Menopause and Hypothalamic-Pituitary Sensitivity to Estrogen FREE

By the year 2030, more than 1.2 billion women in the world will be at least 50 years old.¹ This increasing proportion of the female population will be experiencing the menopausal transition with its accompanying physiology and pathophysiology. Reproductive aging in women and the hormonal changes that occur during the onset of human menopause have been ascribed solely to ovarian failure and oocyte depletion.² However, in other species, the central nervous system is the major regulator of age-related reproductive dysfunction.³

There are 4 events involving the hypothalamic-pituitary-ovarian axis that control the human menstrual cycle: (1) The secretion of follicle-stimulating hormone (FSH), responsible for the development of ovarian follicles and production of estradiol.⁴ Throughout the cycle, estrogen maintains low gonadotropin levels via its negative feedback effect on hypothalamic gonadotropin-releasing hormone and consequently luteinizing hormone (LH) and FSH secretion.⁵ (2) The FSH-induced increase in ovarian estrogen secretion to levels of sufficient strength and duration triggering an LH surge (positive feedback).⁶ (3) The LH surge, a hypothalamic-pituitary response to the estrogen stimulus. This positive feedback response of estrogen on LH secretion has been used as a test of hypothalamic-pituitary function.^7- 8 (4) Ovulation and luteinization of the follicle, triggered by the LH surge, forming a corpus luteum. This is an ovarian response that results in progesterone secretion necessary for the establishment of a pregnancy.⁹

How these events may be altered during menopausal transition has not been well established.

The Study of Women’s Health Across the Nation (SWAN) is a multiethnic observational cohort study of the menopausal transition in 3302 women at 7 sites across the United States designed to enhance understanding of the factors that influence the health of women of diverse race and ethnicity.¹⁰ The details of enrollment have been previously reported.¹⁰ Race/ethnicity was self-determined by study participants and was obtained by asking the following open-ended question: “How would you describe your primary racial or ethnic group?” The responses then were categorized as Caucasian (white), African American, Chinese, Japanese, or Hispanic. This study was approved by all of the sites’ institutional review board, and written informed consent was obtained from each participant.

A subcohort participated in the Daily Hormone Study (DHS) from 1997 to 1999. The women in the DHS included 257 Caucasian (white) women, 175 African American women, 152 Chinese women, 170 Japanese women, and 86 women of Hispanic origin. The cohort has been described previously.^11- 12 Inclusion criteria were age 42 to 52 years; an intact uterus and at least one ovary; at least one menstrual period in the prior 3 months; no use of sex-steroid hormones in the previous 3 months; and not being pregnant. DHS enrollees completed a daily collection of morning voided urine for an entire menstrual cycle ending in bleeding or to 50 days, whichever came first. During the cycle that they collected daily urine specimens, DHS enrollees also completed a daily diary, a questionnaire in which they answered, once a day, whether they had experienced within the preceding 24 hours any trouble sleeping and any hot flashes or night sweats.

Urinary LH, FSH, the estradiol urinary metabolites estrone conjugates (E1c), and the progesterone urinary metabolite pregnanediol glucuronide were measured using chemiluminescent assays as described previously.^11- 12 Concentrations were normalized for creatinine excretion. Previous studies have demonstrated that urinary levels of these hormones, collected and measured by the methods used herein, mirror serum hormone patterns during the menstrual cycle in eumenorrheic controls so closely that patterns of serum and urinary gonadotropins and sex steroids are superimposible.¹³

Of the 840 women who completed the DHS study, 680 women had evidence of luteal activity based on a validated algorithm for pregnanediol glucuronide.¹² The algorithm locates the 5 nadir days of pregnanediol glucuronide in the follicular phase using moving averages throughout the cycle. A 3-fold increase in pregnanediol glucuronide concentrations above this nadir for at least 3 consecutive days was considered evidence of luteal activity. The cycles of these women have been reported previously.¹² The remaining 160 women (19% of the total 840 women) did not have luteal activity. One woman could not be subclassified due to missing data points. The 159 remaining women are the subjects of this report.

All women in the SWAN DHS study had levels of FSH equal to or greater than those previously demonstrated in younger women throughout the entire cycle.¹² This has been described previously and is due to the decreased secretion of ovarian inhibin in older women.^14- 15 Decreased gonadotropin secretion, as is found in some anovulatory cycles in premenopausal young women in their teens to 30s, clearly did not occur in these women.

