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The Journal of Clinical Endocrinology & Metabolism Vol. 88, No. 11 5158-5162
Copyright © 2003 by The Endocrine Society

Obese Patients with Polycystic Ovary Syndrome: Evidence that Metformin Does Not Restore Sensitivity of the Gonadotropin-Releasing Hormone Pulse Generator to Inhibition by Ovarian Steroids

Christine A. Eagleson, Amy B. Bellows, Kathy Hu, Melissa B. Gingrich and John C. Marshall

Division of Endocrinology (C.A.E., J.C.M.) and Center for Research in Reproduction (C.A.E., A.B.B., K.H., M.B.G., J.C.M.), University of Virginia Health System, Charlottesville, Virginia 22908

Address all correspondence and requests for reprints to: John C. Marshall, M.D., Center for Research in Reproduction, University of Virginia Health System, Box 800391, Charlottesville, Virginia 22908. E-mail: jcm9h{at}virginia.edu.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Women with polycystic ovary syndrome (PCOS) have reduced GnRH sensitivity to suppression by ovarian steroids, which can be ameliorated by androgen blockade. We studied nine PCOS women and nine controls to determine whether metformin could change feedback inhibition by estradiol (E2) and progesterone (P). LH was measured every 10 min, and FSH, E2, P, and testosterone (T) were measured every 2 h. Frequently sampled iv glucose tolerance test was performed at the end of each admission. After the first admission, metformin (500 mg, three times a day) was started. The second admission occurred on d 8–11 of the next menstrual cycle in controls and on d 28 in PCOS patients. Patients subsequently took E2 and P for 1 wk until the third admission.

At baseline, PCOS women had higher T, free T, androstenedione, and estrone. After 4 wk of metformin, controls had a slight reduction in total T, but free T was unchanged. However, PCOS patients had reduced insulin, T, and E2, and increased LH mean/amplitude and FSH. After ovarian steroids, controls had a greater reduction in LH pulse frequency than PCOS (61 vs. 25%). These results suggest that the beneficial effects of metformin on ovulatory function in obese PCOS women are probably not mediated by enhanced hypothalamic sensitivity.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
POLYCYSTIC OVARY SYNDROME (PCOS) is a common disorder occurring in 6–10% of women of reproductive age (1, 2). It is characterized by chronic anovulation with either oligomenorrhea or amenorrhea and hyperandrogenism, and is the most common cause of anovulatory infertility and hirsutism (3). The etiology (or etiologies) of PCOS remain uncertain. Evidence exists for insulin resistance, functional ovarian hyperandrogenism, and altered gonadotropin secretion, but the relative roles of each are unclear (4, 5, 6, 7, 8).

Nonetheless, insulin resistance is common, occurring in 30% of lean and 75% of obese PCOS patients, and of significance due to its association with an increased risk of type II diabetes mellitus, dyslipidemia, and potentially cardiovascular disease (9). The impact of insulin resistance was formally evaluated in a study in which oral glucose tolerance test was administered to PCOS women (10). Impaired glucose tolerance and diabetes mellitus was present in 31 and 8%, respectively (10). Both in vitro and in vivo studies have demonstrated the importance of hyperinsulinemia on increasing ovarian androgen production. Insulin directly stimulates ovarian androgen production in in vitro studies, and testosterone (T) levels fall when women with PCOS are given diazoxide to inhibit insulin release (11, 12, 13). Velazquez et al. (14) first used metformin in PCOS and showed a reduction in insulin and T and improvement in menstrual cyclicity. The biguanide metformin inhibits hepatic glucose production and enhances peripheral tissue sensitivity to insulin, resulting in a reduction in insulin secretion (15, 16). Most but not all studies have shown that metformin reduces plasma androgen levels with consequent improvement in ovulatory reproductive function (17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28). Metformin even improves ovulation rate in PCOS women resistant to clomiphene citrate, a group in which ovulation induction is quite difficult (25).

