Thyroid Function Tests in Domesticated Animals: Free Thyroxine Index

Abstract

SUMMARY Normal values for resin sponge uptake of labeled 5,3′-triiodothyronine (T-3) (T-3 test), total serum thyroxine (T-4) by competitive protein binding (T-4 test), and free thyroxine index ( fti ), a product of T-3 and T-4 test results (T-7 test), were determined as a function of age in 7 species of domesticated animals. Results indicate that serum T-4 concentration decreases significantly with age in dog and in swine, and T-3 uptake decreases with age in sheep and goats. In swine, the T-3 values are highest in young adult animals, serum T-4 concentrations seem lower in single-stomached animals than in ruminants, and there seems to be an inverse relationship between serum T-4 concentration and T-3 test results: Those species with higher serum T-4 concentrations having lower T-3 test results and those species with lower serum T-4 concentrations having higher T-3 test results. Results presented indicate that at least in the dog, the T-3 test does not seem to sensitive in detecting changes in thyroid status, and thus, the T-4 test alone could be as valuable as the fti in this species.