FELINE THYROID DISEASE: FOCUS ON NEW APPROACHES AND TREATMENTS

Transcription

1 Vet Times The website for the veterinary profession FELINE THYROID DISEASE: FOCUS ON NEW APPROACHES AND TREATMENTS Author : SARAH CANEY Categories : Vets Date : August 12, 2013 SARAH CANEY discusses this common yet complex disorder in cats, highlighting recent advances in managing it as well as associated complications Summary Hyperthyroidism is the most common feline endocrinopathy and is a frequent diagnosis in clinical practice. There are four management options available antithyroid medication, iodine-restricted food, radioiodine and surgical thyroidectomy. All treatments have potential advantages and disadvantages, so the treatment decision should be based on the individual cat s circumstances and discussions with the owner. Follow-up monitoring is essential to assess treatment success and check for side effects. Many cats suffering from hyperthyroidism have additional medical problems especially chronic kidney disease which may also complicate management and monitoring. Over the years there have been many advances in our understanding of this complex disease. This article will highlight some of the recent important advances in our knowledge concerning treatment of hyperthyroidism. Key words hyperthyroidism, methimazole, carbimazole, radioiodine, iodine-restricted, thyroidectomy THERE are four options for hyperthyroidism management. Tables 1 and 2 summarise some of each option s features. 1 / 13

2 In addition to management of their hyperthyroidism, patients may also require specific treatment for complications associated with this, such as congestive heart failure, urinary tract infections and systemic hypertension. What treatment options are there? Antithyroid medication Medical stabilisation, using antithyroid medications such as methimazole and carbimazole, blocks production of thyroid hormones and symptomatically manages hyperthyroidism. Lifelong treatment is required, unless a curative treatment, such as surgery or radioiodine, is subsequently pursued. In the long term, difficulties with owner and patient compliance may reduce the overall success of this treatment modality. Nevertheless, medical treatment is popular not least since it is a reversible treatment, which is of particular benefit when stabilising patients with concurrent renal disease. The dose of medication can be fine-tuned to suit the individual patient, and withdrawn completely if necessary. A recent survey of owners of cats with hyperthyroidism showed they appreciate discussions with their veterinarian and being fully involved in the decision-making process. The most important treatment priorities for owners were prescribing the most accurate dose of medication and using the lowest possible dose. None ranked once daily treatment as most important to them and 79 per cent of owners said they were, or would be, happy to dose their cat twice daily to control its hyperthyroidism. For 62 per cent of owners, pilling their cat twice daily was not a problem1. Recently, a transdermal methimazole gel has become available in the UK via Summit Veterinary Pharmaceuticals. This is not a veterinary licensed preparation, but can be used under cascade regulations, where appropriate. Dosing is as for oral preparations, with the same range of potential adverse effects. The gel is usually applied to the inside of the pinna on a hairless area. The medication is then absorbed through the skin into the blood stream. Transdermal antithyroid medications can take longer to be effective than oral forms. Iodine-restricted food An iodine-restricted diet is now available. Thyroid hormone production requires iodine molecules, therefore, limiting the amount of iodine fed reduces the amount of thyroid hormone produced and released by the thyroid gland. As with medical management, lifelong treatment (with 100 per cent compliance) is required, unless a curative treatment is subsequently pursued. Patient and owner compliance is essential to success even small deviations from the prescribed feeding can allow escape of thyroid control. Unlike medical treatment, there are no drug-related side effects to worry about. However, 2 / 13

