The menopause, defined as the permanent cessation of menstruation resulting from the loss of ovarian follicular activity, marks the end of natural female reproductive life. It is preceded by a period of menstrual cycle irregularity, the menopausal transition, which usually begins in the mid-40s and is conventionally divided into early and late phases. The endocrine changes, which underlie the transition, are predominantly the consequence of a marked decline in ovarian follicle numbers. The most significant changes include a decrease in early cycle inhibin B and in anti-Mullerian hormone (AMH) levels. The decline in inhibin B results in an increase in FSH, which appears to be an important factor in the maintenance of estradiol (E2) concentrations until late in reproductive life. In the post-menopause, FSH levels are markedly raised, E2 levels are low, whereas inhibin B and AMH are undetectable. The menopausal transition is a time of marked hormonal instability. The Melbourne Women's Midlife Health Project has been an extremely productive study in which it has been possible to describe longitudinal changes in hormone levels throughout the menopause transition and to separate the effects of hormone change from the effects of ageing on a number of endpoints. This review provides the background for an accompanying manuscript in which a novel approach to modelling the hormonal changes during the transition is described.

The menopausal transition is the stage in reproductive life commonly defined as commencing with the onset of menstrual irregularity. Classic studies of the endocrinology of the transition postulated the existence of inhibin in women to explain the observed increase in follicle-stimulating hormone (FSH) levels without a significant decrease in estradiol (E2). Descriptions were provided of cycle characteristics during the transition, emphasizing the unpredictability of the endocrine changes rather than the occurrence of an orderly and progressive decline in ovarian function. Women older than the age of 45 exhibited menstrual irregularity when the average number of primordial follicles per ovary decreased to approximately 100. Inhibin B is a major regulator of FSH secretion and a product of small antral follicles. Its levels respond to the early follicular phase increase and decrease in FSH. The age-related decrease in ovarian primordial follicle numbers, which is reflected in a decrease in the numbers of small antral follicles, leads to a decrease in inhibin B, which in turn leads to an increase in FSH, hypothesized to act as a stimulus to the maintenance of circulating E2 in the follicular phase until late in the transition. Concurrently, the concentrations of testosterone do not change significantly. Early follicular phase FSH levels in women reporting menstrual irregularity fluctuate markedly, with a more uniform increase in levels when no menses have occurred for at least 3 months. Anovulatory cycles occur at increased frequency in the last 30 months before final menses or menopause. In ovulatory cycles, FSH shows little, if any, increase, but anovulatory cycles are usually characterized by low levels of inhibin B, markedly increased levels of FSH, and low levels of E2. Thus, the heterogeneity of follicular phase FSH represents a mixture of ovulatory and anovulatory cycles. Longitudinal data indicate that both ovulatory and anovulatory cycles occur after entry into both the early and late menopausal transition and that ovulatory cycles occur even after final menses. There is no endocrine marker of menopause, which may be primarily an endometrial event. Using the hormonal concentrations in ovulatory cycles observed in women in mid-reproductive age as controls and comparing such concentrations in late reproductive age women older than 45 either continuing to cycle regularly or having entered the early or late menopausal transition, a gradual increase in follicular phase FSH and E2 and a decrease in inhibin B were observed in ovulatory cycles. Anovulatory cycles showed markedly increased FSH with low E2 and inhibin B. No specific endocrine change was characteristic of either the early or late menopausal transition, confirming the observations of previous studies regarding the unpredictability of cycle characteristics and hormone changes with the approach of menopause. Antimüllerian hormone correlates with follicle numbers and shows a large age-related decrease to reach undetectable levels at menopause. Thus, the m...

Progression through the STRAW stages is associated with elevations in serum FSH, LH, and estradiol and decreases in luteal phase progesterone. The marked fall in inhibin B and particularly anti-Mullerian hormone indicate that they may be useful in predicting STRAW stage but future analyses of early cycle measurements on larger cohorts are needed to draw predictive conclusions.

Many of the marked increases in ovulatory cycle E2 and cycle irregularities during the menopausal transition may be due to LOOP events and appear to be triggered by prolonged high follicular phase follicle-stimulating hormone levels.

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