Biomarkers of Ovarian Reserve
Biomarkers of Ovarian Reserve
Abstract
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Introduction
The primary function of the female ovary is the production of a mature and viable oocyte capable
of fertilization and subsequent embryo development and implantation. At birth, the ovary
contains a finite number of oocytes available for folliculogenesis. This finite number of available
oocytes is termed “the ovarian reserve”. The determination of ovarian reserve is important in the
assessment and treatment of infertility. As the ovary ages, the ovarian reserve will decline.
Ovarian reserve (OR) refers to the number and quality of oocytes that, at any given age, are
available to produce a dominant follicle late in the follicular phase of the menstrual cycle. By
estimating the OR, a prediction of the remaining reproductive lifetime could be assessed as well
as the likely success of assisted reproductive techniques (ART) such as in vitro fertilization (IVF)
(Baird et al. 2005). None of the OR tests directly measures the total number of actual oocytes.
Rather, it is assumed that the number of recruitable and developing follicles (pre-antral and
antral) is directly related to the total oocyte pool. During fetal life the ovaries are endowed with
the entire stock of follicles (oocytes surrounded by ovarian granulosa cells) that will serve a
woman’s reproductive life. Because the number of quality oocytes available for recruitment
during folliculogenesis changes markedly during a woman’s lifetime, the tendency is for OR
physical evaluation tests to inaccurately estimate the total pool of “good-viable” oocytes. This
review will look at the physical tools utilized to determine ovarian reserve.
Infertility affects approximately 15%–20% of reproductive aged couples. The most commonly
used biomarker test to assess ovarian reserve is the measurement of day 3 follicle stimulating
hormone (FSH); this blood test determines the level of FSH on day 3 of the menstrual cycle.
Cycle day 3 is the preferred testing day due to the expected low level of estradiol, which in turn
affects FSH levels via negative feedback control. Therefore, Day-3 FSH levels would be
expected to be low, a higher than normal day-3 FSH level would indicate a diminished ovarian
reserve. However, this day still requires standardization to ensure reproducibility. Typically, Day-
3 FSH and estradiol are both measured. However, other blood tests (antimüllerian hormone and,
or inhibin-B) are gaining popularity since they provide more direct determination of ovarian
status, whereas Day-3 FSH and estradiol are indirect measurements. This review will look at the
aforementioned hormones as biomarkers of ovarian reserve.
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Indirect Measures
Day 3 FSH
Day 3 FSH is believed to represent the “basal” level or non-suppressed level of FSH through
ovarian feedback to the pituitary (Barnhart and Osheroff, 1998). Day 3 FSH has been credited
with being a biomarker for ovarian reserve since the late 1980s, as it provides a glimpse of how
well the hypothalamic-pituitary-gonadal axis is functioning (Barnhart and Osheroff, 1998). As
women and their follicles age, the amount of FSH secreted increases due to the lack of
responsiveness of the ovary (Perloe et al. 2000). As day 3 FSH levels climb, it is indicative of a
diminished ovarian reserve. In fact, fluctuation between cycles of day 3 FSH levels is important
to note as it may be reflective of the decline in ovarian reserve (Perloe et al. 2000).
Monitoring day 3 FSH levels may not be the best option for monitoring ovarian reserve, but it is
the most widely recognized OR biomarker and it does provide some insight. Testing is available
on multiple automated platforms and thus is relatively fast, inexpensive, and reproducible. FSH is
proven to increase with the age of follicles and that increase is more dramatic and earlier than that
of LH (Perloe et al. 2000). Historically, FSH has been the biomarker of choice; it is well studied,
documented, and validated which provides a level of comfort to physicians (Sharara et al. 1998).
It is important to recognize some of the issues with using FSH as a biomarker for OR testing.
Between cycle fluctuation in day 3 FSH levels make OR estimation difficult (Perloe et al. 2000).
