Vet360 Vol 3 Issue 04 August 2016 - Page 16

CPD ACCREDITED ARTICLE however the value of such imaging in the diagnosis of hyperthyroidism remains undocumented. Although both CT and ultrasonography can provide information regarding thyroid volume and morphological information, including invasiveness and can guide sampling, they are unable to provide functional information and may not identify ectopic thyroid tissue or metastatic disease. When investigating a cat for possible hyperthyroidism, it is also important to consider all possible differential diagnoses and to look for evidence of multiple interacting diseases. This is because hyperthyroidism is seen most commonly in older cats and this group of patients is often affected by more than one disorder. Diabetes mellitus, renal disease, malassimilation syndromes (including inflammatory bowel disease, early intestinal lymphosarcoma, pancreatitis, and exocrine pancreatic insufficiency), acromegaly, and hyperadrenocorticism are perhaps the most important differential diagnoses. Treatment Spontaneous remission of hyperthyroidism has not been reported and prevention is not possible as the aetiology still remains elusive. Because of the benign nature of the thyroid lesions, if treated appropriately, hyperthyroidism carries a favourable prognosis. Failure to institute therapy will result in insidious progression of clinical signs to emaciation, severe metabolic and cardiac dysfunction and ultimately death. Treatment options include medical management with anti-thyroid drugs, surgical thyroidectomy, thyroid ablation using radioactive iodine (the gold standard in human medicine) or most recently, feeding an iodinerestricted diet. Each treatment option has its own advantages and disadvantages. Selecting one therapy over the other depends on the age of the cat, severity of thyrotoxicosis, presence of concurrent illnesses, facilities available, potential complications, costs and the owner’s lifestyle and willingness to accept the form of treatment advised. There are a few studies directly comparing the outcome of each of the treatment methods available. assess the cat’s renal function (check blood urea nitrogen [BUN], creatinine concentrations, serum SDMA levels and urine specific gravity). This is because resolution of the hyperthyroid state often is associated with an increase in BUN and creatinine concentrations and a decrease in glomerular filtration rate (GFR) and effective renal blood flow. Because of this, some cats without prior evidence of renal insufficiency or with only mild renal impairment develop signs of uraemia after treatment for hyperthyroidism. To determine what effect resolving the hyperthyroid state may have on renal function, a short course of medical therapy is recommended before considering radiotherapy or surgery. Cats which show significant azotaemia should then be maintained on medical therapy and should not be considered suitable candidates for radiotherapy or surgery. Similarly, cats which develop significant azotaemia after radiotherapy or surgery should be given L-thyroxine to maintain a euthyroid or mild hyperthyroid state. Interestingly in a recent study that compared the survival of cats that developed mild azotaemia (> 177 nmol/L) with cats that did not develop azotaemia following medical therapy, no difference was found between the two groups. These results suggest that the importance of the mild azotaemia that commonly develops after treatment for hyperthyroidism may have been overemphasized previously and does not change long-term outcomes in these cats. The same study however did find that in cats that were inadvertently over-treated and made hypothyroid, the development of azotaemia was of significance as it negatively affected prognosis. Medical Management Medical management entails the use of anti-thyroid drugs. It is used in cats for long term control and is advised prior to surgery to decrease the metabolic and cardiac complications associated with anaesthetising hyperthyroid cats and to control clinical signs in cats awaiting radioactive iodine therapy. One study of 167 hyperthyroid cats suggested that the median survival time (MST) in cats treated with radioactive iodine was significantly longer (4 years) compared with that of cats treated with anti-thyroid drug therapy alone (2 years). Owner/cat compliance and adverse drug reactions play a significant role in this shorter survival time. Pre-existing renal failure also adversely affects survival time irrespective of treatment type and results in a MST of about 6 months. Drugs of the thiurylene class, mainly methimazole and carbimazole, are most frequently used for both the pre-operative and long term medical management of hyperthyroidism because of their consistent and potent effect in lowering thyroid hormone concentrations. Methimazole and carbimazole are antithyroid drugs that block T3 and T4 synthesis. They share the same mechanism of action; carbimazole is almost entirely broken down to methimazole in vivo (carbimazole, 5 mg, is broken down to methimazole, 3 mg). It usually takes 1 week of treatment to achieve a significant decrease in tT4 concentration. Before deciding which treatment to use, the cat should be assessed for concurrent disease, especially renal disease, systemic hypertension, and heart disease, all of which occur commonly in association with hyperthyroidism. It is particularly important to The dose for both drugs is 2.5 to 5.0 mg per cat administered orally every 12 to 24 hours initially, increasing to every 8 to 12 hours as necessary, adjusted to maintain a euthyroid state, and given for the rest of the cat’s life. The drugs are most effective given twice vet360 Issue 04 | AUGUST 2016 | 16 VET360 AUGUST 2016 working.indd 16 2016/07/25 11:04 PM