Fluctuation of the plasma TRH level in normal subjects in a 4-hour observation period

Radioimmunoassayable TRH and TSH were measured in plasma samples taken at 5 min intervals for 4 hr (2100-0200 hr) from 4 normal male subjects. Three subjects showed a TSH surge at 2135 hr, 2455 hr and 0150 hr, respectively. The mean plasma TRH level of the 4 subjects was 10.3-11.7 pg/ml. Plasma TRH showed random fluctuation, which did not coincide with the nocturnal increase in plasma TSH.