Vet360 Vol 3 Issue 04 August 2016 - Page 15

CPD ACCREDITED ARTICLE be first managed before proceeding with further thyroid testing. Once concurrent disease is resolved, the “euthyroid sick effect is removed” and most hyperthyroid cats will develop a clearly high tT4, confirming the diagnosis. Conversely, in cats without any overt underlying disease, simply repeating the serum tT4 concentration after 2 weeks may be diagnostic if the tT4 is fluctuating in and out of the reference interval. In some cats with preclinical disease, it may take a number of weeks or even months for the serum tT4 concentrations to increase into the range diagnostic for hyperthyroidism. Test serum free T4 In cats with mild hyperthyroidism and tT4 values within the upper third of the reference interval, serum free T4 (fT4) concentrations can also aid in diagnosis. Serum fT4 concentrations are more consistently elevated (less fluctuation) in hyperthyroid cats than are tT4 concentrations. Although fT4 is more sensitive than tT4 for diagnosing hyperthyroidism, the test specificity for fT4 is poor, with up to 20% of sick (and some clinically normal) euthyroid cats having false-positive fT4 results. Caution is therefore advised in using serum measurements of fT4 as the sole diagnostic test for hyperthyroidism. As a thyroid function test, fT4 should always be interpreted with a corresponding tT4 measurement. A tT4 value within the upper third of the reference range (30-50 nmol/L), combined with a high fT4 concentration, is consistent with mild hyperthyroidism, whereas a low or low– normal tT4 with a high fT4 is usually associated with non-thyroidal illness. Serum fT4 is currently measured by one of two methods: Radio-immuno assay (RIA) using kits designed for use in humans and a modified equilibrium dialysis (MED) technique. The MED technique is the most accurate method for determining serum fT4 concentrations but unfortunately is not available in South Africa. Test serum cTSH The use of canine TSH (cTSH) assays in cats has recently received attention in the diagnosis of feline hyperthyroidism. A reference range for cTSH of 0.03-0.15 ng/ml has been defined for older cats. In cats with subclinical and occult hyperthyroidism, cTSH levels are low or undetectable. Therefore if tT4 levels are in the upper half of the reference range (>30 nmol/l) and cTSH levels are low or undetectable (< 0, 03 ng/ml), hyperthyroidism can be diagnosed. A recent study evaluating the usefulness of cTSH as a diagnostic test for feline hyperthyroidism using thyroid scintigraphy as the gold standard, conclud- ed that measurement of serum cTSH concentration is a very sensitive, but nonspecific, diagnostic test. Approximately 98% of hyperthyroid cats had serum cTSH concentrations that are suppressed below the limit of quantification (<0.03 ng/ml), but approximately 30% of the older euthyroid cats in that study also had undetectable serum cTSH concentrations. The current commercial cTSH assay cannot accurately measure the very low concentrations needed to clearly distinguish between the low-normal serum cTSH concentrations found in some euthyroid cats from the truly low or totally suppressed concentrations found in most hyperthyroid cats. It was shown that combining serum cTSH with tT4 or fT4 concentrations lowered the test sensitivity of cTSH from 98 to 97%, but markedly increased overall test specificity (from 69.9 to 98.8%). The conclusion reached was that testing in parallel by combining serum cTSH concentration with either tT4 or fT4 concentrations improved the ability to correctly differentiate hyperthyroid cats with occult or mild disease from euthyroid cats suspected of having thyroid disease, especially when serum concentrations of tT4, T3, or fT4 were within the upper limits of their reference interval or only marginally increased. Dynamic thyriod tests Other options for diagnosis of suspect cases include dynamic thyroid tests (T3 suppression, TSH/ TRH stimulations tests) and thyroid imaging (scintigraphy, ultrasound and CT). Dynamic thyroid tests are not always straightforward to interpret and are much less frequently performed since they require multiple samples to be collected and may result in side-effects. Thyriod Imaging Scintigraphy is performed to determine whether there is increased activity in the thyroid glands relative to the activity in the salivary glands 20-60 minutes after intravenous or subcutaneous injection of pertechnetate. It is a very accurate and reproducible test used to differentiate between cats with unilateral or bilateral disease and to check for the presence of ectopic thyroid tissue not palpable in the neck. This is extremely helpful when planning surgery. However as it requires access to specialist facilities, it is not routinely available. Ultrasonography has been used to document the dimensions and volume of the thyroid glands in euthyroid and hyperthyroid cats and has been shown to have 85.7% agreement with scintigraphy in defining normal and abnormal thyroid lobes. However, it is technically demanding and very operator dependent. CT has been used to determine the dimensions and volume of thyroid tissue in clinically normal cats; Issue 04 | AUGUST 2016 | 15 VET360 AUGUST 2016 working.indd 15 2016/07/25 11:04 PM