TSH Measurement in Senior Cats--A Prospective Study
ACVIM 2008
J. Wakeling; J. Elliott; H. Syme
Royal Veterinary College
London, UK

Hyperthyroidism is a common disease of older cats and is insidious in onset. In a retrospective study most hyperthyroid cats were found to have had undetectable TSH concentrations (<0.03ng/ml) 1-3 years prior to diagnosis. Also, euthyroid cats with TSH <0.03ng/ml reportedly have a higher frequency of thyroid adenomas and/or hyperplastic nodules than cats with TSH >0.03ng/ml. The purpose of this prospective longitudinal study was to document the frequency with which hyperthyroidism is diagnosed in cats with differing TSH measurements.

Senior cats (>8 years old) presenting for routine health checks were recruited. A complete history, clinical examination, systolic blood pressure, urinalysis, biochemistry, total thyroxine (tT4) and TSH measurement (DPC canine TSH assay) were performed. Cats diagnosed with hyperthyroidism on initial exam (tT4 >55nmol/l) were excluded but all other cats were included irrespective of their actual health status when examined. Cats were re-examined every 6 months (or more often if they were not healthy) with tT4 and TSH concentrations determined at least annually. The probability of hyperthyroidism being diagnosed at follow-up was compared in cats with TSH <0.03ng/ml or TSH >0.03ng/ml, by Fisher's Exact test at first annual check-up (maximum 14 months) and by Kaplan Meier analysis with Log-Rank testing over the total period of follow-up (maximum 3 years). Comparisons between groups were made by the Mann-Whitney test.

Of 106 cats recruited to the study, 17 were diagnosed with concurrent disease (mild chronic kidney disease n=10; hypertension n=6; diabetes n=1). There was no significant difference in tT4 measurements between cats with TSH <0.03 or >0.03 ng/ml at baseline (p=0.14). Cats with TSH <0.03ng/ml were more likely to be diagnosed with hyperthyroidism within a year (p<0.001; see table). Of the 13 cats diagnosed with hyperthyroidism within one year, 12 had baseline TSH concentration <0.03ng/ml and one cat had baseline TSH of 0.08ng/ml. Over the total follow-up period cats with baseline TSH <0.03ng/ml were significantly (p<0.001) more likely to develop hyperthyroidism with a median estimated (95% confidence interval) time to diagnosis of 441 (44-838) days. All cats that became hyperthyroid had a TSH concentration <0.03ng/ml at, and 6 months prior to, diagnosis.

Baseline

First annual check-up

TSH ng/ml

Hyperthyroid
TSH <0.03ng/ml

Euthyroid
TSH <0.03ng/ml

Euthyroid
TSH >0.03ng/ml

Lost to follow up

>0.03 (n=75)

1

11

49

14

<0.03 (n=31)

12

9

5

5

These data show that although TSH is commonly undetectable (<0.03ng/ml) in geriatric cats many of these cats subsequently develop hyperthyroidism. Measurement of TSH by the DPC canine TSH assay may be useful as part of routine feline senior health programmes to assess the risk of incipient hyperthyroidism.

Speaker Information
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Jennifer Wakeling


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