The Influence of the Circadian Thyrotropin Rhythm on the Thyrotropin Response to Thyrotropin-Releasing Hormone in Normal Subjects

200 μg synthetic thyrotropin-releasing hormone (TRH) was administered intravenously to 6 normal men and 5 normal women at 11 a.m. and again at least one week later at 11 p.m. As expected, the basal TSH level was higher at 11 p.m. than at 11 a.m. The absolute peak TSH levels as well as the increases in TSH after TRH stimulation were higher at 11 p.m. than at 11 a.m. A positive correlation between basal TSH level and maximal serum TSH after TRH was found in the 11 a.m. tests as well as in the 11 p.m. tests. When the TSH response was expressed as per cent increase over the low forenoon and the high night basal levels of the circadian cycles, there was no difference in the TSH response to TRH.