Abstract
The rationale that conjoint use of feedback inhibition could alter the effectiveness of a goitrogen, methylthiouracil (MTU), for achieving pharmacological suppression of thyroid gland function was tested in sheep. Experiment 1 determined the appropriate daily dose of T 3 for achieving feedback suppression of thyroid secretion. Experiment 2 tested the hypothesis that prior treatment with T 3 at such dosage would speed up the thyroid suppressive effect of subsequent administration of MTU, and Experiment 3 determined whether T 3 exerted feedback inhibition on the anterior pituitary gland. In Experiment 1, ewes ( n=6) treated with doses of 0, 1, 5 and 25 μg T 3 per kg live weight as a daily s.c. injection for 2 weeks had a dose-related increase in heart rate and decrease in plasma T 4 concentration. Ewes in Experiment 2 received combinations of daily s.c. injections of T 3 at 5 μg per kg and daily oral dosing with 35 mg per kg MTU ( n=6). Suppression of plasma T 4 concentration by T 3 did not improve the effect of subsequent treatment with MTU. In Experiment 3, plasma TSH responses to TRH stimulation (200 μg, i.v.) were reduced by treatment with T 3, confirming its feedback inhibition of the anterior pituitary gland. It is concluded that, in spite of the inhibitory actions of T 3 on plasma T 4 concentration in sheep, its use prior to treatment with MTU did not affect the resultant suppression of thyroid hormone secretion.
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