A Comparison of the Thyroxine: Tri-Iodothyronine Content and Biological Activity of Thyroid from Various Species

Abstract

Samples of ox and sheep thyroid were less potent than pig thyroid in biological assays when the doses were administered either on the basis of B.P. “Thyroxine-iodine” or U.S.P. total combined iodine. Chemical analyisis for iodothyronines revealed that the molar ratio of thyroxine to tri-iodothyronine approximated 2:1 in pig thyroid but approached 3:1 in ox and sheep preparations. Thus the ox and sheep samples which were less potent in the biological assay than pig thyroid also contained less tri-iodothyronine. By using tri-iodothyronine as the dosage basis, there was very little if any difference in physiological efficacy in goitre prevention assay.