Regarding altered gonadotropin secretion, numerous studies have documented elevated LH with increased pulse frequency, and LH is elevated in up to 95% of women with PCOS when recent ovulation is excluded (29, 30). Recent studies have shown that suppression of LH pulse frequency by ovarian steroids is impaired in PCOS compared with ovulatory controls (31, 32). This phenomenon appears to be due to hyperandrogenemia, because it can be ameliorated by pretreatment for 4 wk with flutamide, an androgen receptor blocker (33). The goal of the present study was to determine whether the improved ovulatory function after metformin therapy reflected enhanced sensitivity of the hypothalamic pulse generator to suppression by ovarian steroids.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

Nine women aged 29 ± 1 yr with clinical and laboratory features of PCOS and nine normal controls aged 29 ± 2 yr were studied over 5 wk. The women with PCOS were diagnosed on the basis of a history of oligomenorrhea, amenorrhea, and/or infertility in addition to evidence of hyperandrogenemia. Both PCOS and normal controls were obese and had a similar body mass index (BMI) (38 ± 2 vs. 37 ± 3). All women were screened with LH, FSH, estradiol (E2), total T, dehydroepiandrosterone sulfate, progesterone (P), fasting insulin/glucose, prolactin, T4, and ß-human chorionic gonadotropin. PCOS patients were also screened for elevated morning 17-hydroxyprogesterone levels. Hormonal medications were discontinued for a minimum of 90 d before the study. Women with PCOS were studied at least 60 d from their last menstrual bleed, and normal controls were studied between d 8 and 11 of their menstrual cycle (admissions 1 and 2) to approximate the E2, P, and LH pulse frequency in women with PCOS.

Study protocol

The study was approved by the human investigation committee of the University of Virginia Health Systems, the General Clinical Research Center advisory committee, and the Food and Drug Administration (IND 64126). Informed consent was obtained from all patients. Subjects were admitted to the General Clinical Research Center at 1800 h, which was 2 h before the start of blood sampling. Blood samples were obtained through an indwelling iv forearm heparin lock. A second iv heparin lock was placed in the opposite arm and was used for glucose bolus and iv insulin for the modified frequently sampled iv glucose tolerance test (FSIGT). During each admission, LH was measured every 10 min, and FSH, E2, P, and T were measured every 2 h from 2000–0900 h. GnRH (25 ng/kg) was given at 0800 h. The modified FSIGT was performed after completion of gonadotropin sampling and an overnight fast (34). At -10 and 0 min (0900 h), basal levels of insulin and glucose were drawn. At 0900 h, a bolus of iv glucose (0.3 g/kg 50% dextrose) was injected over 20 sec and flushed with normal saline. Blood samples were drawn at 2, 4, 8, 19, 22, 30, 40, 50, 70, 90, and 180 min after the dextrose injection. At 20 min, regular insulin was injected over 20 sec using 0.03 U/kg for lean subjects and 0.05 U/kg for obese subjects.

After the first admission, patients were given metformin [500 mg, three times a day (tid)] (Bristol-Myers Squibb, Princeton, NJ), which was continued for the entire study. To minimize side effects, patients were advised to escalate the dose by 500 mg/d to reach the goal of 500 mg tid by the third day. Patients complied with all of their medication as assessed by pill counting. Patients had a single plasma measurement of P on d 9, 11, and 13 to confirm ovulatory cycles in normal controls and anovulation in PCOS. The second admission occurred between d 8 and 11 of the subsequent cycle in controls and on d 28 in women with PCOS. Blood sampling, GnRH dosage, and FSIGT were identical with first admission. After the second admission, patients started two E2 patches (0.1 mg/d, changed every 3 d; Estraderm, Novartis Pharmaceuticals, Princeton, NJ) and micronized P. Micronized P was given in variable doses (100–150 mg) either via a capsule (Prometrium, Solvay Pharmaceuticals, Marietta, GA) or via oral suspension (35). The research pharmacy at University of Virginia compounded the P suspension (20 mg/ml) by mixing 2 g micronized P with 2.5 ml glycerin, 25 ml 1% methylcellulose, and 72.5 ml cherry syrup. The third admission occurred after 1 wk of E2 and P. The protocol was identical with that of the first two admissions. Three additional normal controls were studied but not included in the results, because low plasma E2 levels on the third admission indicated noncompliance with the protocol. Two additional PCOS were studied but not included, because they had evidence of ovulation based on elevated plasma P between d 9 and 13.

