General introduction 13 activation of the HPG-axis results in increases in GnRH, LH, FSH and estrogen, which peak two months after birth in female babies and then decline until the age of two (Dwyer & Quinton, 2019). In the female minipuberty, LH and FSH show steady declines in serum levels, but estrogen already shows a cyclic decline in serum levels in this phase (Kuiri-Hänninen et al., 2014). Although the function of this phase is not yet clear, researchers hypothesize that it affects both gonadal and neural development (Hines et al., 2016). After the minipuberty, estrogen and progesterone levels are low during childhood and increase again with the start of puberty, starting the development of secondary sex characteristics such as breast growth and redistribution of body fat. During the reproductive age, most women show predictable and regular changes in estrogen and progesterone levels in a roughly 28-day cycle. Estrogen levels peak around 14 days after the first day of menstrual bleeding, marking ovulation, and progesterone levels peak around 21 days after the first day of menstrual bleeding, after which levels of estrogen and progesterone decline and trigger menstruation. This predictable rhythmicity in hormone levels ends during the menopausal transition, when menstrual cycles first becomes irregular and then stop altogether. During the menopausal transition, levels of estrogen and progesterone decline, but day-to-day levels of estradiol can still strongly fluctuate within this downward slope (Gordon et al., 2016; Joffe et al., 2020). Estrogen and progesterone also play a small part in the male HPG axis. In men, testosterone can be aromatized into estradiol (17β-estradiol, or E2), and this process plays important role in the regulation of gonadal hormones. GnRH can inhibit testosterone production, but the regulation of FSH is mainly regulated via estradiol (Hayes et al., 2001). In men, progesterone serves as a precursor for testosterone, and progesterone levels are similar compared to postmenopausal women (Muneyyirci-Delale et al., 1999). These studies indicate that although estrogen is often overlooked in male physiology, it serves an important function in male sex hormone regulation. Testosterone As seen in female babies, male babies also undergo two phases of hormone exposure during early development. Firstly, exposure to testosterone during fetal development triggers the sexual differentiation of the gonadal system in male babies. Secondly, in the postnatal period male babies also go
RkJQdWJsaXNoZXIy MTk4NDMw