The relationship between autoantibodies to triiodothyronine (T3) and thyroglobulin (Tg) in the dog

The relationship between T3 autoantibodies (T3AA) and thyroglobulin autoantibodies (TgAA) in dogs was investigated by determining the inhibitory effect of triiodothyronine (T3), thyroxine (T4) and thyroglobulin (Tg) on T3AA and TgAA binding activity and by determining the pattern of occurrence of the two activities in canine serum samples. Strong similarity in binding characteristics between the two activities, as one might expect if T3AA activity were merely a cross-reactivity of TgAA, was not observed. Canine T3AA activity exhibited a cross-reactivity to purified canine Tg that was intermediate between that of T3 and T4, indicating an antigenic relationship to an epitope of Tg. Average affinity constants of canine T3AA (N = 11) for T3, Tg and T4 were 1.76 x 10(10) M-1, 2.29 x 10(9) M-1, and 1.02 x 10(8) M-1, respectively. Canine TgAA activity, however, did not cross-react significantly with T3 or T4. Canine TgAA (N = 21) binding to canine Tg was not inhibited by T4 or T3 at concentrations up to 2 x 10(-4) M. Each of 23 canine serum samples containing T3AA also exhibited TgAA activity, although there was poor correlation between the magnitudes of the two activities. Neither T3AA nor TgAA activity was observed in serum samples from 16 euthyroid dogs; however, 46.7% of the samples from 15 hypothyroid dogs had detectable TgAA activity. T3AA is so rare that is was not observed in this small population of samples from hypothyroid dogs. The [125I] T3 binding in serum from hypothyroid dogs was elevated compared to that in euthyroid dogs, but was considerably lower than in samples generally designated as containing T3AA. These results suggest that T3AA found in occasional canine serum samples are due to the presence of autoantibodies recognizing a T3 containing epitope of Tg that is different from the epitopes involved in eliciting the predominant population of canine Tg autoantibodies.