THE RELATIVE ACTIVITIES OF TRIIODOTHYRONINE AND THYROXINE

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Abstract
SUMMARY: The biological activity of 3: 5: 3′-l-triiodothyronine (TIT) has been compared with that of l-thyroxine (THX) by three different methods. 1. The first compared increases in oxygen consumption rate in rats and showed that TIT had 3·7 and 7·5 times the activity of THX when administered subcutaneously and orally. Although the onset of action appeared to be more rapid with TIT, the overall effects of the two compounds on oxygen consumption in intact rats were qualitatively similar. 2. The second, a mouse anoxia method involving subcutaneous injection, gave a ratio of 4·5 in favour of TIT. 3. In the third, the effects of the two compounds in preventing goitre formation were studied in rats given thiouracil. The relative activity of TIT established by this method was 7·4 times that of THX.