After completion of the study, all PCOS patients were given the option of continuing metformin (500 mg tid). Telephone interviews were conducted at 1- to 3-month intervals for up to 6 months to evaluate for changes in menstrual cyclicity.

Hormonal measurements

All samples from an individual woman were analyzed in duplicate in the same assay for each hormone. Plasma LH and FSH were measured by chemiluminescence (Nichols Institute Diagnostics, San Juan Capistrano, CA). The assay sensitivities were 0.15 and 1.7 mIU/ml for LH and FSH, respectively; intraassay coefficients of variation (CVs) were 7.5 and 8.4%, and interassay CVs were 12 and 13%, respectively. E2, P, and T were measured by RIA (Diagnostics Systems Laboratories, Inc., Webster, TX). The assay sensitivities were 4.7 pg/ml, 0.12 ng/ml, and 0.08 ng/ml for E2, P, and T, respectively; intraassay CVs were 8.8, 9.1, and 8.4%, and interassay CVs were 14.7, 16.4, and 16.3%, respectively. Insulin was measured by RIA (Diagnostics Systems Laboratories, Inc). The assay sensitivity was 1.3 µIU/ml; intraassay CV was 5.5%, and interassay CV was 16.4%. Androstenedione was measured by RIA (Diagnostics Systems Laboratories, Inc). The assay sensitivity was 0.03 ng/ml; intraassay CV was 6.0%, and interassay CV was 13.8%.

Data and statistical analysis

Data are presented as mean ± SEM. A value of P < 0.05 was considered significant. LH pulses and amplitude were identified and characterized using the computer algorithm Cluster 7 with parameters of threshold change corresponding to a t statistic of 2.45 for both peak upstroke and downstroke (36, 37). Based on our studies, pulses defined by the Cluster program were accepted if the increment in LH was greater than 0.25 for a peak of less than 1 mIU/ml, greater than 0.5 for a peak measuring 1–5 mIU/ml, and greater than 1 for a peak of greater than 5 mIU/ml (38). Missing values represented less than 0.1% of the total and were not replaced.

Data comparing admissions for each group were analyzed using paired t tests. Data comparing PCOS to controls in each admission were analyzed with unpaired t tests. Due to limited data, adjustments for multiple comparisons within a response or for multiple responses could not be made. Regression analysis was performed to determine whether the change in LH pulse frequency as a function of increasing P was different in normal controls and women with PCOS. The SAS (SAS Institute, Inc., Cary, NC) general linear model procedure was used for the regression analysis. The model was based on ordinary least squares regression.

Free T (picomoles per liter) was calculated as follows: T determined by RIA (nanomoles per liter)/[K x SHBG (nanomoles per liter) + 1] x 1000, where K is the equilibrium constant for T binding to SHBG (1.6 x 109 liter/mol) (39).

Insulin sensitivity index (Si)

The Si was calculated using the MINMOD program from Dr. Richard N. Bergman (University of Southern California–Keck School of Medicine, Los Angeles, CA) (34). The Si represents the increase in net fractional glucose clearance rate per unit change in plasma insulin concentration after the iv glucose load.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Data from screening laboratories showed normal levels of T4, 17-hydroxyprogesterone, and dehydroepiandrosterone sulfate in all subjects. Mean data from all admissions are shown in Table 1Go. At baseline, both groups had similar plasma concentrations of LH (mean, frequency, and amplitude), FSH, E2, P, insulin, and SHBG. Women with PCOS had higher T (1.26 ± 0.12 vs. 0.44 ± 0.04 ng/ml; P < 0.001), higher calculated free T (59.3 ± 11.4 vs. 20.2 ± 4.9 pmol/liter; P = 0.01), androstenedione (2.9 ± 0.6 vs. 1.4 ± 0.2 ng/ml; P < 0.05), and estrone (75.1 ± 7.9 vs. 47.7 ± 6.3 pg/ml; P < 0.05).