Conceptually, an estrogen increase is a high level compared with baseline, in absolute terms and relative to observed variability, followed by substantial decline. The specific criteria used here were adapted from previously established definitions for mid-reproductive age women.¹³ An estrogen increase was defined as an E1c level of at least (1) 50 pg/mg creatinine, (2) twice the baseline level (the mean of 5 consecutive days starting 9 days earlier), and (3) 3 standard deviations of the baseline levels above the baseline mean. In addition, the estrogen peak was required to culminate in a drop to no more than 1.5 times baseline at some time within the next 5 days. An LH surge is considered present when a high level is observed relative to baseline in absolute terms and in excess of day to day variability both before and after the peak, established by a drop in levels. An LH surge was defined as an LH level of at least (1) 6 mIU/mg creatinine, (2) 3 times the mean baseline level (of 4 consecutive days starting 5 days earlier), (3) baseline mean plus 3 standard deviations of the 4-day baseline levels, (4) baseline mean plus 2 standard deviations of levels on days 2 through 6 after the peak, and (5) 0.8 times the maximum level in that cycle. In addition, the LH surge was required to culminate in a drop to no more than 1.5 times baseline within 6 days following the peak.

Cycles were classified by these algorithms as falling into 1 of 3 distinct patterns: (1) both estrogen increase and LH surge (coincident within 2 days), (2) estrogen increase only, and (3) neither. Visual inspection of the cycle data plots by 2 observers (G.W., J.H.S.) revealed that 20 cycles had apparent estrogen increases that were missed by the defining algorithm due to an increase too early in a short cycle to establish a baseline or a slow decline. Four cycles with an algorithmically determined estrogen increase were reclassified to “neither.” Thus, of 159 cycles, 29 were classified as “both,” 32 as “estrogen increase only,” and 98 as “neither.” These cycle classifications were based solely on hormone levels and not age or menopausal symptoms.

Analysis of variance was conducted to compare cycle classification groups on women’s ages and body mass index; χ² tests were conducted to compare groups on ethnicity, reason for ending collection, and experience of symptoms during the cycle. Rank-sum tests were conducted to compare E1c levels of groups 1 and 2 by cycle day and to compare the 3 groups on women’s percentage of cycle days with symptom occurrence. Reported P values are 2-sided, without adjustment for multiple comparisons. Statistical analyses were performed with SAS version 8.2 (SAS Institute, Cary, NC). Statistical significance was set at 2-sided P<.05.

Women in the 3 classification categories presented here were compared by ethnicity, age, and body mass index. As shown in Table 1, there was no significant correlation between category of cycle and any of these characteristics.

The results from the 29 individuals who had an estrogen increase followed by an LH surge (group 1, “both”) are shown in Figure 1, left panels, in which hormone levels are synchronized to the LH peak. As seen in ovulatory cycles, these LH surges are accompanied by FSH surges. Hormone levels are similar to those of the previously reported women in DHS who had luteal phases,¹² indicating an adequate hypothalamic-pituitary response. In women in group 1, the follicle or follicles that secreted sufficient estrogen to elicit an LH and FSH surge did not luteinize as documented by lack of an increase in pregnanediol glucuronide levels. This is a defect at the ovarian level because hypothalamic-pituitary responses were similar to those of women with apparently normal cycles.

The results from the 32 women who had clear estrogen increases but no LH surges (group 2) are shown in Figure 1, middle panel. Hormone levels are synchronized to the E1c peak because no LH surges were seen in these women. The estrogen increases in these women were equivalent to those seen in ovulatory women and to those in group 1. However, in contrast to those of group 1 women, these estrogen increases did not produce an LH surge. Comparisons of the E1c levels at each cycle day in group 1 women with those in group 2 women were performed using rank-sum tests. At all cycle days, E1c levels in group 2 women who had no LH surge were not lower than E1c levels in group 1 women who had LH surges. In fact, no differences at any cycle day were observed, with the exception of day –15, when E1c levels in group 2 women were higher (P = .04) than those in women in group 1.

That the secretion pattern of E1c in group 2 women was equivalent to that of women in group 1 is not apparent in Figure 1, in which hormone levels are synchronized to the LH surge as is convention. Therefore, the E1c levels synchronized to the E1c peak are presented in Figure 2. For ease of illustration of this similarity (between E1c levels in the women with gonadotropin surges and those in women without surges), the corresponding data for group 2 from Figure 1 are also provided in Figure 2. These E1c patterns in women in groups 1 and 2 are also similar to E1c secretion in the previously reported SWAN DHS study in women with luteal activity.¹² Thus, in group 2 women there is adequate ovarian response, but the LH surge, a hypothalamic-pituitary phenomenon, did not occur in the presence of an estrogen stimulus that is adequate to elicit an LH surge in ovulating women and in younger women. This clearly demonstrates unresponsiveness of the hypothalamic-pituitary axis to an estrogen peak. Gonadotropin levels dropped in the latter part of the cycles, likely due to negative feedback from the estrogen increase.

Hormone secretion in the 98 women who had no estrogen peaks or LH surges (group 3) are shown in Figure 1, right panels. Levels of LH are higher than those seen in either SWAN perimenopausal women with luteal phases or in the other 2 groups presented here. Estrogen levels are comparable to those in the early follicular phase of DHS luteal women¹² and group 1 or 2 women. Thus, group 3 women still have ovarian function but are unable to produce an estrogen peak.

