Diurnal variation of testosterone and estradiol: a source of bias in comparative studies on breast cancer

In this paper we present a study of the diurnal variation of testosterone (T) and estradiol (E2) in women, carried out as part of the design of a prospective study on the hormonal and nutritional etiology of breast cancer. Blood samples were obtained 5 times on the same day, in the morning and early afternoon, from 23 women aged between 25 and 63 yr. Twelve were sampled within the first days following daylight-saving time (SUMTI) introduction. In postmenopause, T mean values decreased from 08:00 h to 15:00 h and the effect of blood drawing time was statistically significant (p <0.01), with no significant effects of SUMTI. For E2 mean values, no significant effect was found for either blood drawing time or SUMTI. In premeno-pause, T mean values decreased from morning to afternoon (p <0.01 ), while no effect of SUMTI was found. A significant decrease was observed for E2 during the day (p <0.01), with no significant influence of SUMTI. These results indicate that diurnal variation of T and E2 are such, that one must not neglect the possible effects of timing procedures on hormonal measurements, when hormonal hypotheses are tested in comparative studies on cancer etiology.