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  		TABLE 1. Hormonal parameters in controls and women with PCOS before and after metformin alone (admits 1 and 2) and after the addition of E2 and P for 7 d (admit 3)

 
After 4 wk of metformin (admission 2), there was a slight reduction in total T (0.44 ± 0.04 vs. 0.36 ± 0.02 ng/ml; P = 0.045) in controls, although free T remained unchanged. None of the other hormonal parameters changed significantly in the controls. In contrast, PCOS women experienced a reduction in E2 (57.2 ± 7.0 vs. 49.6 ± 6.7 pg/ml; P < 0.005), T (1.26 ± 0.12 vs. 1.06 ± 0.10 ng/ml; P = 0.01), and insulin (25.8 ± 4.7 vs. 19.4 ± 4.5 µIU/ml; P < 0.005). Mean LH increased from 6.4 ± 1.2 to 7.6 ± 1.4 mIU/ml, pulse amplitude increased from 1.7 ± 0.3 to 2.3 ± 0.3 mIU/ml, and mean FSH increased from 4.5 ± 0.5 to 5.4 ± 0.5 mIU/ml in PCOS subjects (P < 0.005).

After 1 wk of ovarian steroid administration, mean E2 and P values increased in both groups, but to a greater degree in normal controls. LH pulse frequency fell by 61% in normal controls (10.3 ± 0.5 to 4.0 ± 0.9 pulses/12 h; P < 0.001), but PCOS subjects had less (25%) reduction in pulse frequency (9.6 ± 0.4 vs. 7.2 ± 0.4 pulses/12 h; P < 0.001). Both groups had a significant reduction in FSH. Fasting insulin levels fell between the second and third admission in normal controls (24.2 ± 6.3 vs. 15.9 ± 4.0 µIU/ml; P = 0.01), but not in PCOS (19.4 ± 4.5 vs. 22.3 ± 6.6 µIU/ml; P = 0.36). The calculated Si did not change in controls (0.9 ± 0.2 vs. 1.7 ± 0.6 x 10-4 min-1·µU-1·ml-1; P = 0.24) or PCOS (1.1 ± 0.3 vs. 1.1 ± 0.3 x 10-4 min-1·µU-1·ml-1; P = 0.82).

Figure 1Go shows the change in LH pulse frequency between admissions 2 and 3 as a function of plasma P. Linear regression analysis was performed and compared with prior results when subjects were treated with E2 and P alone for 7 d but without metformin (shaded area; Ref.31). Normal controls had similar changes in LH pulse frequency for a given P value with and without metformin. In contrast, PCOS subjects had less reduction in LH pulse frequency at a given P value compared with normal controls both with and without metformin (P < 0.001). Overall, metformin did not alter the effect of P on LH pulse frequency in PCOS subjects. However, four PCOS subjects did show a reduction in LH pulse frequency, which overlaps that seen in normal controls (LH pulses reduced by >=3 pulses every 12 h). Subgroup analysis of these four patients showed a more marked reduction in T (25 vs. 20%), insulin (44 vs. 16%), and free T (26 vs. 17%) than the other five nonresponding PCOS patients. Whereas baseline mean T values were similar in both subgroups (1.2 vs. 1.3 ng/ml), mean free T (46.9 vs. 69.2 pmol/liter) and insulin levels (18.2 vs. 31.9 µIU/ml) were lower in PCOS patients who suppressed LH pulses into the normal range. These responsive PCOS subjects were less obese with a mean BMI of 35 compared with 41 in nonresponsive patients.



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  		FIG. 1. The change in LH pulse frequency (decrement between admits 2 and 3) after E2 and P administration for 7 d in the presence of metformin. Data are shown as a function of the mean plasma P level during admit 3 for controls (left) and PCOS patients (right). The shaded area shows the range of change in an identical protocol performed in the absence of metformin (31 ). The slopes for linear regression analysis are as follows: controls with metformin, -0.24; controls without metformin, -0.24; PCOS with metformin, -0.08; and PCOS without metformin, -0.35. Conversion factors to obtain SI units are as follows: LH (1.0 for international units per liter) and P (3.18 for nanomoles per liter).

 
After completion of the study, four of nine PCOS patients elected to continue metformin (500 mg tid). Two patients had improvement in menstrual cyclicity (oligomenorrhea to regular cycles and amenorrhea to oligomenorrhea). In the PCOS subjects with improved menstrual cyclicity, one subject was in the responsive group, and the other was in the nonresponsive group.


