Parameters of Thyroid Function in Serum of 16 Selected Vertebrate Species: A Study of PBI, Serum T₄, Free T₄, and the Pattern of T₄ and T₃ Binding to Serum Proteins

Sera from 2-4 adult animals from each of 16 selected vertebrate species [man, rhesus monkey, cattle, sheep, goat, horse, dog, rat, mice (white and brown), guinea pig, rabbit, pigeon, chicken, trout and perch] were examined for total protein, PBI, serum total T₄, T₄ dialyzable fraction, resin T₃ uptake, the maximal T₄ binding capacity of T₄ binding proteins and the concentration of free T₄ calculated. The pattern of T₄ and T₃ binding to serum proteins was also examined qualitatively by starch gel, paper and agarose gel electrophoreses in 4 different buffer systems. All animals exhibited T₄ and T₃ binding to one or more serum proteins. Binding of T₄ to prealbumin occurred in sera from man, rhesus monkey, horse, pigeon and chicken. T₃ bound to the prealbumin zone only in the pigeon. T₄ binding to prealbumin was inhibited by 0.075M barbital in all species capable of binding T₄ to this protein except for the pigeons. There was no inter- or intraspecies correlation between the total serum protein concentration and the maximal T₄ binding capacity to serum proteins, total T₄ or free T₄. With the exception of the rhesus monkey, serum free T₄ concentration in most mammals was within the accepted normal range for human, despite a wide range of variability in the PBI, total T₄ and resin T₃ uptake. Avians had moderately raised, and fish very high, free T₄ levels. The interspecies and to some extent intraspecies constancy of the free T₄ concentration despite wide variability in other parameters of thyroid function suggests that this test may, as in the human, more accurately correlate with the peripheral tissue effect of thyroid hormones. (Endocrinology86: 793, 1970)