The aim of this research was to obtain basic values for the evaluation of thyroid function in nondomestic felids. Serum thyroid hormone concentrations (thyroxine, T4; triiodothyronine, T3) were measured by radioimmunoassay in 145 cats, representing nine species of captive nondomestic felids: jaguar (Panthera onca), n = 49; puma (Puma concolor), n = 10; ocelot (Leopardus pardalis), n = 22; oncilla (Leopardus tigrinus), n = 12; geoffroy (Oncifelis geoffroyi), n = 4; jaguarundi (Herpailurus yaguarondi), n = 8; margay (Leopardus wiedii), n = 7; lion (Panthera leo), n = 26; and tiger (Panthera tigris), n = 7. For each species, mean ± SEM of T3 and T4, respectively, were as follows: jaguar, 0.56 ± 0.03 and 9.7 ± 0.8 ng/ml; puma, 0.67 ± 0.04 and 11.2 ± 1.2 ng/ml; ocelot, 0.48 ± 0.03 and 13.8 ± 1.5 ng/ml; oncilla, 0.43 ± 0.03 and 10.0 ± 1.6 ng/ml; geoffroy, 0.44 ± 0.04 and 8.0 ± 0.16 ng/ml; jaguarundi, 0.7 ± 0.03 and 5.0 ± 1.0 ng/ml; margay, 0.48 ± 0.04 and 12.2 ± 2.3 ng/ml; lion, 0.43 ± 0.02 and 5.7 ± 2.6 ng/ml; and tiger, 0.66 ± 0.03 and 12.6 ± 0.9 ng/ml. Within species, T3 and T4 concentrations did not differ (P > 0.05) between males and females.