranted, including more comprehensive blood work and urinalysis. Schoeman (2011) advised caution to practitioners when handling potential hyperthyroid cats as they can be particularly resistant to handling and restraint for examination; they may appear abnormally aggressive and fractious when trying to restrain them when, in fact, physiologically, they are extremely fragile and are at an increased risk of collapse and death if handled inappropriately. Less is most definitely more when it comes to handling these patients. The vet will decide which blood tests are required to reach diagnosis and the VN must ensure the correct samples are obtained for analysis. Total T4 (tt4) tests are usually the first performed and the majority of patients can be diagnosed in this manner (Caney, 2012; Barry, 2013). Caney (2012) explained some hyperthyroid cats may have normal tt4 levels due to hormone fluctuations in the early stages of the disease, or concurrent disease processes previously mentioned can result in the tt4 levels being suppressed to within normal limits. Free T4 by equilibrium dialysis is a highly sensitive test available for diagnosing hyperthyroidism, but should not be used as the initial screening test due to the small potential for falsepositive results (Caney, 2012). Wakeling et al (2008) discussed canine thyroid stimulating hormone (ctsh) assays have proven effective in diagnosing subclinical or occult hyperthyroidism in conjunction with the tt4 results, so the vet may request the VN obtains bloods for this test as well. Treatment options The three main accepted ways reported in literature (Neiger, 2007; Church, 2009; Scherk, 2009; Manchado, 2010; Caney, 2012; and Barry, 2013) to treat hyperthyroidism are: Medical management the use of mercaptoimidazole antithyroid medications is common in veterinary practice, with the main two being carbimazole (Vidalta, Schering-Plough) and thiamazole (methimazole; Felimazole, Dechra). These drugs inhibit thyroid peroxidase, which is required to synthesise thyroid hormone from tyrosine and iodine. Depending on the medication and the patient s condition, the tablets need to be administered once, twice or potentially three times daily, with careful monitoring of the tt4 levels to adjust the doses until the patient is euthyroid and stable (Barry, 2013). The main problems associated with medicating the patient are the owner s ability to successfully treat the cat as frequently as required and some of the side effects that may be seen in the first few weeks after commencement of the drug, including anorexia, vomiting and lethargy (Nelson, 2009). This is when the clients will require a lot of support from the VN team in practice. Surgical management this is considered a highly effective treatment method, but it is not without significant risk to the patient, which the owners should be made fully aware of when 4 / 9

5 obtaining consent for anaesthesia. Hyperthyroid patients can prove to be very unstable and high risk when contemplating anaesthesia, and time should be taken to stabilise them prior to surgery. Due to both thyroid gland lobes typically being affected (in more than 70 per cent of hyperthyroid cats) they will both need to be removed for a successful outcome. Barry (2013) and Peeters (2011) discussed how the vet will always attempt to preserve the parathyroid glands during surgery, but in some cases there is a risk of the glands being removed or damaged or having their blood supply compromised. The VN must remember the consequences of this type of surgery, including lifethreatening hypocalcaemia secondary to hypoparathyroidism, Horner s syndrome, laryngeal paralysis and persistent hypothyroidism that will affect the patient s immediate recovery, but these things may also significantly impact on the long-term management of patients and owners must appreciate the seriousness of the situation (Peeters, 2011). Radioactive iodine the success rate of this treatment is around 95 per cent but there are few veterinary practices in the UK that offer such treatments, although it is becoming more widely used. Caney (2012) discussed research studies to date that demonstrated this treatment modality offered the best long-term prognosis for hyperthyroid patients, and Barry (2013) warned while it is deemed safe and effective, there is a chance the cat may become permanently hypothyroid following treatment, so it still requires life-long monitoring and potentially supplementary thyroxine. This, coupled with the issues surrounding availability and cost, may result in clients opting for surgery or medical management instead. Caney (2012) and Barry (2013) also contemplated nutritional management as a more novel approach to treatment. Iodine-restricted diets are relatively new to the veterinary market; naturally, if iodine intake is restricted, so is its availability for thyroid hormone synthesis, and that is the premise of these diets. Hill s launched a prescription diet called y/d, which is an iodine-restricted diet, and it is also phosphate-restricted and contains moderate protein levels, thus making it suitable for cats with mild to moderate renal disease as well. The studies conducted by Hill s and its scientists have shown promising results on the efficacy of the diet, so it is definitely a treatment option worth considering in future. However, longer term studies are also required to prove the efficacy of the diet over a prolonged period. Nursing management As mentioned, the VN plays a large role in the management of hyperthyroid patients, both in the veterinary practice and also in the support of clients. Some important nursing considerations reflecting on the aetiology and pathogenesis include: Minimal restraint, patience and the use of anaesthetic numbing cream are strongly advised; stress should be minimised. Consider patients inability to thermoregulate effectively when preparing their environment for hospitalisation. 5 / 9

6 Try to ensure patients are housed in a quiet environment free from too many disturbances, as they find sleeping and resting difficult. Perhaps try a pheromone diffuser to minimise environmental stressors and ease handling. Barrier nursing is prudent due to the immunosuppressive effects of drugs. Choose the cat s diet carefully and try not to modify the diets too quickly to avoid gastrointestinal upsets. Feed little and often and ensure the cat always has access to fresh water. Regular ocular examinations are required, especially before every examination and clinical intervention, so as to not exacerbate existing damage. Never crush Vidalta to administer it this can increase its toxic effects. Cats may appreciate help grooming, or they may detest this intervention judge each patient individually. Monitor them regularly for changes in demeanour and the development of any neurological signs, and ensure their vital signs are monitored frequently. It would be advisable to have an oxygen source nearby. Utilise a model of nursing, such as the Ability Model, alongside the nursing process to ensure comprehensive, holistic management of these patients. Remain up-to-date with new developments in the field to ensure evidence-based veterinary interventions are being performed. Conclusion Hyperthyroidism is evidently a complex disease, which significantly impacts on a cat s health and well-being; its ability to perform many normal daily functions is impaired and it needs the support of the vet and VN. It is not just a disease of the thyroid glands, but is often associated with concurrent problems of the circulatory and renal systems too, which makes these even more difficult cases for vets to manage. There are many well-established and effective treatment options, as well as newer treatments that VNs need to be aware of to support clients and their cats through what is essentially a life-limiting condition; it is quality of life that is most important, and a good quality of life is achievable for many of these patients if early detection is achieved and they are managed optimally. 6 / 9

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