SERUM-FREE THYROXINE CONCENTRATIONS, MEASURED BY CHEMILUMINESCENCE ASSAY BEFORE AND AFTER THYROTROPIN ADMINISTRATION IN HEALTHY DOGS, HYPOTHYROID DOGS, AND EUTHYROID DOGS WITH DERMATHOPATHIES

The purpose of this study was to determine the usefulness of free thyr oxine (FT4) measured by chemiluminescence in evaluating thyroid functi on in dogs. Total thyroxine (TT4) concentration measured by radioimmun oassay (RIA) and FT4 measured by chemiluminescence were evaluated in 3 0 healthy dogs, 60 euthyroid dogs with concurrent dermatopathies, and 30 hypothyroid dogs before and after intravenous stimulation with 1 or 2 IU of thyrotropin (TSH). Median basal TT4 and median TT4 concentrat ions at 4 h post-TSH administration were not significantly different ( P < 0.0001) between healthy dogs and euthyroid dogs with dermatopathie s, but were significantly higher than those in hypothyroid dogs. In he althy dogs, the median TT4 concentrations at 4 and 6 h post-TSH admini stration were not significantly different. Median basal FT4 and median FT4 concentrations at 4 h post-TSH administration in healthy dogs wer e significantly lower (P < 0.0001) than those in euthyroid dogs with d ermatopathies, but significantly higher than the same parameters in hy pothyroid dogs. There was a significant difference between the median FT4 concentrations at 4 h post-TSH administration and median basal FT4 concentrations for healthy dogs and euthyroid dogs with dermatopathie s, but not for hypothyroid dogs. Lastly, in healthy dogs, median FT4 c oncentrations at 4 and 6 h post-TSH administration were not significan tly different. Free thyroxine measured by chemiluminescence was highly correlated (P < 0.0001; Spearman r = 0.98) with FT4 measured by the r eference method for free hormone analysis, namely, equilibrium dialysi s, when sera from 56 dogs were used. It was concluded that FT4 measure d by chemiluminescence could be an alternative to TT4 measured by RIA when performing TSH stimulation tests on dogs. By doing so, the hazard ous effects of radioactive materials on the handlers and the environme nt could be avoided. Because of recent difficulty in obtaining a comme rcial source of TSH, it will be important to look more carefully at th e diagnostic potential of basal FT4 determination alone, measured by c hemiluminescence.