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INVITED REVIEW
Inhibin B in male reproduction: pathophysiology and clinical
relevance
S J Meachem, E Nieschlag and M Simoni
Institute for Reproductive Medicine of the University, MuÈnster, Germany
(Correspondence should be addressed to E Nieschlag, Institute for Reproductive Medicine of the University, Domagkstrasse 11, D-48129 MuÈnster,
Germany; Email: nieschl@uni-muenster.de)
(S J Meachem is now at Prince Henry's Institute of Medical Research, PO Box 5152, Clayton, Victoria 3168, Australia)
Abstract
The recent availability of speci®c inhibin assays has demonstrated that inhibin B is the relevant
circulating inhibin form in the human male. Inhibin B is a dimer of an a and a b
B
subunit. It is
produced exclusively by the testis, predominantly by the Sertoli cells in the prepubertal testis, while
the site of production in the adult is still controversial. Inhibin B controls FSH secretion via a negative
feedback mechanism. In the adult, inhibin B production depends both on FSH and on spermatogenic
status, but it is not known in which way germ cells contribute to inhibin B production. The regulation
of inhibin B production changes during life. There is an inhibin B peak in serum shortly after birth
only partly correlated with an increase in serum FSH, probably re¯ecting the proliferating activity of
the Sertoli cells during this phase of life. Afterwards, inhibin B levels decrease and remain low until
puberty, when they rise again, ®rst as a consequence of FSH stimulation and then as a result of the
combined regulation by FSH and the ongoing spermatogenesis. In the adult, serum inhibin B shows a
clear diurnal variation closely related to that of testosterone. The administration of FSH increases the
secretion of inhibin B in normal men, but is much more pronounced in males with secondary
hypogonadism. The treatment of infertile men with FSH, however, does not result in an unequivocal
inhibin B increase. There is a clear inverse relationship between serum inhibin B and FSH in the
adult. Serum inhibin B levels are strongly positively correlated with testicular volume and sperm
counts. In infertile patients, inhibin B decreases and FSH increases. In general, there is very good
correlation with the degree of spermatogenetic damage, with the arrest at the earlier stages having
the lowest inhibin B levels. However, for unknown reasons, there are cases of Sertoli-cell-only
syndrome with normal inhibin B levels. Inhibin B and FSH together are a more sensitive and speci®c
marker for spermatogenesis than either one alone. However, the inhibin B concentrations are not a
reliable predictor of the presence of sperm in biopsy samples for testicular sperm extraction.
Suppression of spermatogenesis with testosterone and gestagens leads to a partial reduction of inhibin
B in serum but it is never completely suppressed. In contrast, testicular irradiation in monkeys or
humans leads to a rapid and dramatic decrease of inhibin B, which becomes undetectable when germ
cells are completely absent. In summary, although inhibin B is a valuable index of spermatogenesis,
the measurement of serum inhibin B levels is still of limited clinical relevance for individual patients.
European Journal of Endocrinology 145 561±571
Introduction
The existence of a testicular, non-steroidal, endocrine
factor controlling pituitary function was postulated in
1932, around the time of gonadotrophin discovery and
isolation, when McCullagh (1) reported that aqueous
testicular extracts prevented the formation of `castra-
tion cells' in the rat pituitary. Moreover, McCullagh was
able to show that the selective destruction of the
seminiferous epithelium was suf®cient to cause pitui-
tary hyperfunction without inducing atrophy of the
secondary sex glands, i.e. hypoandrogenism (1). Thus,
in this early period, inhibin (latin inhibere, to suppress)
was de®ned in its essence: a non-steroidal testicular
substance, produced by the seminiferous epithelium,
speci®cally inhibiting follicle-stimulating hormone
(FSH) secretion. Decades of research have led to the
discovery and characterisation of a large family of
proteins, the inhibin family (2, 3), and to the partial
de®nition of the function of its members. However, it
was only the development of speci®c immunoassays for
biologically active, dimeric forms of inhibin that
provided new insights into our understanding of the
physiological relevance of inhibin in male reproductive
ISSN 0804-4643European Journal of Endocrinology (2001) 145 561±571
q 2001 Society of the European Journal of Endocrinology Online version via http://www.eje.org
function. Today, it is clear that inhibin B is the only
inhibin form present in male circulation. Serum inhibin
B levels re¯ect the functional state of the seminiferous
epithelium and are involved in the feedback of the
pituitary±gonadal axis, as shown by many clinical
studies investigating inhibin B in a variety of physio-
logical and pathological states. Although these studies
have provided insight into inhibin biology, many
aspects of inhibin B synthesis, secretion, function and
mechanism of action remain elusive and, sometimes,
contradictory. Perhaps not unexpectedly, inhibin B
serum levels correlate quite well with FSH concentra-
tions in the vast majority of cases and the measurement
of serum inhibin B, initially regarded as the long-
awaited tool which could render obsolete more invasive
diagnostic interventions, has not yet found unequivocal
application in clinical routine. Nevertheless, the possi-
bility of measuring serum inhibin B has enriched our
knowledge of male gonadal function. The primary aim
of this review is to discuss the recent clinical data that
have added to our understanding of inhibin patho-
physiology and the usefulness of inhibin B as a
diagnostic tool in the assessment of testicular function.
