Relative Potencies of Thyroxine and Triiodothyronine Based on Various Criteria in Thiouracil-Treated Chickens

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Abstract
Relative sensitivities of various criteria, which were found to be suitable qualitative indicators of thyroxine and triiodothyronine, were determined by calculating and comparing the amounts of each hormone necessary to maintain the control level of each criterion in thiouracil-treated chicks. Relative sensitivities of the criteria used for determining action of thyroxine were: feather length > rectal temperature > weight of the thyroid > oxygen consumption rate > heart rate > suffocation time; of triiodothyronine: feather length > rectal temperature > suffocation time > thyroid weight > oxygen consumption rate > heart rate. Relative potencies of thyroxine and triiodothyronine were determined by comparing the magnitudes of responses of each of the above criteria to equal or equivalent quantities of these two hormones. Thyroxine and triiodothyronine exhibited equal potency in affecting the above criteria in thiouracil-treated chicks except for a) antigoiterogenic action in which case thyroxine was more potent than triiodothyronine and possibly b) elevation of rectal temperature for which action triiodothyronine appeared to be the more potent hormone.