Feline hyperthyroidism has become the most common endocrine disorder in older cats (mean 13 years of age, range 4 to 20 years of age (Nelson, 2007)) since the first case was described in 1979 (Peterson et al, 1979). The most recent epidemiological survey reported a prevalence of 21% of 508 cats 10 years of age or older in the greater Dublin area, Ireland (Gallagher and Mooney, 2013). Pedigree cats are under represented and there is no gender bias (Nelson, 2007). Feline hyperthyroidism is the result of over-production of thyroxine by one or more tumours of thyroid tissue. The normal feline thyroid gland is composed of two lobes located laterally on either side of the proximal trachea. Rarely, ectopic thyroid tissue can arise from the base of the tongue to the mediastinum (Patnaik et al, 2000). The loose attachments to the trachea allow enlarging adenomas or carcinomas to reach and enter the cranial thoracic inlet as a result of the effect of gravity. Most tumours are benign adenomas, with carcinomas accounting for fewer than 2% of cases (Peterson and Broome, 2014). However, a recent study of 2096 hyperthyroid cats referred for radioiodine treatment has suggested that this prevalence can reach 19.3% in cats treated with oral methimazole for 4 or more years (Peterson et al, 2015). The authors concluded that their study provides evidence that feline hyperthyroidism is a condition that progresses over time, with enlargement of tumours and transformation to malignancy, which cannot be arrested by medical management. The role of methimazole in this process, if any, requires further study. However, they acknowledged that their study population may not be representative of the whole population of cats managed with methimazole.
Benign adenomas vary in size from 1 mm to 30 mm, sometimes showing extensive cystic change (Figure 1), and between 70 and 75% of cases are bilateral. The potential for mediastinal thyroid tissue, and for cervical thyroid tumours to reach appreciable size and gravitate through the cranial thoracic inlet, accounts for between 2 and 20% of cases having ectopic tumours within the thoracic cavity (Hibbert et al, 2009; Peterson and Broome, 2014). Clinical signs of thyrotoxicosis include weight loss, polyphagia, tachycardia, palpable goitre, polydipsia and polyuria, diarrhoea and vomiting. Secondary changes commonly encountered include hepatopathy, secondary left ventricular hypertrophy and behavioural changes including restlessness and aggression. However, these changes are not readily apparent in the early stages and feline hyperthyroidism is a condition frequently underdiagnosed with up to 10% of older cats subclinically hyperthyroid (Sparkes, 2012; Gallagher and Mooney, 2013). Left untreated, hyperthyroidism will shorten lifespan through congestive heart failure, thromboembolic disease, starvation, behavioural change (such as accidental injuries resulting from extreme restlessness) or from other causes. A palpable goitre is observed in most cases, but this is not pathognomonic of feline hyperthyroidism (Norsworthy et al, 2002). Feline hyperthyroidism is more likely as the size of the goitre increases (Boretti et al, 2009), and with practice good correlation between observers can be achieved (Paepe et al, 2008). A straightforward diagnosis can be confirmed by demonstration of total T4 above normal range. Some diagnoses may rely on demonstration of free T4 above normal range, when total T4 is in high euthyroid (normal) range. Severely depressed levels of thyroid stimulating hormone (TSH) can support a diagnosis of feline hyperthyroidism, but this is not yet validated in the cat (Peterson, 2013). Specialist imaging including scintigraphy and ultrasound are useful but not commonly used (Broome, 2006; Barberet et al, 2010). Fine-needle aspirate biopsy can be mis-representative since both benign and malignant tumours can be found in the same thyroid lobe (Hibbert et al, 2009).
The focus of the remainder of this article is on current treatment options and the role of the veterinary nurse.
Treatment options
The goal of treatment is to return the hyperthyroid patient to euthyroidism and this is achieved either by the curative methods of radioiodine or surgery, or the lifelong control of thyroxine overproduction by using either oral or transdermal medication, or by severe restriction of dietary iodine. These treatments (and others) are summarised in Table 1 and are discussed in turn.