Inhibin B structure, synthesis, secretion
and action
Inhibin is a glycoprotein hormone of gonadal origin
which was ®rst identi®ed by its ability to negatively
regulate FSH (4). Inhibin consists of two covalently
linked subunits, a common a subunit and a b subunit,
the latter of which exists in two forms, A and B
(denoted b
A
and b
B
respectively). Although many
molecular forms are found in circulation, biological
activity resides only in the dimeric forms, inhibin A (a-
b
A
) and B (a-b
B
) (5).
Over the last three decades there have been
numerous studies on inhibin synthesis and secretion
in a number of in vivo and in vitro models; however,
such studies failed to demonstrate an unequivocal or
systematic association between serum inhibin levels
and spermatogenesis (6, 7). One reason for this was the
lack of speci®c and sensitive assays to measure the
variously sized inhibin forms. Levels of inhibin in blood
were measured using heterologous assays, e.g. the
Monash radioimmunoassay (6) which could not
distinguish between the bioactive, dimeric inhibin
form (a-b
A
and a-b
B
) and the inactive forms such as
free a subunit (pro-aN-aC) and its precursor (pro-aC)-
related immunoactive peptides (8, 9). However, the
production of highly speci®c antibodies and the advent
of a new two-site, enzyme-linked immunoabsorbent
assay (ELISA) for speci®c measurement of the bioactive
inhibin A and B (10) has overcome this problem and
has led to the realisation that inhibin B is the major
form produced in the foetal and adult human male,
where inhibin A is undetectable (8, 11±13). This is in
agreement with animal studies including the rhesus
monkey (14) and the rat (15).
The Sertoli cell is considered the predominant source
of inhibin B (16, 17). This notion is currently
challenged and confounded by a mounting number of
sometimes contradictory studies, based on immuno-
localisation of inhibin subunits and on the in vitro and/
or in vivo production of inhibin B in various experi-
mental settings. These studies indicate that germ cells
and possibly even Leydig cells would produce inhibin as
well (18, 19). The human male inhibin subunits seem
to be differentially localised and secreted depending on
age and cell type. For example, in the foetal testis, a-
and b
B
subunits, but not b
A
were immunolocalised in
the Sertoli and Leydig cells (20). In cultured testicular
cells from prepubertal boys, both highly puri®ed
luteinizing hormone (LH) and recombinant FSH
stimulated inhibin B secretion (21). In the adult
human testis, b
A
and b
B
subunits were found in Sertoli
and Leydig cells but not in germ cells (22). In a recent
intriguing study, positive staining for the inhibin a
subunit was evident in Sertoli cells, while b
B
subunit
was evident in germ cells from pachytene spermato-
cytes to early spermatids but not in Sertoli cells (19),
suggesting that the two subunits that constitute inhibin
B could be contributed by different cell types in adult
men. Electron microscopic evidence reports the transfer
of inhibin a subunit from Sertoli cells to spermatocytes
in the human testis (23), suggesting that inhibin
subunit dimerisation could occur in the spermatocytes.
Extracellular dimerisation may also take place (24). In
any case, it should be considered that the different
antibodies used in these studies are recognising
different epitopes which could be shared by intermedi-
ate products of the inhibin family and/or they could
identify b subunits destined to become part of activins.
The development of an activin B immunoassay, not
available at the moment, should help in resolving this
question.
If the a and the b
B
subunits localised in Leydig cells
are assembled into mature inhibin B, Leydig cells
should contribute to the levels of inhibin B in
circulation. However, administration of recombinant
LH to hypogonadal men or human chorionic gonado-
trophin to normal men is unable to raise serum inhibin
B levels, demonstrating that, in man, Leydig cells do
not contribute to the pool of circulating inhibin B (25,
26). In summary, although the contribution of germ
cells in the regulation of inhibin B secretion is no longer
subject to question (see below), Sertoli cells should still
be considered to be the main source of inhibin B in the
human male, until its assembly and secretion by other
cell types is unequivocally proved.
