Vet360 Vol 3 Issue 04 August 2016 - Page 14

CPD ACCREDITED ARTICLE Clinical signs The clinical signs of hyperthyroidism vary in severity and cats diagnosed earlier these days and so often show less of the classic clinical signs. The disease is insidiously progressive and owners may consider signs, especially when mild, part of the “normal” aging process. Thus months may pass before veterinary care is sought. General features of the disease include: • Weight loss (one of the most common clinical signs and seen in more than 80% of cats with the disease) • Hyperactivity • Polyuria and polydipsia • Unkempt hair coat • Cardiovascular signs (very commonly seen) including tachycardia, heart murmurs, gallop rhythm, increased apical pulse, evidence of systemic hypertension (e.g. retinal detachment), left ventricular hypertrophy seen on echocardiography and evidence of congestive heart failure (e.g. dyspnoea, pulmonary crackles in cats with pulmonary oedema and/or pleural effusion) • Palpable goitre • Gastrointestinal signs such as polyphagia or anorexia (seen in apathetic hyperthyroidism), vomiting and less commonly diarrhoea • Muscle weakness and atrophy (rarely seen) It is now thought that most cats with hyperthyroidism go through a 1-3 year period of subclinical hyperthyroidism before developing overt disease cal signs or ruling out the disease in cats presenting with varied problems that may or may not be related to hyperthyroidism. General screening laboratory tests, diagnostic imaging (echocardiography, radiography) and electrocardiography may provide supportive evidence of hyperthyroidism or detail the extent of cardiac involvement. Screening laboratory tests are also useful in eliminating other diseases with similar clinical signs or in depicting concurrent disorders potentially masked by hyperthyroidism that may be important in treatment decisions and ultimate prognosis, such as the presence of chronic renal disease, diabetes mellitus or neoplasia. Cats showing depression, lethargy and reduced appetite are referred to as apathetic hyperthyroid cases and have traditionally accounted for less than 10% of all hyperthyroid cases. Some of these cats are overweight rather than thin and most of them are suffering from either severe cardiac complications associated with their hyperthyroidism or have underlying neoplastic disease. Changes found on routine screening tests include: • Elevated liver enzyme activities (alanine aminotransferase (ALT) and alkaline phosphatase (ALP). At least one of these liver enzymes is elevated in 90% of hyperthyroid cats. • Leukocytosis, eosinopenia and erythrocytosis • Mild hypokalaemia and hyperphosphataemia, in the absence of azotaemia, are also seen in a small number of affected cats • Hyperthyroid cats have also been shown to be more vulnerable to bacterial lower urinary tract infections, with a prevalence of 12% in one study, so cystocentesis and urine culture is indicated. Urinalysis is also indicated as part of detailed screening for concurrent illnesses such as chronic kidney disease and diabetes mellitus. The overwhelming majority of hyperthyroid cats have a palpable goitre. Increasingly however, cats with ‘cold’ thyroid nodules have been identified (i.e. the presence of palpable thyroid nodules without detectable hyperthyroidism). Many of these cats are in fact suffering from subclinical hyperthyroidism. It is now thought that most hyperthyroid cats go through a 1-3 year period of subclinical hyperthyroidism before developing overt hyperthyroidism. During this period, the cat has a total plasma T4 concentration within the normal reference range in combination with persistently low levels of TSH (< 0.03 ng/ml). Many of the cats later go on to develop hyperthyroidism, so close monitoring of these patients is justified. In most cats, the diagnosis can be confirmed by measuring resting serum total thyroxine levels (tT4). Serum tT4 concentration includes both the proteinbound and free levels of T4 circulating in the blood and in most cats, hyperthyroidism can be diagnosed on the basis of high resting serum tT4 concentration (tT4 > 50nmol/L). Measurement of serum total T3 (tT3) alone is not usually recommended because it is less sensitive than measuring tT4 (about 30% of hyperthyroid cats have normal tT3 values). Occasionally normal resting serum tT4 concentrations are recorded for cats with hyperthyroidism. This could be due to within or between day variations in mildly affected animals or the effects of concurrent non-thyroidal illness. Diagnosis As hyperthyroid cats are being diagnosed earlier, when showing fewer clinical signs than in the past, there has been a change in clinical emphasis. Instead of simply conf \Z[HXYۛ\[H]\[[]H\X[[X[YۜH[\XYۛH\\\Y\H[]]]Y[K[KBY\\\Y\H\\XY\]HHܛX[ [Z]Y  ۘ[][ۈ   ML[ K\B\HH\وXH[\Y]]H[ۜ΂]\H [][Xݙ\ X[YXXH[\Z[\X\H\Y[YYY Xۘ\[\X\H[] ͌\YH UQT M MU ͌UQT Mܚ[˚[ M M ̍H LN B