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Management
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Cure
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Other (1.3%)
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Iodine-restricted diet
Available in the UK since 2011, Hill's y/d remains the only commercially available restricted-iodine diet for cats. Iodine is an essential component of thyroid hormones and by severely restricting availability this diet prevents over production. After transitioning to the diet 64% of cats are euthyroid after 4 weeks, 75% after 8 weeks (van der Kooij et al, 2014) and 90% after 12 weeks (Melendez, 2012). However, most cats find y/d unpalatable so that for 75% of cases this diet is not an effective option (van der Kooij et al, 2014). The manufacturers claim a new formulation in 2013 has improved palatability (Hill's Pet Nutrition Ltd, 2013). Best results are obtained when y/d is eaten exclusively for the remainder of the cat's life. However, cats with potential access to iodine sources other than restricted-iodine diet, by having uncontrolled access to the outdoors, also became euthyroid in a recent study (van der Kooij et al, 2014). The suitability of this diet for the long-term nutritional needs of older cats has been questioned (Peterson, 2012b), concerns citing its dependence on carbohydrates and low energy density, making maintenance of body muscle mass difficult in the older cat (see below). The long-term effect of an iodine deficient diet on aspects of physiology other than thyroid hormones is not fully understood. It has been suggested that the high demand for iodine by thyroid tumours may make hyperthyroid cats relatively iodine deficient. For example, stomatitis has been observed in hyperthyroid cats with resolution once euthyroidism is achieved, suggesting that iodine may play a part in maintaining oral health as a constituent of saliva (Peterson, 2012b). However, there are patients for which Hill's y/d may be the most appropriate option, such as the drug intolerant or difficult to medicate cat for whom surgery and radioiodine are excluded for reasons of cost, availability or clinical suitability (see below).
Medication
Medication is most appropriate when a curative option is not available or appropriate, or if a reversible option is preferred such as for a cat with concomitant advanced renal disease. Methimazole (also known as thiamazole) is the active ingredient provided directly, or after metabolism, by currently available medications (Table 1). Some preparations are not licensed for veterinary species and the responsibility to comply with the dispensing cascade lies with the prescribing veterinary surgeon. By blockading thyroid peroxidase, methimazole prevents thyroglobulin and dietary iodine combining, a key step in the production of thyroxine. However, the underlying thyroid tumour remains unaffected. Euthyroidism is achieved in approximately 80% of new cases within 3 weeks, but although mean total T4 is within euthyroid range for the test population, individuals are often over- or under-treated (Peterson et al, 1988). The prevalence of carcinoma has been shown to rise from 2 to 20% after prolonged thiamazole use (Peterson and Broome, 2012) and this is one factor making medication less suited to a newly-diagnosed younger cat. Side effects are frequently encountered and although many can be managed some can be fatal (Table 2). Most side effects are reversible within 6 weeks of medication being withdrawn (Peterson et al, 1988). It has been shown that as few as 75% of cat owners are able to medicate their cat consistently (Caney, 2013) and this, together with the prevalence of adverse effects, have been suggested as an explanation for the life expectancy of medicated cats being half that of those treated with radioiodine (Milner et al, 2006). A recent survey reported that 98% of UK veterinary general practitioners regarded owner compliance with medication as ‘important’ or ‘very important’ (Table 3) (Higgs et al, 2014). For clients, sometimes overlooked is the potential hazard in handling these drugs. Methimazole is a potential teratogen and gloves should be worn when handling medication or the litter tray. One drug manufacturer produces a blister-pack of thiamazole product to reduce direct handling of the drug (Thiafeline, Animalcare).
