Plasma unconjugated oestriol in late pregnancy: circadian variation and the effect of meals and a glucose load.

Plasma unconjugated oestriol (E3) concentrations were determined by radioimmunoassay in 10 normal subjects in late pregnancy, throughout a normal day and night, and after a 50 g oral glucose load. There was a circadian pattern in E3 concentration characterized by an abrupt increase of 15 per cent at night. There was no significant rhythm during the day or night, taken separately. However there was a 10 to 11 per cent reduction in E3 concentrations soon after some meals and after a glucose load, possibly due to expansion of the plasma volume. Excluding the effect of meals, the fluctuation of E3 concentrations in individuals (median of the coefficients of variation) was 11.5 per cent in the day. Overall, it was 13.2 per cent in the day, 12.1 per cent at night and 15.9 per cent over the whole period studied. Since the variation in plasma unconjugated E3 concentrations through 24 hours was no greater than random fluctuations or day-to-day variation there is no need to restrict the time of blood sampling in clinical practice.