Endocrine Abstracts

Cats are the only species, other than humans, in which naturally occurring, non-malignant hyperthyroidism has been recorded. Feline hyperthyroidism has many similarities to toxic nodular goitre, including age at risk, presentation, histological features and similar ‘switch-on’ mutations of the TSH receptor and linked G-proteins. Subclinical hyperthyroidism is well recognised in humans but has only recently been described in cats. We present preliminary data from a prospective study documenting the prevalence and progression of subclinical hyperthyroidism in senior (>8 years) cats.

Cats presenting for routine health checks were recruited for the study. A full history, standardised clinical examination, cystocentesis and blood sampling were performed at each visit. Owners were requested to complete a questionnaire detailing the cat’s environment, diet and exposure to potential toxins. Full biochemistry and urinalysis were performed every 6 months and total thyroxine (tT4) and thyrotropin (TSH) concentrations (by DPC chemiluminescent assay) determined at least every 12 months. For the purposes of this study, subclinical hyperthyroidism was defined as tT4 <45 nmol/L (reference range 19–55 nmol/L) in conjunction with TSH <0.03 ng/ml.

One hundred senior cats have been recruited to date of which 6% had mild clinical hyperthyroidism (tT4>55 nmol/L), 20% had subclinical hyperthyroidism and 13% had other problems (chronic kidney disease n=7; hypertension n=5; diabetes n=1). To date, 6/38 (16%) previously non-hyperthyroid cats have been diagnosed hyperthyroid within 12 months; 5/6 were subclinically hyperthyroid at the initial visit and one had within reference range TSH concentrations. Remaining cats (n=3) with persistently low TSH (<0.03 ng/ml) but tT4 <45 nmol/L at follow up had a mean 85% increase in tT4 concentrations. Only one cat with low TSH concentration (<0.03 ng/ml) when first examined, subsequently showed an increase in TSH concentration.

These data confirm a high prevalence of subclinical hyperthyroidism, as defined in this study, in senior cats, with a high risk of progression to clinical disease within 12 months.