Serum Binding of Radiothyroxine in Sheep and Cattle2

The serum protein binding of thyroxine was studied in 19 bovine and 25 ovine sera. For this measurement the serum proteins and an ion exchange resin compete for a negligible amount of thyroxine labeled with 1-thyroxine-I131. The percent of the thyroxine bound to the serum proteins of ewes (71%) and cow (78%) was greater than that reported for women (pregnant, 40%; nonpregnant, 30%). No effect of first trimester pregnancy in cows or ewes was observed. Rambouillet, Columbia and Hampshire ewes all bound the same proportion of the thyroxine. By paper electrophoresis it was found that the thyroxine binding protein of ewes and cows migrated with the alpha-globulins.