| Of 262 cats, typically within the first 1–2 months of starting oral medication, adverse effects were observed in the % of cats shown (Peterson et al, 1988): | In a survey of 603 UK general practitioners, within the previous 12 months the following side effects were observed by the % of veterinary surgeons shown (Higgs et al, 2014): | ||
|---|---|---|---|
| 18.1 | Anorexia | 69 | Vomiting |
| Vomiting | 47 | Anorexia | |
| Lethargy | 44.8 | Facial pruritus | |
| Excoriation of face and neck | 22.7 | Azotaemia | |
| BLEEDING | 11.8 | ANAEMIA | |
| Hepatopathy | 10.9 | LEUKOPENIA | |
| 16.4 | Eosinophilia | 9.6 | HEPATIC DAMAGE |
| Lymphocytosis | 8.4 | NEUTROPENIA | |
| Leukopenia (IF SEVERE)(if mild) | 8.4 | THROMBOCYTOPENIA | |
| 3.8 | Agranulocytosis | 4.7 | Lymphadenopathy |
| THROMBOCYTOPENIA | 0.9 | Sudden death | |
| 21.8 | Antinuclear antibodies (significance uncertain) | General awareness of the prevalence of these adverse events would be enhanced by improved participation within the VMD pharmacovigilance scheme. The GPs reported at the following frequencies: | |
| 1.9 | RED CELL AUTOANTIBODIES | ||
| 49.6% never reported an adverse event | |||
| 36.5% reported up to 25% of adverse events | |||
| 8% reported 26–99% | |||
| 5.9% reported 100% | |||
| Recommended clinical response; red discontinue methimazole permanently – BOLD BLOCK CAPITALS are life threatening; blue monitor (BOLD BLOCK CAPITALS may become life threatening), green try lower dose and continue if tolerated | |||
| Client concerns ranked from 1 (least) to 10 (most) (Boland et al, 2014) | % of UK GP veterinary surgeons reporting the stated concern as ‘important’ or ‘very important’ when devising long-term treatment plans (Higgs et al, 2014) | ||
|---|---|---|---|
| 7 | Hospitalisation period | 98.0 | Owner compliance with medication |
| 3 | Side effects to the cat following radioiodine treatment (RIT) | 97.7 | Ease of drug administration |
| 2 | Travel to radioiodine centre | 96.5 | Co-morbid disease |
| 1 | Health risks to themselves | 80.9 | Cost of treatment |
| 1 | Waiting period for referral | 78.6 | Cost of monitoring |
| 1 | Cost of radioiodine | 76.3 | Risk of surgical complications |
| 66.7 | Age | ||
| 65.3 | Risk of drug side effects | ||
| 48.1 | Ease of referral for RIT | ||
| 30.5 | Whether cat is insured | ||
| 19.7 | Indoor vs outdoor | ||
Off-license, methimazole is also available in the UK as a transdermal gel (Summit Veterinary Pharmaceuticals) and as well as improving the reliability of medication compared with oral formulations some side effects such as vomiting and diarrhoea are reported much less frequently. Pinnal dermatitis (at the application site) can be encountered. However, although effective in keeping the mean total T4 within euthyroid range for a population of hyperthyroid cats, individuals commonly are either under or over treated (Boretti et al, 2013).
