1 Division of Reproductive Medicine, Department of Obstetrics and Gynaecology, 2 Centre for Clinical Decision Sciences, Department of Public Health and 3 Department of Internal Medicine, Erasmus University Medical Centre Rotterdam, Rotterdam, The Netherlands
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Abstract |
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Key words: androgens/gonadotrophins/inhibin B/normogonadotrophic anovulatory infertility/PCOS
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Introduction |
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The polycystic ovarian syndrome (PCOS), characterized by chronic anovulation, hyperandrogenaemia and sometimes hyperinsulinaemia (Dunaif, 1999), is the most common cause of normogonadotrophic normo-oestrogenic anovulatory infertility. Certainly, PCOS is part of the WHO 2 group, with a variable reported incidence based on differences in inclusion criteria used (van Santbrink, 1997). A previous National Institute of Health (NIH) consensus workshop concluded, but by no means unanimously, that hyperandrogenaemic chronic anovulation should be considered the hallmark criterion for the diagnosis of PCOS (Dunaif et al., 1992
). More recently, it was reported (and agreed by the last NIH consensus workshop) that the occurrence of polycystic ovaries should be added (Dewailly, 2000
).
Inhibin is a dimeric non-steroidal glycoprotein hormone that selectively inhibits FSH production and/or release from the pituitary. It consists of two partially homologous sub-units, combined with either ßA (inhibin A) or ßB (inhibin B) (Robertson et al., 1997
). Recently, it was reported that inhibin A serum concentrations are high around ovulation and during the mid-luteal phase of the normal menstrual cycle. In contrast, inhibin B seems to be the predominant form secreted by developing small antral follicles during the early follicular phase (Groome et al., 1996
). Recent reports on serum and follicular fluid inhibins in PCOS are conflicting. According to some authors, concentrations of total immunoreactive inhibins (Mizunuma et al., 1994
),
inhibin (Pigny et al., 1997
), or inhibin B (Lambert-Messerlian et al., 1994
; Anderson et al., 1998
; Lockwood et al., 1998
) are raised. Others found normal concentrations of total immunoreactive inhibin (Buckler et al., 1988
; Pache et al. 1992
) or inhibin B (Magoffin and Jakimiuk, 1998
).
Since inhibin B selectively inhibits FSH, high inhibin B could be held responsible for an elevated LH/FSH ratio characteristic for some patients (Magoffin and Jakimiuk, 1998). Moreover, inhibin may directly stimulate theca cell androgen biosynthesis (Hillier et al., 1991
). Inhibin B concentrations might also represent the extent of ovarian dysfunction in these patients, since an increased number of healthy follicles (Hughesdon, 1982
; Pache et al., 1991a
) may result in increased serum concentrations. The present cross-sectional study in a large cohort of normogonadotrophic anovulatory infertile patients was conducted to investigate whether inhibin B concentrations were correlated with endocrine and ultrasound findings in WHO 2 and PCOS patients.
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Materials and methods |
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For sonographic imaging, a 6.5 MHz vaginal transducer (model EUB-415; Hitachi Medical Corporation, Tokyo, Japan) was used. The ovaries were localized and scanned as described previously (Pache et al., 1991b). Ovarian volume, stroma echogenicity (arbitrarily scored from 1 to 3 per ovary) as well as the mean follicle number were assessed as described earlier (van Santbrink et al., 1997
).
The control group consisted of 30 healthy volunteers selected by advertisement and paid for participation as previously published (Schipper et al., 1998). Inclusion criteria were a regular menstrual cycle (2630 days), 2035 years of age, normal body mass index (BMI 1825 kg/m2) and no previous use of medication or oral contraceptives during at least 3 months prior to the study. Transvaginal ultrasound and blood sampling were performed during the early follicular phase (cycle day 3, 4 or 5).
Hormone assays
Blood samples were obtained by venepuncture and processed within 2 h after withdrawal. Serum was stored at 20°C and assayed for LH, FSH, androstenedione, testosterone, inhibin B, oestradiol and progesterone. Serum LH and FSH concentrations were measured by immunofluorometric assay (Amerlite, Ortho-Clinical Diagnostics, Amersham, UK), while serum oestradiol, progesterone, testosterone, androstenedione and SHBG concentrations were measured by radioimmunoassay (RIA) provided by Diagnostic Products Corp. (DPC, Los Angeles, CA, USA), as described previously (Imani et al., 2000). Intra- and inter-assay coefficients of variation were <5 and 15% for LH, <3 and 8% for FSH, <8 and 11% for androstenedione, <3 and 5% for testosterone, <5 and 7% for oestradiol, <16 and 17% for progesterone, and <4 and 5% for SHBG respectively. Dimeric inhibin B concentrations were assessed using an immuno-enzymometric assay obtained from Serotec (Oxford, Oxon, UK), as described previously (Schipper et al., 1998
). The detection limit of the assay, defined as the amount of inhibin equivalent with the signal of the blank +3 SDs of this signal, was 3.4 ng/l. Intra- and inter-assay coefficients of variation for inhibin B were <9 and 15% respectively.