Twenty-eight of the 29 women with LH surges (group 1), 31 of the 32 women without LH surges (group 2), and 97 of the 98 women with neither estrogen nor LH increases (group 3) participated in the daily diary component of the study. For each symptom— trouble sleeping or hot flashes or night sweats—the percentage of days a woman reported that she experienced the symptom was computed as the total number of days she reported presence of the symptom divided by the total number of days she reported either yes or no for that symptom × 100. These percentages of days with positive reports for women in the 3 categories were compared by rank-sum tests.

There were no differences in the percentages of days with trouble sleeping among the 3 groups (Table 2). Comparison of the percentages of days with hot flashes or night sweats among the 3 groups revealed significant group differences (Table 2). The percentages of days with hot flashes or night sweats were significantly higher for group 3 women than for either group 1 women (P = .01) or group 2 women (P = .02). The percentages of days with hot flashes or night sweats in group 1 women and group 2 women did not differ (P = .73).

Hormone secretion patterns in older reproductive-age women demonstrate significant alterations of hypothalamic-pituitary feedback mechanisms in addition to decreased ovarian function. Cycles exist in which failure to mount an LH surge occurs in the face of adequate estrogen stimulation. These findings support the hypothesis that there is a relative hypothalamic-pituitary insensitivity to estrogen in aging women that is manifested by both positive and negative feedback mechanisms. Estrogen levels and patterns that produce LH surges in younger women fail to do so in some older women. In addition, levels of estrogen similar to those in younger women, which cause negative feedback of LH in normal ovulatory women and in group 1 women, fail to do so in group 3 women, who have elevated LH in the presence of early follicular-phase levels of E1c. This situation may represent a later stage of the menopausal transition because there is opening of the negative feedback loop between ovarian estrogen and pituitary LH, as is seen in postmenopausal women. Levels of estrogen capable of lowering LH in cycling women were not able to cause negative feedback of estrogen on LH secretion. Because control of FSH secretion is more complex than LH and includes major influences by inhibins and activins, FSH is not a good marker for estrogen-negative feedback control of gonadotropin secretion. Decreased LH pulse frequency has been observed in the presence of normal, midreproductive sex steroid levels in both the follicular¹⁶ and luteal¹⁷ phases of the menstrual cycles of older premenopausal women, supporting this hypothesis as well.

A predominant hypothesis to explain the onset of puberty is the occurrence of a gradual decrease in estrogen sensitivity of the hypothalamic-pituitary axis, such that small levels of circulating estrogen, which suppress gonadotropin secretion prepuberty, are unable to do so in the pubertal transition.¹⁸ Decreased sensitivity in later life may simply be a continuation of the same pattern of progressive age-related estrogen insensitivity. Levels of LH are higher in perimenopausal women than in younger women, even in the presence of estrogen concentrations that result in lower LH levels in younger women.¹⁴ Symptoms such as hot flashes and sleep disturbances occur more commonly in perimenopausal women than in postmenopausal women.¹⁹ Yet, the perimenopausal transition is a time when circulating estrogen levels are equivalent or higher than levels observed in younger women.¹⁴ Additionally, exogenous estrogen is therapeutic in perimenopausal women.²⁰ These observations are consistent with the hypothesis that a decrease in estrogen sensitivity occurs as women age through the menopausal transition.

We found no differences in reported symptoms of sleep disturbances or vasomotor changes in women with different LH-positive feedback responses to equivalent circulating estrogen peaks (categories 1 and 2). Although group 3 women did not differ in the prevalence of sleep disturbances, they had a significantly higher prevalence of hot flashes or night sweats. While all women in this study had similar baseline estrogen levels, the group 3 women did not have mid-cycle estrogen peaks. Since younger women with estrogen levels similar to these women¹² do not get hot flashes, the flashes may be due to opening of the negative feedback loop of estrogen on gonadotropin secretion.

All women have hormonal changes during their menopausal transition, but not all women experience symptoms. It is likely that the gonadotropin control centers in the brain differ from the areas involved in the symptoms assessed in our study. These regions may have different steroid sensitivities and control mechanisms, accounting for our findings. However, certain changes in gonadotropin levels may be permissive to alterations in central nervous system function, which result in symptoms. Physicians should be aware of the central nervous system changes involved in the menopausal transition because these changes best explain their patients’ symptoms. An appreciation of these changes may assist patients in understanding and dealing with their own menopause. Other symptoms, such as mood and changes in affect, may also have similar explanations.

Demonstration of hypothalamic-pituitary insensitivity to estrogen in perimenopausal women shows that certain human menopausal responses are similar to those of other species, including rats. Age-related decreased expression of estrogen receptor β in some hypothalamic areas in the rat has been described.²¹ These results suggest that the rat may be a useful model for the study of human central nervous system aging and that the mechanisms of reproductive aging may be more similar in diverse mammalian species than previously thought.