HPA axis activity across the menstrual cycle - a systematic review and meta-analysis of longitudinal studies.

Differential HPA axis function has been proposed to underlie sex-differences in mental disorders; however, the impact of fluctuating sex hormones across the menstrual cycle on HPA axis activity is still unclear. This meta-analysis investigated basal cortisol concentrations as a marker for HPA axis activity across the menstrual cycle. Through a systematic literature search of five databases, 121 longitudinal studies were included, summarizing data of 2641 healthy, cycling participants between the ages of 18 and 45. The meta-analysis showed higher cortisol concentrations in the follicular vs. luteal phase (d = 0.12, p =.004, [0.04 - 0.20]). Comparisons between more precise cycle phases were mostly insignificant, aside from higher concentrations in the menstrual vs. premenstrual phase (d = 0.17, [0.02 - 0.33], p =.03). In all included studies, nine samples used established cortisol parameters to indicate HPA axis function, specifically diurnal profiles (k = 4) and the cortisol awakening response (CAR) (k = 5). Therefore, the meta-analysis highlights the need for more rigorous investigation of HPA axis activity and menstrual cycle phase.