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 less than 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 premenopause, T mean values decreased from morning to afternoon (p less than 0.01), while no effect of SUMTI was found. A significant decrease was observed for E2 during the day (p less than 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.