Validation of an automated enzyme immunoassay for the measurement of serum total thyroxine in cats

Background Hyperthyroidism is common in older cats, which necessitates frequent screening of serum total thyroxine ( TT 4) concentrations. Fast, cheap, and reliable methods to measure TT 4 in cats are needed. Objectives The purpose of the study was the validation of a human TT 4 enzyme immunoassay ( EIA ) for use with feline serum, and derivation of a TT 4 reference interval ( RI ) for cats aged 9 years and older. Methods Assay precision, reproducibility, and linearity were evaluated. Interference by hemolysis was also assessed. Method comparison studies between the human EIA and a previously validated radioimmunoassay ( RIA ) and chemiluminescent‐enzyme immunoassay ( CEIA ) were performed. Healthy cats (> 9 years) were recruited from 3 UK first opinion practices. Results The human TT 4 EIA demonstrated good precision and reproducibility, and adequate linearity. Hemolysis did not significantly alter measured TT 4 concentrations until HGB > 8 g/L. Method comparison revealed proportional and constant errors between EIA and RIA / CEIA . The TT 4 RI for cats (> 9 years) was calculated as 7.1–45.1 nmol/L ( n = 49). Conclusions The human TT 4 EIA was successfully validated for use with feline serum and offers a rapid, cheap, and reliable method for determination of serum TT 4 concentrations in cats.