Serum prolactin and thyrotrophin responses to thyrotrophin-releasing hormone at different times of the day in normal women

In order to determine whether or not the pituitary responsiveness to thyrotrophin-releasing hormone (TRH) changes during the nyctohemeral cycle, 10 healthy regularly cycling women were given 200 microgram of TRH at 02.00 h, 10.00 h and 18.00 h with at least a 32 h interval between each test. Serum prolactin (Prl) and thyrotrophin (TSH) in 7 of the 10 women were measured serially before and after TRH administration. The mean basal Prl levels were significantly higher (P less than 0.01) at 02.00 h than at 10.00 h and 18.00 h. The mean basal TSH levels were higher, although not significantly, at 02.00 h than at 10.00 h and 18.00 h. Although a higher TSH release occurred at 02.00 h than at 10.00 h and 18.00 h, the mean serum TSH and Prl peak responses to TRH were statistically similar in the three groups of tests. The integrated changes scores, calculated as the difference between the average post-TRH hormonal release and the average baseline levels, although higher in the 18.00 h test for Prl and the 02.00 h test for TSH, were not statistically different among the three tests.