Abstract
Summary
Thyroxine secretion rate (TSR) of 100 cross-bred cockerels was determined at 7 weeks of age without tapazole to block I131 uptake. At 9 weeks, 4 groups of equal TSR were separated. One group served as a control, whereas other groups were fed 0.025, 0.0375, and 0.05% tapazole in ration. TSR of groups were .74±.04, 1.11±.08, 1.18±.08, and 1.27±.07 μg/100 g body weight, respectively. Group not fed tapazole was significantly lower than where included, but differences between tapazole-fed birds were not significant. Tapazole appeared to increase TSR 63.5%. Analysis of these data showed that tapazole increased TSR to greatest extent in groups showing low TSR without tapazole. It is suggested that tapazole does not actually increase TSR but in its absence there is a tendency to underestimate the true TSR due to the slow release rate of thyroidal-I131. It is recommended that in determination of TSR sufficient goitrogen be fed to completely block I131 recycling.
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