An explanation for the difference between the responses of mammals and birds to thyroxine and tri-iodothyronine

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Abstract
The similar potencies of L-thyroxine and 3:5:3[image]-triiodo-L-thyronine in the chicken have been further studied by comparing the overall rate of metabolism of the two thyroid hormones in both mammals and birds and by comparing binding of thyroxine by serum proteins in man, chicken and duck. Thyroxine and triiodothyronine have indistinguishable rates of disappearance from the whole body in the chicken with half-lives of 22.5 [plus or minus] 1 hr. Thyroxine-binding protein as found in human serum is absent from, or present only in small amounts in, chicken and duck sera. Both endogenously labelled and exogenously labelled thyroid hormones are mostly bound to albumin in the sera of these birds. Quantitative studies show that both affinity and capacity of chicken serum to bind thyroxine are lower than those found for human serum. The major thyroxine-binding protein fractions in human serum, Cohn fraction IV-4 and prealbumin, bind thyroxine about 3-4 times more firmly than triiodothyronine. Human-, chicken- and duck-serum-albumin fractions bind both hormones with almost identical intensities. It is concluded that the difference in the response of birds and mammals to triiodothyronine, as compared with thyroxine, is due to a fundamental difference in the binding of thyroid hormones to avian-serum proteins and mammalian-serum proteins respectively.