Surgery
Thyroidectomy can be a successful curative option when radioiodine is unavailable (Naan et al, 2006). Surgery is the only treatment method from which meaningful biopsies can be obtained. Cats without a palpable goitre or poor anaesthetic or surgical risks are not suitable for surgery. Challenges for the anaesthetist include those associated with the older age group typical of hyperthyroid cats, often with a combination of poor body condition score and tachycardia with secondary left ventricular hypertrophy. Hypo and hyperkalaemia are both associated with hyperthyroidism. Medication with methimazole pre- surgery can help to reduce anaesthetic risk. In cats intolerant of methimazole, or when tachycardia persists despite use of methimazole, beta-blockers also help to reduce anaesthetic risk (Naan et al, 2006). For the surgeon challenges include the difficulties of visualising micro-adenomas, and removing the entire tumour (whether adenoma or carcinoma) to prevent recurrence while avoiding iatrogenic damage to adjacent parathyroid glands. Parathyroid glands form part of the mechanism achieving calcium homeostasis and, if damaged, hypocalcaemia can occur usually within 72 hours of bilateral or second thyroidectomies. If unrecognised and untreated, hypocalcaemia can be fatal. Clinical signs of hypocalcaemia include reduced appetite, depression, weakness and twitching, progressing to tetany and seizures. Clinicians commonly hospitalise cats for several days post bilateral or second thyroidectomies using periodic blood testing and clinical observation to detect falling blood calcium levels. Treatment aims to achieve immediate correction of the deficiency and then medium-term supplementation with a combination of oral calcium and vitamin D (to aid absorption). Long-term supplementation is only rarely required. Ectopic (intrathoracic) thyroid tumour can be another reason for surgical failure. Different techniques have been described aiming to overcome the challenges listed above (Flanders, 1999). These include staged surgeries (meaning that in cases of bilateral thyroid tumour the second unilateral thyroidectomy is undertaken often several weeks after the first), intra-capsular techniques (to reduce the risk of parathyroid damage by incising the capsule enclosing the thyroid lobe, to avoid damaging adjacent parathyroid tissue) and extra-capsular techniques (to avoid retention of thyroid tumour cells by removing the thyroid lobe within its capsule).
Radioiodine
Radioiodine is the treatment of choice in most cases, especially the newly diagnosed (Peterson and Becker, 1995; Peterson, 2006; Nelson, 2007; Daniel and Neelis, 2014). Cases requiring intensive nursing care or other clinical support may not be suitable if staff radiation safety would be compromised (see below). Sodium iodide 131 is a dual-emitter of both β and γ radiation, and is administered usually as a subcutaneous injection (Peterson and Becker, 1995; Theon et al, 1994). Once administered it follows the same physiological pathway as dietary iodine. The normal production of thyroxine requires stimulation of the thyroid gland by TSH, produced by the pituitary. Increasing thyroxine levels result in a reduction in TSH levels by a negative feedback loop, thereby achieving homeostasis. Thyroid tumour tissue produces thyroxine to excess without requiring stimulation by TSH. The above-normal thyroxine levels that result suppress TSH below the level needed to stimulate normal thyroid tissue and disuse atrophy results. The atrophied normal thyroid tissue usually takes up insignificant amounts of iodine, and when radioiodine is administered to uncontrolled hyperthyroid cats it is this which usually prevents ablation of normal thyroid tissue. Once taken up by the thyroid tumour (at whatever location) the β radiation causes severe damage to cells within a radius of up to 0.5 mm. It is this very short path length that prevents iatrogenic hypoparathyroidism from occurring. Most tumour cells are destroyed but some are only damaged, prevented from being able to divide but still functional until apoptosis (normal cell death). Most cats become euthyroid within 1 to 3 weeks of radioiodine treatment (RIT) but the full effect is not seen in every case until up to 6 months later. At the end of this period approximately 95% of cats are permanently cured, with de novo episodes of hyperthyroidism affecting approximately 2% with a mean interval of 3.4 years. Approximately 1.5% of cases are still hyperthyroid 6 months post RIT and most of these respond to a second treatment (Peterson and Becker, 1995). Thyroid carcinoma is suspected in hyperthyroid cats unresponsive to two treatments. Approximately 2.1% of cats require lifelong supplementation with oral thyroxine following iatrogenic hypothyroidism (Peterson and Becker, 1995) (Table 4). Recovery of atrophied normal thyroid tissue can take a period of weeks to months and hypothyroidism can be encountered immediately following RIT, resolving in most cases. Persistent hypothyroidism results either from inadvertent ablation of normal thyroid tissue, or failure of atrophied normal thyroid tissue to resume normal function. In cats RIT has no side effects other than iatrogenic hypothyroidism. However, the γ-radiation from iodine 131, which has the same characteristics as x-ray radiation, creates substantial radiation safety challenges for staff and cat owners. For this reason cats are commonly sedated for treatment, and specialist hospitalisation is required before cats can be returned to their owners. In the UK cats can return home from 5 days after treatment although the minimum period varies between centres. Pre-RIT screening intends to avoid either patient welfare or staff safety being compromised if close handling of a radioactive cat was required, for example following progression of significant concomitant disease. Pre-RIT protocols vary between centres. Cases with life-threatening concomitant disease such as congestive heart failure may therefore be unsuitable for RIT, while non-critical illnesses may be managed and RIT provided when resolved or stable (Puig et al, 2015). Minimising the hospitalisation period is significant as a catowner survey highlighted this as the biggest concern when considering RIT (Table 3, Figure 2) (Boland et al, 2014). Providing cat-friendly accommodation with appropriate behavioural enrichment is key in managing cat needs and client concerns (Figure 3).