Since lower day 3 FSH levels represent satisfactory ovarian reserve and higher levels represent
declining OR, a single day 3 FSH measurement may not be very accurate. It may be better to
look at subsequent cycle day 3 FSH levels (Perloe et al. 2000). Additionally, an increased day 3
FSH level is considered a late indicator of marked decreased fertility potential (Sharara et al.
1998). It may be better to look for an early indicator of declining OR and/or decreased fertility
potential.
Estradiol
Day 3 estradiol has been assessed for OR testing as well, but is not as extensively relied upon.
Estradiol is a product of the granulosa cells and can be considered a reflection of follicular
activity. As with FSH, estradiol testing is also available on multiple automated platforms and thus
is relatively fast, inexpensive and reproducible. However, it is never used alone as a biomarker
for OR.
An increased estradiol level early in the menstrual cycle suggests that follicular development is in
an advanced stage that is inappropriate for day 3 (Perloe et al. 2000). However, estradiol levels
can be increased for two very different reasons. Estradiol levels can become elevated due to the
occurrence of rapid folliculogenesis. Alternatively, an increased estradiol level can be due to an
enhanced OR, such as in women afflicted with polycystic ovarian syndrome (PCOS), where a
small amount of estradiol is being produced by a large number of antral follicles (Toner, 2003).
FSH:LH ratio
The literature gives some honorable mention to looking at the ratio of measured FSH to LH,
which is most frequently determined on day 3 of the cycle. An elevated or exaggerated FSH:LH
ratio can be a signal of diminished OR (Toner, 2003). By looking at the ratio an elevation can be
detected, even with an FSH level that appears to be within the reference interval (Toner, 2003).
Since FSH begins rising before LH as OR diminishes, using two measurements may be more
reliable and will catch a small increase in FSH faster (Perloe et al. 2000). The FSH:LH ratio is an
early indicator of ovarian ageing and could be the first of diminished OR (Mukherjee et al. 1996).
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Direct Measures
Inhibin B
Inhibin B is a peptide hormone that is a member of the transforming growth factor-β (TGF-β)
superfamily (Perloe et al. 2000). It is produced from small antral follicles and selectively inhibits
FSH release (Perloe et al. 2000; Broekmans et al. 1998). Inhibin B levels during the early
follicular phase decrease prior to the increase in FSH levels (Toner, 2003). As the follicle cohort
size is decreased, which is expected as women age, a decrease in inhibin B levels will be
observed (Broekmans et al. 1998).
Unfortunately, documentation in the literature is minimal and also variable. Inhibin B levels do
hold promise, but need more study and validation (Sharara et al. 1998). This could be attributed
to a variation between assays used and there may be concerns about cycle-to-cycle variability
(Sharara et al. 1998). One study of interest found inhibin B to have the best positive association
with the number of oocytes collected from patients undergoing gonadotropin ovarian stimulation
tests to assess ovarian reserve (Muttukrishna et al. 2005). This study looked at 108 women and
the change in inhibin B between days 3 and 4, as well as other biomarkers. Currently, there is
only one commercially available assay for inhibin B. Unfortunately, it is still being optimized and
is currently available for research use only.
AMH
Like inhibin B, antimüllerian hormone (AMH) is also a member of the TGF-β superfamily. AMH
is produced by the granulosa cells of pre-antral and small antral follicles. Follicular growth is
modulated by AMH, which inhibits recruitment of follicles from the primordial pool by
modifying the FSH sensitivity of those follicles (La Marca et al. 2006; Visser et al. 2006). AMH
is considered to be reflective of the non-FSH dependant follicular growth. As a follicle matures,
AMH production disappears allowing the follicle to complete the development process during the
FSH-dependant stages of growth (Visser et al. 2006). There is a linear decline of AMH levels
over time (Visser et al. 2006; La Marca and Volpe, 2006). This decline is attributed to a
decreasing number of follicles in the primordial pool.
