Lability of Plasma TSH Levels in the Rat in Response to Nonspecific Exteroceptive Stimuli. ∗

Summary

Mild nonspecific exteroceptive stimuli such as handling, transferring from one room to another, and I.P. injections, lead to a rapid decrease of plasma TSH concentrations which become undetectable within 15 minutes following the beginning of the stressing procedure. There is evidence for a circadian rhythm of plasma TSH levels in opposite phases to those of the variations of plasma ACTH or corticosteroid variations. These observations are considered to be of importance to define resting levels of plasma TSH concentrations. For rats kept at 22°C ± 0.5, rapidly decapitated at 7:00 a.m., resting levels of circulating TSH are between 25–30 mU/100 ml plasma in this laboratory.