The route of secretion of inhibin B in the bloodstream
is still poorly understood. Inhibin B is secreted in
seminal plasma and it has been shown that Sertoli cells
secrete inhibin in a highly vectorial manner. In
cultured primate Sertoli cells .90% of inhibin is
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S J Meachem and others EUROPEAN JOURNAL OF ENDOCRINOLOGY (2001) 145
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secreted into the adluminal compartment in the
presence of FSH (17). Studies in the rat have shown
that the major route via which inhibin reaches the
bloodstream is through secretion into the seminiferous
tubular ¯uid, thus supporting the idea of intratubular
production of inhibin (27). The presence of bioactive
inhibin in seminal plasma has been long recognised
(28). Indeed, one of the ®rst studies supporting the
inhibin hypothesis showed that the subcutaneous
injection of seminal plasma from normal men into
castrated rats signi®cantly inhibited serum FSH, while
seminal plasma from azoospermic men did not (29).
Seminal plasma of normozoospermic men contains
high but extremely variable levels of inhibin B (from
undetectable up to .50 000 pg/ml) while inhibin is
undetectable in vasectomised patients (13, 30). The
reason for this large variability in seminal plasma
concentrations in the presence of rather constant
serum levels is unknown, but suggests that the
transport of inhibin B into the blood might involve
some active, yet unknown mechanism.
Serum inhibin B shows a clear diurnal rhythm
parallel to that of testosterone. Individual diurnal
pro®les of serum inhibin B in 13 subjects, in whom
frequent serum samples were withdrawn continuously
for 24 h, suggested that testosterone and oestradiol
might play a role in the diurnal rhythm of inhibin B,
which is independent of FSH (31). In fact, administra-
tion of a high FSH dose (3000 IU) to healthy volunteers
results in a signi®cant increase of serum inhibin B
which, however, maintains its typical diurnal rhythm
with the lowest concentrations in the evening/night
hours (32). Since circadian testosterone ¯uctuations
are independent of LH as well, both inhibin B and
testosterone rhythms could be regulated by the same,
gonadotrophin-independent, local mechanism.
The main function of dimeric inhibin (both A and B) is
the negative control of FSH secretion (33). In the mouse,
inhibin might have an intratesticular action, e.g. in the
control of Sertoli and Leydig cell neoplastic proliferation
(34), but there is no evidence of any clinically relevant,
intratesticular inhibin action so far. The mechanism by
which inhibin B exerts its biological effect is still the
subject of intensive research. Inhibin has been shown to
bind to the activin type II receptor (ActRII) and thereby
inhibit activin action. However, inhibin does not antag-
onise activin action in all the tissues in which this
hormone acts, suggesting the existence of a speci®c
binding factor required for inhibin action. In addition,
evidence has accumulated that, besides being an activin
antagonist, inhibin has speci®c binding sites in tissues
where it is thought to act, such as the pituitary (35).
Indeed, two putative, speci®c inhibin receptors have been
isolated and molecularly characterised last year: beta-
glycan and p120 (36,37). Betaglycan, also known as type
III transforming growth factor-b receptor, functions as
inhibin co-receptor in the presence of ActRII, resulting in
enhanced antagonism of activin action (36). p120,
isolated with a classical biochemical approach from
bovine pituitaries based on its capacity to bind inhibin
A, is localised to the gonadotropes of the pituitary gland
and Leydigand Sertoli cells (37). p120,alsocalled inhibin-
binding protein, is a speci®c inhibin B receptor and, via
association with the type IB activin receptor, appears to be
necessary for inhibin B-induced antagonistic effect on
activin-stimulated signal transduction (38).
Control of inhibin B secretion
The ®rst speci®c assay for the measurement of serum
inhibin B was available in 1996 (39). After the ®rst
demonstration that inhibin B is the physiologically
relevant inhibin form in man (8), this assay was used to
study the changes in serum inhibin B levels in a
number of pathophysiological conditions, such as
maturation and senescence, infertility, hypogonadism,
gonadotrophin administration, hormonal contracep-
tion and testicular damage resulting from radiation or
chemotherapy. All these studies provided very relevant,
although sometimes contradictory, information about
inhibin physiology, explaining in part the complex
interplay between FSH, inhibin B and spermatogenesis.