AMH will in all probability become the hormone of choice for assessing OR. It has been
suggested that AMH is the single best predictor of poor response for ART (Muttukrishna et al.
2005). The fact that AMH is secreted without dependence on other hormones, particularly the
gonadotropins, and that AMH is expressed at a constant level, independent of cycle day make
AMH very attractive as a direct measurement of OR (La Marca et al. 2006; Feyereisen et al.
2006; Hehenkamp et al. 2006; La Marca et al. 2007). The freedom that AMH testing offers both
clinicians and patients by allowing collections to be performed on any day during the menstrual
cycle is a vast logistical advantage over other biomarkers.
One recent study demonstrated not only a strong relationship between AMH and AFC, but that
this relationship was stronger than the other typical biomarkers relationships with AFC
(Feyereisen et al. 2006). In addition to being a good biomarker for the quantity of follicles,
another study illustrated that AMH is also suggestive of the quality of the remaining oocytes
(Ebner et al. 2006). Women with normal reproductive performance were examined twice within
an average of four years and assessed the AFC and various endocrine markers, demonstrating that
serum AMH, followed by the AFC showed the most consistent correlation to the age-related
decline of reproductive capacity (van Rooij et al. 2005).
Additional research is needed to recognize all the roles AMH plays. It will be important to
recognize the mechanisms that control production of AMH within the granulosa cells (Feyereisen
et al. 2006). To better understand if AMH is truly reflective of quality of the follicles, the fate of
oocytes and embryos from individual follicles will need to be assessed (Feyereisen et al. 2006).
GAST
The Gonadotropin-releasing Hormone Agonist Test (GAST) is another stimulation test that is
fairly well documented in the literature. It evaluates the change in estradiol levels on cycle days 2
and 3 following a subcutaneous administration of a gonadotropin-releasing hormone agonist (1
mg leuprolide acetate) (Perloe et al. 2000). The dose of the agonist causes a massive, temporary
release of FSH and LH from the pituitary, which in turn increases estradiol production within a
24-hour timeframe (Broekmans et al. 1998). A robust increase or flare of estradiol in response to
this stimulation is reflective of recruitable follicles in the early follicular phase which is in turn
representative of OR (Perloe et al. 2000; Broekmans et al. 1998).
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Summary
   a. All tests have some benefits. There will always be a combination of markers used to get
      the best answer. The best candidate for a single biomarker is AMH.
   b. We could recommend the need of a reference interval study using a “normal” population
      in addition to an infertile population. Reference intervals for each population should be
      stratified by age and body mass index and for ART patients, further stratified by
      gonadotropin response, i.e. poor versus good responder.
   c. Bottom line: AMH is the focus for the future of ovarian reserve assessment. Further
      clinical studies are needed, but today, AMH appears to be representative of the hands of
      the biological clock that we have been hearing tick for years.
Figure 1
Schematic of the reproductive endocrinology in the female. Please see notations within the Figure for
detailed description of the relationships between the hormones.
Table 1
Comparison of the different physical tools to assess ovarian reserve.
 Ovarian volume      Confirms menopausal status reliable     Significant changes in ovarian volume not
                     predictor of declining OR status        discernable during end of reproductive years
                     independent of advancing age            Highly variable in younger infertile patients
    Physical              Advantages                                   Disadvantages
    measurement tool
    Antral follicle count Consistent correlation to the age-related    Performance for predicting failure to achieve
                          decline of reproductive capacity             pregnancy is poor
                          There is no significant difference           Highly variable in younger infertile patients
                          between right-sided and left-sided antral    AMH may be a better/equivalent predictor of
                          follicle counts within the same individual   age-related decline of reproductive capacity
                          The AFC is a useful prediction tool for
                          poor IVF response or hyper IVF response
    Ovarian stromal       Results are highly predictive of ovarian     Results do not always correlate with advancing
    blood flow velocity   responsiveness                               age
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