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 metabolic clearance rate (MCR) of TSH was measured to determine whether altered rates of TSH metabolism could acount for the 30-100-fold greater TSH concentration observed after 24 h in T3-injected hypothyroid rats as compared to T3-injected euthyroid rats. Neither a mean 40% decrease in the MCR of TSH nor a 4-fold increase in number of thyrotrophs in hypothyroid rats could account for the large difference in residual TSH concentration. The TSH secretion rate per thyrotroph in non-injected or T3-injected hypothyroid rats appears to be 4-8-fold greater than in euthyroid rats.
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