Changes in diurnal rhythms of free cortisol secretion during different phases of menstrual cycle.

The effect of the menstrual cycle on the diurnal cortisol rhythm was investigated in 15 normally cyclic healthy women during reproductive life. Salivary cortisol was measured by radioimmunoassay in samples collected every 2 h for 24 h during the four phases of the menstrual cycle: menstrual phase, late follicular/peri-ovulation phase, early to mid luteal phase and late luteal phase, respectively. Distinct diurnal rhythms of free cortisol were found throughout the menstrual cycle by using a nonlinear periodic regression model. The model was characterized by an asymmetrically peaked diurnal cycle and ultradian harmonics. There was a trend to higher troughs and significantly shorter peak-width in phase II and phase IV compared to phase I. The ultradian amplitude in phase IV was significantly lower compared with phase I and showed a trend of decrease compared with phase II. The results suggest that the daily cortisol secretion is modulated by the phase of the menstrual cycle.