Hyperthyroidism in cats: should we be routinely testing for early diagnosis?
Abstract
Background:
Routine testing for hyperthyroidism could become standard practice in all cats, before the onset of clinical signs.
Aims:
To describe serum total thyroxine (T4) concentration levels in a population of clinically normal cats; to determine the range of T4 levels in clinically normal cats; to estimate the prevalence of hyperthyroidism in clinically normal cats and to identify risk factors for elevated T4 levels in clinically normal cats tested for hyperthyroidism and are diagnosed.
Methods:
Total T4 records of 202 clinically normal cats, tested for routine pre-anaesthetic bloods, using a Quantum Saturno 100 Vet wet chemistry analyser, were analysed retrospectively. Any result above or below the normal reference range was classified as being hypo/hyperthyroid. Statistical Package for Social Sciences (SPSS) software assisted in the analysis of blood results and highlighted risk factors associated with the disease.
Results:
Out of the 202 clinically normal cats, 76 did, in fact, have hyperthyroidism. There was a statistically significant association between breed and T4 result, with a much higher prevalence of hyperthyroidism and higher median T4 result in purebreed cats compared with crossbreed cats. Thus providing evidence that routine testing on all purebreed cats could be worthwhile. Age, weight, gender, neutered status and colour were not significantly associated with hyperthyroidism or T4 result.
Conclusion:
Purebreed cats are a group that could be routinely tested for hyperthyroidism, as results showed a statistically significant association between breed and T4 result.
Hyperthyroidism is one of the most commonly diagnosed endocrine diseases affecting older cats (Wakeling et al, 2011; Graves, 2017). The clinical signs of hyperthyroidism have now been well described (Capen and Martin, 2003; Peterson, 2012) with an increase in incidence of cats being diagnosed with hyperthyroidism since the late 1970s. According to Capen and Martin (2003) this is the result of a larger number of cats receiving veterinary medical care, an improvement in assays for thyroid hormones, detailed characteristics of hyperthyroidism and increased awareness of its occurrence in cats by veterinary surgeons (VS).
Research indicates that there has been a change in emphasis from simply confirming a diagnosis in a cat presenting with classical clinical signs to diagnosing hyperthyroidism in cats with no clinical signs (Graves, 2017). In light of this, there has been a direct improvement in the efficacy of the diagnostic tests used to confirm hyperthyroidism with the opportunity to routinely test those cats with early or mild signs of the disease. More veterinary practitioners have improved their resources and are now able to run inhouse blood tests that previously would have been sent to an external laboratory. This, in turn, allows more tests to be performed easily producing immediate results with a huge increase in offering clients the option of pre-anaesthetic blood tests. Consequently, testing for hyperthyroidism has become easier and this paper recommends it to be considered as part of the routine blood tests that are run on a daily basis within the practice.
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