   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
This study was performed in follow-up to prior studies in which women with PCOS were found to have reduced sensitivity of the GnRH pulse generator to inhibition by P (31, 32). We aimed to determine whether the beneficial effects of metformin on ovulatory function reflected enhanced feedback inhibition of LH (GnRH) pulse secretion by ovarian steroids.

As expected, PCOS subjects had higher plasma levels of T, androstenedione, and estrone at baseline. Both groups were obese and had similarly elevated mean fasting insulin levels. After 4 wk of metformin, PCOS subjects experienced a reduction in insulin and T levels. Insulin levels fell by 25 ± 6%, whereas total T levels fell by 16 ± 5%. In controls, fasting insulin levels fell between admits 2 and 3 (23 ± 4.0% below baseline), but it is unlikely that this reflects a delayed effect of metformin. In reviewing individual data, two control subjects had a greater than 20% increase in insulin from admits 1–2, but then a 35% decrease by admit 3, which suggests noncompliance with fasting during admit 2. Metformin in doses of 500 mg twice daily up to 850 mg tid has been studied in PCOS patients over variable periods between 4 wk and 23 months. Those studies using similar regimens to the present investigation [metformin (500 mg tid) given for 30–60 d] found similar reductions in insulin and total T (28 and 18%, respectively) (17, 25, 26, 28). Although insulin levels fell by admission 2 in PCOS and by admission 3 in normal controls, Si were all low as measured by FSIGT and did not change significantly. This may reflect the fact that metformin is less effective in enhancing insulin sensitivity when the BMI is greater than 30 kg/m2 (40).

After E2 and P for 7 d, both groups had reduction of LH pulse frequency, but this reduction was greater in controls than in PCOS (61 ± 8 vs. 25 ± 4%). The fall in LH pulse frequency as a function of plasma P was compared with previous results in the absence of pretreatment with metformin (31). The reduction in LH pulse frequency in controls was similar regardless of whether they had been pretreated with metformin. In the earlier study by Pastor et al. (31), normal controls had a greater reduction in LH pulse frequency for any given P level, suggesting underlying insensitivity of GnRH to suppression by ovarian steroids in PCOS subjects. In the current study, metformin did not enhance the sensitivity of the GnRH pulse generator in these obese PCOS subjects.

These results stand in marked contrast to our previous study in which normal controls and PCOS were given flutamide, an androgen receptor blocker, for 4 wk before treatment with ovarian steroids (33). In that study, we followed an identical protocol. In controls, reduction in LH pulse frequency after E2 and P was similar with and without flutamide. However, PCOS subjects showed significant enhancement of the ability of E2 and P to reduce LH pulse frequency after flutamide pretreatment, such that the sensitivity of the GnRH pulse generator was restored to normal.

These data confirm that the GnRH pulse generator is less sensitive to suppression by ovarian steroids in PCOS compared with controls and indicate that metformin does not enhance hypothalamic sensitivity in markedly obese PCOS subjects. This may reflect the degree of reduction in plasma T, because flutamide is an effective androgen receptor blocker and hypothalamic androgen exposure may be completely prevented. In contrast, metformin only moderately reduces plasma T, which likely results in less reduction in hypothalamic androgen exposure.

Several studies have shown improvement in the frequency of ovulation after metformin, both with and without clomiphene citrate in PCOS patients (14, 17, 20, 22, 23, 24, 25, 27, 28). The improvement in ovulatory cyclicity ranges from 18–96%, indicating that metformin is not universally effective in improving ovulatory function. In our study, four of nine (44%) PCOS patients had hypothalamic sensitivity to ovarian steroids, which overlapped that of normal controls. It is interesting that these four responsive subjects were less obese, had lower baseline free T, and had more marked reduction in T and insulin. The responsive patients may therefore have had less hypothalamic androgen exposure allowing for improvement in sensitivity to ovarian steroids. Thus, increased ovulatory function in PCOS patients on metformin may occur in those who have restoration of hypothalamic sensitivity to P. However, the actions of intraovarian androgen excess on follicular maturation may in part be responsible for anovulation by promoting follicular atresia (41, 42). Thus, metformin may also exert actions directly on the ovary to improve ovulatory function by reducing the effects of increased intraovarian androgens.

