The Relationship Between Autoantibodies to Triiodothyronine (T₃) and Thryoglobulin (Tg) in the dog

Abstract

The relationship between T₃ autoantibodies (T₁ AA) and thyroglobulin autoantibodies (Tg AA) in dogs was investigated by determining the inhibitory effect of triiodothyronine (T₃), thyroxine (T₄) and thyroglobulin (T_g) 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 T_g that was intermediate between that of T₃ and T₄, indicating an antigenic relationship to an epitope of T_g. Average affinity constants of canine T₃ AA (N = II) for T₃, Tg and T₄ were 1.76 × 10¹⁰M^-1, 2.29 × 10⁹M^-1, and 1.02 × 10^gM^-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 × 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 T₃ AA 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.

Key Words:

• T₃ autoantibodies
• Tg autoantibodies
• canine hypothyroidism