Biphasic thyrotropin suppression in euthyroid and hypothyroid rats

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.