These data do not suggest a predominant hypothalamic site of action for metformin in enhancing ovulatory function. However, in some patients the decrease in plasma T and reduced impairment of steroid feedback may be part of the mechanisms involved in improved ovulatory and menstrual function.


   Acknowledgments
 
We acknowledge and extend our gratitude to the General Clinical Research Center (GCRC) staff, GCRC nurses, the Ligand Core Lab, and the GCRC statistician, Rob D. Abbott, Ph.D., without whom this research and publication would not have been possible.


   Footnotes
 
This work was supported by National Institute of Child Health and Human Development, National Institutes of Health, through Cooperative Agreement U54-HD-28934 as part of the Specialized Cooperative Centers Program in Reproduction Research, by HD-34179 (to J.C.M.), by General Clinical Research Center Grant M-01-RR-00847, and by Eli Lilly Pituitary Scholarship (to C.A.E.).

Abbreviations: BMI, Body mass index; CV, coefficient of variation; E2, estradiol; FSIGT, frequently sampled iv glucose tolerance test; P, progesterone; PCOS, polycystic ovary syndrome; Si, sensitivity index; T, testosterone; tid, three times a day.

Received February 3, 2003.

Accepted July 19, 2003.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

  1. Asuncion M, Calvo RM, San Millan JL, Sancho J, Avila S, Escobar-Morreale HF 2000 A prospective study of the prevalence of polycystic ovary syndrome in unselected Caucasian women from Spain. J Clin Endocrinol Metab 85:2434–2438[Abstract/Free Full Text]
  2. Knochenhauer ES, Key TJ, Kahsar-Miller M, Waggoner W, Boots LR, Azziz R 1998 Prevalence of the polycystic ovary syndrome in unselected black and white women of the southeastern United States: a prospective study. J Clin Endocrinol Metab 83:3078–3082[Abstract/Free Full Text]
  3. Solomon CG 1999 The epidemiology of polycystic ovary syndrome: prevalence and associated disease risks. Endocrinol Metab Clin North Am 28:247–263[CrossRef][Medline]
  4. Franks S 1995 Polycystic ovary syndrome. N Engl J Med 333:853–861[Free Full Text]
  5. Utiger RD 1996 Insulin and the polycystic ovary syndrome. N Engl J Med 335:657–658[Free Full Text]
  6. Dunaif A 1999 Insulin action in the polycystic ovary syndrome. Endocrinol Metab Clin North Am 28:341–359[CrossRef][Medline]
  7. Poretsky L, Piper B 1994 Insulin resistance, hypersecretion of LH, and a dual-defect hypothesis for the pathogenesis of polycystic ovary syndrome. Obstet Gynecol 84:613–621[Medline]
  8. Ehrmann DA, Barnes RB, Rosenfield RL 1995 Polycystic ovary syndrome as a form of functional ovarian hyperandrogenism due to dysregulation of androgen secretion. Endocr Rev 16:322–353[Abstract/Free Full Text]
  9. Morales AJ, Laughlin GA, Butzow T, Maheshwari H, Baumann G, Yen SS 1996 Insulin, somatotropic, and luteinizing hormone axes in lean and obese women with polycystic ovary syndrome: common and distinct features. J Clin Endocrinol Metab 81:2854–2864[Abstract/Free Full Text]
  10. Legro RS, Kunselman AR, Dodson WC, Dunaif A 1999 Prevalence and predictors of risk for type 2 diabetes mellitus and impaired glucose tolerance in polycystic ovary syndrome: a prospective, controlled study in 254 affected women. J Clin Endocrinol Metab 84:165–169[Abstract/Free Full Text]
  11. Barbieri RL, Markis A, Randall RW, Daniels G, Kistner RW, Ryan KJ 1986 Insulin stimulates androgen accumulation in incubations of ovarian stroma obtained from women with hyperandrogenism. J Clin Endocrinol Metab 62:904–910[Abstract/Free Full Text]
