Circadian rhythm of free oestradiol in relation to plasma cortisol in late human pregnancy.

The present study investigates the diurnal variations of free plasma oestradiol levels in late human pregnancy. The oestradiol levels are correlated to the maternal adrenal function as reflected by the plasma levels of cortisol. According to the half life time of oestradiol, blood samples were collected at short time intervals of 30 and 60 min, respectively. Three pregnant women volunteered in the study. Free oestradiol was measured by radioimmunoassay and cortisol was quantitated by a protein binding method. All patients exhibited a circadian rhythm for free oestradiol with significantly higher values in the early morning (28.3 +/- 7.2 ng/ml) than in the afternoon and early night (21.2 +/- 3.6 ng/ml, P less than 0.001). The course of oestradiol followed in a moderate but significant correlation plasma cortisol (r = 0.34, P less than 0.001). During the period of increasing cortisol the oestradiol levels demonstrated the phenomenon of episodic secretion. The results obtained suggest that the maternal adrenals predominantly regulate the diurnal rhythm of free oestradiol in late human pregnancy.