Abstract
To determine whether thyrotropin (TSH) suppression was characterized by a rapid and slow component in euthyroid rats, TSH concentrations were measured in concentrates of plasma pooled from groups of 8 euthyroid rats 24 h after injection of a single dose of triiodothyronine (T3). Plasma TSH decreased after T3 injection to values 3.2–5.9% of pre-T3-injection concentrations. The presence of TSH in plasma after single T3 injection and its absence (<0.25% of euthyroid values) after euthyroid rats were treated with T3, 2-4 μg/day for 43 days, indicate that, as in the hypothyroid rat, TSH suppression has both a rapid and slow component in the euthyroid rat. A small but statistically insignificant increase in TSH secretion occurred after injection of a high dose of thyrotropin-releasing hormone (TRH; 1 μg/100 g BW) into T3-treated euthyroid or hypothyroid rats. This suggested that endogenous TRH did not play an important role in maintaining TSH secretion in the presence of high plasma T3 concentrations. The metab...
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