The effects of thyroxine and triiodothyronine on the thyrotrophin response to thyrotrophin-releasing hormone in normal and iopanoic acid-treated euthyroid rats

in European Journal of Endocrinology
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The present work studied the TSH response to TRH in T4- and T3-treated normal rats and in rats with depressed intrapituitary conversion of T4 to T3. Euthyroid normal rats were injected iv with a single dose of 6 μG T4/100 g body weight, 1 μg T3/100 g or normal saline. Another group treated with iopanoic acid (IOP) received 1 or 10 μg T4/100 g, 0.21 or 2.5 μg T3/100 g, or saline as control. Twenty min later 1 μg TRH/100 g was injected iv. Blood samples were drawn at times 0, 20 min (immediately preceding the injection of TRH) and 30 min (10 min post-TRH) for measurement of plasma T4, T3 and TSH. Other groups of IOP-treated rats were injected iv with 150 μCi of [3',5'-125I]T4; the hypophyses were removed at 30 min and homogenized in PBS buffer, extracted and chromatographed in tertiary amyl alcohol:hexane:ammonia. The control group of rats had a 13-fold increase in plasma TSH 10 min after the TRH injection, while rats treated with 6 μg T4/100 g had a 7.6-fold increase (P < 0.01 vs control increment) and rats treated with 1 μg T3/100 g had no change in TSH response as compared to controls. In IOP-treated rats injected with 10 μg T4/100 g there was a smaller increase in the TSH response to TRH as compared to control (P < 0.01), and a similar smaller response was seen in rats treated with 2.5 μg T3/100 g (P < 0.01 vs control). IOP-treated animals given 1 μg T4 or 0.21 μg T3/100 g had no significant changes in TSH response to TRH. It seems probable that T4 possesses an intrinsic hormonal capacity to regulate the secretion of TSH before it monodeiodinates to T3.


     European Society of Endocrinology

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