TSH SYNTHESIS AND RELEASE IN THE THYROIDECTOMIZED RAT:

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Abstract
The time- and dose-dependent effects of T3 [triiodothyronine] on the anterior pituitary (AP) TSH [thyroid stimulating hormone] release and synthesis were studied in rats 5 days after thyroidectomy (.hivin.T-5d). T3, in various doses, was injected i.v. to animals which were killed later at various times . Plasma and AP TSH were measured. T3 lowered plasma TSH to a minimum within 3 h. An inverse linear relationship was found between T3, up to 634 .+-. 134 ng/100 g b.w. [body weight] (mean .+-. SD) and plasma TSH. Higher doses of T3 did not lower plasma THS any further. At T3 of 634 .+-. 134 ng/100 g b.w. the specific AP binder was 68-90% saturated, thus suggesting a relationship between occupancy by T3 of the AP binder and the regulation of TSH release. The TSH release and synthesis rates were compared after 0.5 .mu.g, 1 .mu.g and 25 .mu.g T3. All 3 doses of T3 suppressed release and synthesis. The effect of T3 on the synthesis rate, however, lagged behind the effect on the release both in time and magnitude.