Absence of Circadian Rhythms of Gonadotropin Secretion in Women

Abstract

Diurnal rhythms of LH and FSH have been reported in normal women, but it is unclear whether these reflect underlying circadian control from the suprachiasmatic nucleus and/or external influences. The aim of this study was to determine whether endogenous circadian rhythms of LH, FSH, and the glycoprotein free α-subunit (FAS) are present in reproductive-aged women. Subjects were studied in the early follicular phase using a constant routine protocol in a Clinical Research Center at an academic medical center. Subjects were healthy, normal-cycling women aged 23-29 yr (n = 11). Temperature data were collected, and blood samples were assayed for LH, FSH, FAS, and TSH. Core body temperature and TSH were best fit by a sinusoid model, indicating that known circadian rhythms were present in this population. However, the patterns of FSH, LH, and FAS over 24 h were best fit by a linear model. Furthermore, there were no differences in LH and FAS interpulse intervals or pulse amplitudes between evening, night, and morning. Under conditions that control for sleep/wake, light/dark, activity, position, and nutritional cues, there is no circadian rhythm of LH, FSH, or FAS in women during the early follicular phase despite the presence of endogenous rhythms of TSH and core body temperature. These studies indicate that endogenous circadian control does not contribute to previously reported diurnal rhythms in reproductive-aged women and emphasizes the importance of environmental cues in controlling normal reproductive function.