Vet360 Vol 3 Issue 04 August 2016 - Page 17

CPD ACCREDITED ARTICLE or three times daily. The dose should initially be low (especially when renal insufficiency is suspected), and the renal values should be monitored as the dose is gradually increased. When cat and owner compliance is good, the successful response rate is approximately 85%. Difficulty in administration causes many treatment failures. In an attempt to make medicating simpler, many clinicians are now using topical transdermal applications of methimazole, and initial studies seem to show promise. Advantages of medical therapy are numerous, including easy implementation, no requirements for special facilities, good availability and a reasonable cost to the client. Almost all cats are potential candidates and very few contra-indications, other than presence of thyroid carcinoma, exist. The main disadvantages of medical therapy in this species, is the high degree of owner and patient compliance required and the rapid recurrence of signs if compliance is lost. In addition adverse and serious side effects occur in about 8-15% of cats treated with carbimazole or methimazole, which may require complete drug withdrawal. The most common is vomiting, anorexia and depression. Blood dyscrasias, facial excoriation, hepatotoxicity, renal decompensation, coagulation abnormalities, generalised lymphadenopathy and even acquired myasthenia gravis occur less commonly. These side effects normally occur within the first 3 months of treatment. Other medical treatments available include the use of iopanoic acid and beta blockers. The results of studies evaluating the efficacy of cholecystographic agents such as iopanoic acid, in the treatment of feline hyperthyroidism have been disappointing and the drug is not recommended for long term management of the disease but may be suitable for short -term management prior to surgery or radioiodine therapy. Beta blockers such as propanolol and atenolol as well as potassium iodate have also been used successfully in pre-surgical stabilisation but longer term studies are required to evaluate their use for long-term medical management. Control of hyperthyroidism by percutaneous injection of ethanol into solitary thyroid nodules as well as ultrasound-guided thyroid radio-heat ablation has been described in a small number of cats, but these techniques have been associated with serious side-effects and are unlikely to be a viable option for the majority of hyperthyroid cats. Surgical Thyroidectomy Surgical thyroidectomy is highly effective, almost always curative and widely available for cats, but the procedure can be associated with significant morbidity and mortality. Hyperthyroid cats also represent an anaesthetic risk due to the systemic effects of the disease. The most common serious post-surgical complications in cats include hypocalcaemia, due to hypoparathyroidism when the parathyroid glands are inadvertently removed, laryngeal paralysis, voice changes and Horner’s syndrome. Treatment failure can occur due to inappropriate unilateral thyroidectomy where there is bilateral disease, incomplete removal of tissue or less commonly the presence of hyper-functional ectopic thyroid tissue. This can be avoided by performing thyriod scintingraphy pre-operatively. Radioactive Iodine Treatment Radioactive iodine treatment, the therapy of choice in human toxic nodular goiter, is also successfully used in the cat. It is simple, safe and effective and is possibly the best curative treatment for most hyperthyroid cats. Permanent hypothyroidism is rare as is recurrence after successful treatment and other side effects are minimal. The main disadvantages of such treatment are that it is potentially extremely hazardous to personnel, requires prolonged hospitalisation, is costly and is only available in a limited number of centres. Iodine Restricted Diet A novel option for treatment of feline hyperthyroidism now exists in the form of a commercially available iodine restricted diet (Hills Y/D®). An iodine restricted food was developed based on the hypothesis that feline hyperthyroidism can be managed nutritionally by limiting the amount of dietary iodine available for production of thyroid hormones. Feeding an iodine restricted food has been shown in multiple feeding trials over the past 10 years, in over 100 cats with naturally occurring disease, to decrease thyroid hormone concentration and alleviate clinical signs of hyperthyroidism. Three additional studies documenting the safety and efficacy of iodine restricted food as the sole therapy of hyperthyroid cats showed that a food with iodine levels at or below 0.32 ppm per dry matter basis was able to convert 90% of the hyperthyroid cats in the study to a euthyroid state within 8-12 weeks. In all these studies biochemical features of renal function remained stable and no other biochemical abnormalities were observed. The only drawback in using an iodine restricted diet is that feeding compliance has to be 100% for the diet to have an effect. Any exposure to products or foods containing iodine, such as compounding agents for medications, treats, supplements, cleaning agents, other pet food, prey food or home prepared meals or treats and even in some cases municipal water, should be discontinued. The diet has been shown not to have any adverse effects if fed to healthy cats and as such, the maintenance of normal thyroid concentrations and lack of clinical signs of hypothyroidism in these cats indicates that adequate amounts of iodine is provided to healthy animals fed the diet. Issue 04 | AUGUST 2016 | 17 VET360 AUGUST 2016 working.indd 17 2016/07/26 9:34 AM