  12. Nestler JE, Jakubowicz DJ, de Vargas AF, Brik C, Quintero N, Medina F 1998 Insulin stimulates testosterone biosynthesis by human thecal cells from women with polycystic ovary syndrome by activating its own receptor and using inositolglycan mediators as the signal transduction system. J Clin Endocrinol Metab 83:2001–2005[Abstract/Free Full Text]
  13. Nestler JE, Barlascini CO, Matt DW, Steingold KA, Plymate SR, Clore JN, Blackard WG 1989 Suppression of serum insulin by diazoxide reduces serum testosterone levels in obese women with polycystic ovary syndrome. J Clin Endocrinol Metab 68:1027–1032[Abstract/Free Full Text]
  14. Velazquez EM, Mendoza S, Hamer T, Sosa F, Glueck CJ 1994 Metformin therapy in polycystic ovary syndrome reduces hyperinsulinemia, insulin resistance, hyperandrogenemia, and systolic blood pressure, while facilitating normal menses and pregnancy. Metabolism 43:647–654[CrossRef][Medline]
  15. Nagi DK, Yudkin JS 1993 Effects of metformin on insulin resistance, risk factors for cardiovascular disease, and plasminogen activator inhibitor in NIDDM subjects: a study of two ethnic groups. Diabetes Care 16:621–629[Abstract]
  16. DeFronzo RA, Barzilai N, Simonson DC 1991 Mechanism of metformin action in obese and lean noninsulin-dependent diabetic subjects. J Clin Endocrinol Metab 73:1294–1301[Abstract/Free Full Text]
  17. la Marca A, Egbe TO, Morgante G, Paglia T, Cianci A, De Leo V, Ciani A 2000 Metformin treatment reduces ovarian cytochrome P-450c17{alpha} response to human chorionic gonadotropin in women with insulin resistance-related polycystic ovary syndrome. Hum Reprod 15:21–23[Abstract/Free Full Text]
  18. Moghetti P, Castello R, Negri C, Tosi F, Perrone F, Caputo M, Zanolin E, Muggeo M 2000 Metformin effects on clinical features, endocrine and metabolic profiles, and insulin sensitivity in polycystic ovary syndrome: a randomized, double-blind, placebo-controlled 6-month trial, followed by open, long-term clinical evaluation. J Clin Endocrinol Metab 85:139–146[Abstract/Free Full Text]
  19. De Leo V, la Marca A, Ditto A, Morgante G, Cianci A 1999 Effects of metformin on gonadotropin-induced ovulation in women with polycystic ovary syndrome. Fertil Steril 72:282–285[CrossRef][Medline]
  20. Glueck CJ, Wang P, Fontaine R, Tracy T, Sieve-Smith L 1999 Metformin-induced resumption of normal menses in 39 of 43 (91%) previously amenorrheic women with the polycystic ovary syndrome. Metabolism 48:511–519[CrossRef][Medline]
  21. Pirwany IR, Yates RW, Cameron IT, Fleming R 1999 Effects of the insulin sensitizing drug metformin on ovarian function, follicular growth and ovulation rate in obese women with oligomenorrhea. Hum Reprod 14:2963–2968[Abstract/Free Full Text]
  22. Unluhizarci K, Kelestimur F, Bayram F, Sahin Y, Tutus A 1999 The effects of metformin on insulin resistance and ovarian steroidogenesis in women with polycystic ovary syndrome. Clin Endocrinol (Oxf) 51:231–236[CrossRef][Medline]
  23. Diamanti-Kandarakis E, Kouli C, Tsianateli T, Bergiele A 1998 Therapeutic effects of metformin on insulin resistance and hyperandrogenism in polycystic ovary syndrome. Eur J Endocrinol 138:269–274[Abstract]
  24. Morin-Papunen LC, Koivunen RM, Ruokonene A, Martikainen HK 1998 Metformin therapy improves the menstrual pattern with minimal endocrine and metabolic effects in women with polycystic ovary syndrome. Fertil Steril 69:691–696[CrossRef][Medline]
  25. Nestler JE, Jakubowicz DJ, Evans WS, Pasquali R 1998 Effects of metformin on spontaneous and clomiphene-induced ovulation in the polycystic ovary syndrome. N Engl J Med 338:1876–1880[Abstract/Free Full Text]