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Post-treatment nutrition
In addition to treating hyperthyroidism consideration should be given to appropriate dietary management. The normal ageing process includes loss of muscle mass (sarcopenia of ageing) after 12 years of age (to the extent that 15 year olds may have one third less muscle mass than 7 year olds), an increase in energy requirements above 11 years of age and a reduced ability to digest protein. Most hyperthyroid cats are over 10 years of age and their increased metabolic rate compounds normal ageing changes. As obligate carnivores, 40–60% of dietary energy must be supplied as protein to prevent the cat's own protein being consumed (Peterson and Eirmann, 2014).
Chronic renal disease (CRD) is commonly encountered in older cats (Williams et al, 2010a) and is more common in hyperthyroid cats than in euthyroid cats (30% of hyperthyroid cats with mean age 12–13 years of age) (Williams et al, 2010b). It is now understood that hyperthyroidism hastens the progression of renal disease (Higgs et al, 2014), hence the value of achieving stable permanent euthyroidism soon after hyperthyroidism is diagnosed. Cats with CRD often have elevated serum phosphorous levels and managing this often relies on reducing dietary protein. Paradoxically this restriction of dietary protein compounds normal ageing changes and worsens the catabolism of hyperthyroidism.
It is also known that a proportion of hyperthyroid cats show pre-diabetic change including insulin resistance and mild hyperglycaemia, for which a diet where <10% of energy is provided by carbohydrates is advised. Given that up to 50% of energy will then be provided by fat, care must be taken to avoid obesity which again predisposes to diabetes. Cats with advanced CRD will require phosphate restriction and this can be difficult to achieve while maintaining energy intake in the proportions recommended above, and dietary phosphate binders may be required.
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Diet
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Drinking water
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Cat litter
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Packaging
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Environment
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Risk factors for the development of feline hyperthyroidism
Currently the aetiology of feline hyperthyroidism is poorly understood. Epidemiological studies and others have suggested an association of feline hyperthyroidism with the feeding of canned cat food, inappropriate or inconsistent concentrations of dietary iodine, exposure to goitrogens (such as bisphenol A (found in some packaging such as cat food tins), isoflavones (a component of soy protein, used as a cheaper protein source in some cat foods) and flame retardants), the use of litter trays, herbicides and some ectoparasiticide spot-on's (Peterson, 2012a). Genetic predisposition has also been suggested (Kass et al, 1999). However, recent work sponsored by a pet food manufacturer challenges the associations described above, as there is limited evidence of causation (van Hoek et al, 2014).
Some suggested strategies to limit the risk factors for feline hyperthyroidism have been proposed (Table 5) and although it is not known whether they will have any benefit they are unlikely to be harmful and are based on current thinking (Peterson, 2012a).
The role of the veterinary nurse in feline hyperthyroidism
Feline hyperthyroidism is the most common endocrinopathy in older cats, but is still underdiagnosed. The role of the veterinary nurse impacts at all stages:
Conclusion
The increasing prevalence of feline hyperthyroidism, still without an understanding of aetiology or effective prevention strategies, makes this a very frequent presentation in most companion animal practices. The veterinary nurse is ideally placed to facilitate early detection, enhance concordance between the practice team and the cat owner, and aid long-term management of feline hyperthyroid patients.