3 compliance may be an issue especially if using this treatment long-term. The food is phosphate restricted and moderate in protein, making it a suitable nutrition for cats with mild to moderate renal disease, but not recommended for cats in International Renal Interest Society (IRIS) Stages 3 and 4 renal disease. Surgical thyroidectomy A potentially curative treatment, surgical thyroidectomy has the disadvantages of requiring general anaesthesia (which may be contraindicated in some patients) and is only suitable for cases with easily accessible hyperfunctional thyroid tissue. Up to 20 per cent of patients may have ectopic hyperfunctional thyroid tissue, commonly located in the anterior thorax not an area suited to straightforward thyroidectomy. In routine cases, side effects of thyroidectomy, such as damage to the parathyroid glands resulting in hypocalcaemia, are possible. Radioiodine treatment Usually administered by subcutaneous injection, radioactive iodine targets abnormal thyroid tissue, resulting typically in a 95 per cent success rate. Published studies have so far shown the best longterm prognosis for treatment of hyperthyroidism is achieved with radioiodine2. How should patients be managed initially? All hyperthyroidism treatments have the potential to worsen kidney function. This is because the hyperthyroid condition increases renal blood flow and glomerular filtration rate (GFR). The reduction in renal blood flow and GFR occurring with successful treatment of hyperthyroidism, may unmask previously unknown cases of renal disease and/or worsen pre-existing disease. Assessing kidney function in hyperthyroid cats is difficult for many reasons. It is also not possible to predict which patients will deteriorate into chronic kidney disease, following treatment of their thyroid disease. It is estimated that possibly one third to one half of all patients suffer a clinically significant decline in renal function, following achievement of euthyroidism. For this reason, most clinicians prefer to treat all hyperthyroid cats with reversible treatments in the first instance. If kidney function deteriorates, the hyperthyroid treatment can be reduced or stopped to help the cat to cope again. Reversible treatments include the thioureylenes methimazole and carbimazole, and iodine-restricted foods. If a cat is known to have kidney disease before medical treatment is started, more gentle treatment for example, lower doses of antithyroid medication and closer monitoring is recommended. If any problems are seen, the dose of treatment can be lowered or even stopped. It can be a delicate balance between treating the thyroid disease and yet not compromising kidney function. If the kidney disease remains stable on hyperthyroid treatment, and the cat remains well, more permanent treatments for the hyperthyroidism, such as surgery and radioiodine, can be 3 / 13

4 considered. Understanding hypothyroidism and renal complications Iatrogenic hypothyroidism is a potential complication with all of the different treatment modalities. Until relatively recently, hypothyroidism was thought to be a very uncommon consequence of treatment for hyperthyroidism. Iatrogenic hypothyroidism is thought to be most common following treatment with radioiodine. Although one publication suggested as many as 30 per cent of radioiodine-treated hyperthyroid cats suffered from hypothyroidism3, most publications have suggested less than 10 per cent of cats treated with radioiodine suffer from hypothyroidism4,5,6. In most cases, the hypothyroidism is not permanent, although treatment is still indicated during this period. Unfortunately, little information is published on iatrogenic hypothyroidism, for those cats treated surgically or using antithyroid medication. Clinical signs of iatrogenic hypothyroidism include lethargy, weight gain, hypothermia, seborrhoea sicca, alopecia (especially affecting the pinnae) and myxoedema. Routine laboratory profiles may reveal hypercholesterolaemia and a mild, non-regenerative anaemia. Hypothyroidism cannot be diagnosed solely on the basis of persistently low total thyroxine (tt4) results, since these can be suppressed by nonthyroidal illness. A diagnosis of hypothyroidism can be confirmed by measuring thyroid stimulating hormone (TSH) levels (using the canine assay, hence ctsh) and/or by performing a thyrotropin releasing hormone (TRH) or TSH stimulation test. Cats with hypothyroidism have persistently low levels of tt4, elevated levels of ctsh and fail to respond to stimulation with exogenous TRH or TSH. In those cats developing hypothyroidism following radioiodine therapy, ctsh levels of more than 0.5ng/ ml are often found (reference range 0.03ng/ml to 0.15ng/ ml). Measuring free thyroxine (T4) can also be helpful as levels are expected to be low in cats suffering from hypothyroidism. However, levels of free T4 can also be suppressed by concurrent illnesses, such as chronic kidney disease. A recent study showed development of iatrogenic hypothyroidism is associated with a significantly worse prognosis. A significantly higher proportion of cats with iatrogenic hypothyroidism suffered from azotaemia, compared to cats that were euthyroid, following treatment of their hyperthyroidism. In addition, hypothyroid cats that developed azotaemia post-treatment suffered significantly shorter survival times than those that remained non-azotaemic7. Hypothyroidism reduces renal blood flow and GFR, which is likely to be an important mechanism for negatively impacting on renal function in these cats. Given this information, it is now important for all clinicians to consider iatrogenic hypothyroidism in their patients following treatment of hyperthyroidism. Advice for detection and management of hypothyroidism varies, according to the treatment given. 4 / 13