  26. Nestler JE, Jakubowicz DJ 1997 Lean women with polycystic ovary syndrome respond to insulin reduction with decreases in ovarian P450c17{alpha} activity and serum androgens. J Clin Endocrinol Metab 82:4075–4079[Abstract/Free Full Text]
  27. Velazquez E, Acosta A, Mendoza SG 1997 Menstrual cyclicity after metformin therapy in polycystic ovary syndrome. Obstet Gynecol 90:392–395[CrossRef][Medline]
  28. Nestler JE, Jakubowicz DJ 1996 Decreases in ovarian cytochrome P450c17{alpha} activity and serum free testosterone after reduction of insulin secretion in polycystic ovary syndrome. N Engl J Med 335:617–623[Abstract/Free Full Text]
  29. Taylor AE, McCourt B, Martin KA, Anderson EJ, Adams JM, Schoenfeld D, Hall JE 1997 Determinants of abnormal gonadotropin secretion in clinically defined women with PCOS. J Clin Endocrinol Metab 82:2249–2256
  30. Taylor AE 1998 Polycystic ovary syndrome. Endocrinol Metab Clin North Am 27:877–902[CrossRef][Medline]
  31. Pastor CL, Griffin-Korf ML, Aloi JA, Evans WS, Marshall JC 1998 Polycystic ovary syndrome: evidence for reduced sensitivity of the gonadotropin-releasing hormone pulse generator to inhibition by estradiol and progesterone. J Clin Endocrinol Metab 83:582–590[Abstract/Free Full Text]
  32. Daniels TL, Berga SL 1997 Resistance of gonadotropin releasing hormone drive to sex steroid-induced suppression in hyperandrogenic anovulation. J Clin Endocrinol Metab 82:4179–4183[Abstract/Free Full Text]
  33. Eagleson CA, Gingrich MB, Pastor CL, Arora TK, Burt CM, Evans WS, Marshall JC 2000 Polycystic ovarian syndrome: evidence that flutamide restores sensitivity of the gonadotropin-releasing hormone pulse generator to inhibition by estradiol and progesterone. J Clin Endocrinol Metab 85:4047–4052[Abstract/Free Full Text]
  34. Steil GM, Volund A, Kahn SE, Bergman RN 1993 Reduced sample number for calculation of insulin sensitivity and glucose effectiveness from the minimal model: suitability for use in population studies. Diabetes 42:250–256[Abstract]
  35. 1998 Progesterone oral suspension (40 mg/mL). IJPC 2:57
  36. Veldhuis JD, Carlson ML, Johnson ML 1987 The pituitary gland secretes in bursts: appraising the nature of glandular secretory impulses by simultaneous multiple-parameter deconvolution of plasma hormone concentration. Proc Natl Acad Sci USA 84:7686–7690[Abstract/Free Full Text]
  37. Veldhuis JD, Johnson ML 1986 Cluster analysis: a simple, versatile, and robust algorithm for endocrine pulse detection. Am J Physiol 250:E486–E493
  38. McCartney CR, Gingrich MB, Hu Y, Evans WS, Marshall JC 2002 Hypothalamic regulation of cyclic ovulation: evidence that the increase in gonadotropin-releasing hormone pulse frequency during the follicular phase reflects the gradual loss of the restraining effects of progesterone. J Clin Endocrinol Metab 87:2194–2200[Abstract/Free Full Text]
  39. Ekins RP 1984 Free hormones in blood: the concept and the measurement. J Clin Immunoassay 4:163–180[CrossRef]
  40. Kahn SE, Prigeon RL, McCulloch DK, Boyko EJ, Bergman RN, Schwartz MW, Neifing JL, Ward WK, Beard JC, Palmer JP, Porte Jr D 1993 Quantification of the relationship between insulin sensitivity and ß-cell function in human subjects: evidence for a hyperbolic function. Diabetes 42:1663–1672[Abstract]
  41. Hillier SG, Ross GT 1979 Effects of exogenous testosterone on ovarian weight, follicular morphology and intraovarian progesterone concentration in estrogen-primed hypophysectomized immature female rats. Biol Reprod 20:261–268[Abstract]
  42. Louvet JP, Harman SM, Schreiber JR, Ross GT 1975 Evidence of a role of androgens in follicular maturation. Endocrinology 97:366–372[Abstract/Free Full Text]



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