Abstract
Summary
In a study of the relationship between blood thyroxine concentration and thyrotrophin (TSH) secretion, the time course of thyroid response to thyroxine injection has been examined. As measured by thyroid I131 release curves pituitary-thyroid inhibition occurred immediately following sudden increase in blood thyroxine concentration in 2 rabbits and within 3 hours in a third. Groups of rats were pre-treated with radioiodine and thyroid inhibition produced by injection of thyroxine. Protein bound iodine (PBI) levels in the blood were followed serially as the inhibitory effects of the injected material gradually decreased. Pituitary-thyroid activation (as measured by sharp increase in PBI131) was observed as PBI levels fell from 5.2 to 4.0 μg% (control PBI 4.0 μg%) over a 6-hour period.
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