5 With cats receiving hyperthyroidism management with antithyroid medication, medical treatment should be titrated to maintain euthyroidism the aim being to suppress tt4 levels so they remain within the lower half of the reference range in treated cats. The dose of antithyroid medication should be reduced if tt4 levels fall below the reference range. Cats with hyperthyroidism managed by feeding an iodinerestricted food may need to have this food withdrawn if they develop hypothyroidism, although, so far, this has not been reported in any cats receiving this management. In cats that have had permanent hyperthyroidism treatment (such as iodine or surgery), transient hypothyroidism lasting up to six months is common and probably affects up to 30 per cent of patients. A diagnosis of hypothyroidism can be confirmed by performing diagnostic tests, as described earlier. L-thyroxine hormone supplementation (available under the cascade at an initial dose of 10 µg/kg to 20 µg/kg orally, daily) is recommended in confirmed cases of iatrogenic hypothyroidism. The dose is adjusted according to clinical response and post-pill thyroxine levels. Hyperthyroidism and urinary tract infections A number of recent publications have indicated cats with hyperthyroidism are more vulnerable to bacterial lower urinary tract infections (UTIs). For example, one study reported bacterial lower UTIs were diagnosed in 12 per cent of hyperthyroid cats8. Unfortunately, bacterial UTIs are clinically silent in a high proportion of older cats, with no haematuria, dysuria or other signs to indicate their presence. Where possible, bacterial culture of cystocentesis-obtained urine samples is recommended in hyperthyroid cats at time of diagnosis and periodically thereafter especially if indicated by clinical signs or previous history. The author performs urine bacteriology twice a year in her hyperthyroid patients. Hyperthyroidism and systemic hypertension Around 10 to 20 per cent of hyperthyroid cats are diagnosed with concurrent systemic hypertension. Interestingly, a proportion of hyperthyroid cats develop systemic hypertension after their hyperthyroidism has been treated, so continued monitoring of blood pressure and ocular findings is indicated during and following treatment. The author recommends blood pressure measurement using Doppler methodology every three to six months in hyperthyroid patients. References and further reading 1. Caney S M A (2013). An online survey to determine owner experiences and opinions on the management of their hyperthyroid cats using oral anti-thyroid medications, J Feline Med Surg 15(6): / 13

6 2. Milner R J, Channell C D, Levy J K and Schaer M (2006). Survival times for cats with hyperthyroidism treated with iodine 131, methimazole, or both: 167 cases ( ), J Am Vet Med Assoc 228(4): Nykamp S G, Dykes N L, Zarfoss M K and Scarlett J M (2005). Association of the risk of development of hypothyroidism after iodine 131 treatment with the pretreatment pattern of sodium pertechnetate Tc 99m uptake in the thyroid gland in cats with hyperthyroidism: 165 cases ( ), J Am Vet Med Assoc 226(10): 1,671-1, Slater M R, Komkov A, Robinson L E and Hightower D (1994). Longterm follow-up of hyperthyroid cats treated with iodine-131, Vet Radiol Ultrasound 35(3): Peterson M A and Becker D V (1995). Radioiodine treatment of 524 cats with hyperthyroidism, J Am Vet Med Assoc 207(11): 1,422-1, Slater M R, Geller S and Rogers K (2001). Long-term health and predictors of survival for hyperthyroid cats treated with iodine-131, J Vet Intern Med 15(1): Williams T L, Elliott J and Syme H M (2010). Association of iatrogenic hypothyroidism with azotaemia and reduced survival time in cats treated for hyperthyroidism, J Vet Intern Med 24(5): 1,086-1, Mayer-Roenne B, Goldstein R E and Erb H N (2007). Urinary tract infections in cats with hyperthyroidism, diabetes mellitus and chronic kidney disease, J Feline Med Surg 9(2): Caney S M A (2013). Caring for a Cat with Hyperthyroidism (2nd edn), Cat Professional Ltd, Edinburgh. 6 / 13

7 Weight loss is a common feature of hyperthyroidism and can be dramatic. 7 / 13

8 Iatrogenic hypothyroidism, following treatment with radioiodine. Pinnal alopecia is clearly visible. 8 / 13

9 A recent survey of owners of cats with hyperthyroidism indicated that 79 per cent were, or would be, happy to dose their cat twice daily in order to control its hyperthyroidism. 9 / 13

10 Blood pressure monitoring is important at the time of diagnosis and following treatment of hyperthyroidism. 10 / 13

11 TABLE 1. Treatment considerations for each hyperthyroidism management option 11 / 13

12 12 / 13

13 TABLE 2. Advantages and disadvantages of each management option 13 / 13 Powered by TCPDF (