Serum free thyroxine concentrations measured by chemiluminescence in hyperthyroid and euthyroid cats

Serum free thyroxine (FT4) concentrations using chemiluminescence were measured in hyperthyroid cats (n = 72) and clinically normal cats (n = 129) to establish reference values and to determine if this method could be a useful alternative to total T4 (TT4) measurement by radioimmunoassay (RIA). Mean serum FT4 concentration (68.3 +/- 26.8 pmol/L) of hyperthyroid cats was significantly higher than that of euthyroid cats (22.9 +/- 4.8 pmol/L). Reference values for basal FT4 of hyperthyroid and healthy cats were 33 to 114 pmol/L and 16 to 30 pmol/L, respectively. The results of the present study suggest that FT4 measured by chemiluminescence could be a useful alternative to TT4 measured by RIA when evaluating thyroid function in cats, since the hazardous effects of radioactive materials on the manipulators and the environment could be avoided. Further studies are required to corroborate these preliminary findings.