Data analysis
Data are presented as median and range. Due to the non-parametric distribution, groups were compared using the MannWhitney rank-sum test. Non-parametric cross correlations and analysis of co-variance was performed using a commercially available software package (Statistics Package for Social Sciences, Chicago, IL, USA). A P value < 0.05 was considered to indicate statistical significance.
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Results |
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Discussion |
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Dominant follicle selection is disturbed in polycystic ovaries, resulting in an increased number of follicles per ovary and presumably a variable number of healthy early antral follicles (Fauser, 1994). Because inhibin B is produced predominantly by healthy small follicles it has been postulated that inhibin B concentrations are increased in at least some PCOS patients. Elevated concentrations of inhibin B could be established in two preliminary reports (Anderson et al., 1998
; Lockwood et al., 1998
) in 9 and 10 PCOS patients respectively. In contrast, in the present study, inhibin B concentrations in PCOS patients were similar compared with WHO 2 and control patients.
It has also been postulated that most of the inhibin B produced in the ovary during the follicular phase originates from the dominant follicle due to the observed major difference in the amount produced by large antral follicles (with androstenedione/oestradiol ratios 4) compared with remain- ing cohort follicles with androstenedione/oestradiol ratios >4 (Magoffin and Jakimiuk, 1998
). Follicular fluid oestradiol, androstenedione and total immunoreactive inhibin concentrations as well as androstenedione/oestradiol ratios were similar in PCOS patients compared to controls (Pache et al., 1992
). More recently, inhibin B concentrations in these 48 mm follicles from polycystic ovaries were also found to be similar to size-matched normal follicles (Magoffin and Jakimiuk, 1998
). Increased inhibin secretion by polycystic ovaries could augment LH-stimulated androgen production by theca cells (Hillier et al., 1991
). Recent observations in PCOS patients supporting this concept include elevated
inhibin concentrations, which were correlated with increased circulating concentrations of androstenedione (Pigny et al., 1997
). It was not possible to demonstrate a consistent and strong correlation between inhibin B and LH, testosterone or its precursors in the current study. It therefore appears that alterations in inhibin B concentrations do not play an important local role in vivo in LH-induced androgen production in these patients.
If serum inhibin B concentrations represent the extent of ovarian dysfunction in PCOS, one would expect a good correlation between inhibin B and endocrine hallmarks associated with ovarian abnormalities in these patients, such as elevated serum LH or androgen concentrations. Although ultrasound parameters are weakly correlated with inhibin B, serum oestradiol concentrations were not. In fact, serum oestradiol concentrations were fairly constant at different inhibin B concentrations. The observed lack of a strong, consistent relationship between inhibin B and endocrine markers of PCOS may suggest that inhibin B does not represent the magnitude of ovarian dysfunction in these patients. These results are consistent with recent observations from our group indicating that inhibin B serum concentrations do not predict ovarian responsiveness to ovulation induction (Imani et al., 2000).
It has been postulated that if inhibin is prematurely produced in excessive amounts relative to the developmental stage of the follicle, FSH secretion by the pituitary might become suppressed and concomitantly follicle development could be retarded (Magoffin and Jakimiuk, 1998). If this is true, inhibin B concentrations should be negatively correlated with serum FSH. Based on the observed weak positive correlation between serum inhibin B and FSH concentrations, the current data do not support this concept. The present study supports the contention that FSH is the primary drive of inhibin B synthesis, in line with recent observations during pubertal development (Crofton et al., 1997
). Since inhibin B concentrations in PCOS patients (produced by small arrested follicles) are similar to control concentrations, one might speculate that inhibin B exerts limited negative feedback on pituitary FSH production at the beginning of the cycle. Moreover, intra-follicular inhibin B concentrations in PCOS do not significantly decrease with increasing follicle diameters also indicating that FSH is the primary drive of the feedback (Lambert-Messerlian et al., 1997
).
In conclusion, inhibin B serum concentrations are normal in most normogonadotrophic anovulatory infertile women, including PCOS, suggesting a normal number of healthy early antral follicles despite increased overall follicle numbers in PCOS. Serum inhibin B assays cannot be recommended for routine screening in normogonadotrophic anovulatory infertility.
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Notes |
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References |
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Submitted on December 7, 2000; accepted on March 19, 2001.