It should be considered that all the data published so far
have been obtained using only one assay, which still
has a rather poor interassay precision (Fig. 1) and
cannot be properly evaluated in terms of accuracy since
internationally recognised reference preparations are
not available. Considering the extreme heterogeneity of
immunoreactive species of inhibin in the circulation,
appropriate standardisation is crucial and the avail-
ability of alternative assays would be very valuable.
Childhood and pubertal maturation
Serum inhibin B levels as well as the relationship
between FSH and inhibin B vary throughout life in the
human male, with a major switch in inhibin regulation
Figure 1 Measurement of serum inhibin B by a commercially
available ELISA. Results from the measurement of control serum
with normal inhibin B values in 111 consecutive assays are shown.
The interassay coef®cient of variation was 18.9%.
Inhibin B and male reproduction
563EUROPEAN JOURNAL OF ENDOCRINOLOGY (2001) 145
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occurring around puberty. While Sertoli cell prolifera-
tion and FSH govern inhibin activity in childhood,
germ cells are a major determinant of inhibin B
production in adulthood.
Inhibin B levels are low but detectable in cord blood
and increase into the adult range as early as in the ®rst
week after birth in male newborns (40), continue to
rise up to levels higher that those observed in adults by
3±6 months of age (30, 41, 42) and then decline
progressively to reach their nadir towards 3±6 years
(30, 42). This early postnatal increase is presumably
due to the activation of the hypothalamic±pituitary
testicular axis (41) and mirrors the proliferating
activity of Sertoli cells. Intriguingly, the early postnatal
rise of inhibin B is better correlated with LH and
testosterone than with FSH (42, 43), raising the
possibility that Sertoli cell proliferation in neonatal
life depends more on LH/testosterone than on FSH.
Inhibin B levels remain relatively high for several
months while FSH concentrations are low, suggesting
that the negative feedback control of FSH secretion
might already be established between the ®rst and
second years of age and that inhibin B production,
probably initially activated by FSH and/or by LH,
continues without gonadotrophin stimulation under
the in¯uence of yet unknown factors (41). It has been
suggested that inhibin B might be a marker of the
presence of the testis in case of sexual ambiguity with
cryptorchidism but no studies have been carried out yet
to verify this hypothesis. Inhibin B can be elevated by
recombinant FSH administration in prepubertal hypo-
gonadal boys (44).
After the initial postnatal rise, serum inhibin B levels
are low until puberty but still higher than in adults
with impaired spermatogenesis. This suggests that the
regulation of inhibin B secretion in childhood is
different from that in the adult. A critical change in
the regulation of inhibin B secretion occurs at puberty
(45) or just prior to mid puberty (44). Prepubertal boys
lacking spermatogenesis and the relevant germ cell
types secrete inhibin B in readily measureable amounts
(41, 46), whereas adult men with impaired spermato-
genesis (Sertoli-cell-only; SCO) have low to undetect-
able levels of serum inhibin B in the presence of
elevated FSH concentrations (11). Obviously, prior to
puberty, inhibin B levels are independent of the
presence of actively proliferating germ cells, while
after puberty serum inhibin B levels are closely related
to the spermatogenic status (41). During puberty, the
main control of inhibin B secretion switches from FSH
to spermatogenesis. Basal inhibin B increases under
FSH stimulation in the ®rst pubertal stages when the
last wave of Sertoli cell proliferation occurs and a
positive correlation between inhibin B and FSH is
observed. At Tanner stages G3 and G4, however, FSH
and inhibin B levels correlate negatively, suggesting
that the negative feedback regulation loop is fully
established at this stage of development (46±48).
Altogether, the existing data suggest that the stimula-
tory activity of FSH on inhibin B is the predominant
form of control as long as Sertoli cells proliferate (49±
52), although it is not known whether LH can also
in¯uence inhibin B production in childhood and
puberty. At puberty, when Sertoli cell proliferation
ceases and spermatogenesis starts, the basal, adult
inhibin B level is set and can be considered to be an
index of Sertoli cell density. Once full spermatogenesis is
ongoing, changes in inhibin B levels re¯ect mainly the
status of germ cell proliferation and development and
depend only secondarily on FSH. Therefore, the main
regulation of inhibin B production seems to change at
puberty. For instance, puberty could induce a switch in
the control of inhibin subunit expression, and it has
been postulated that testosterone might be responsible
for this maturational switch (53) in early pubertal boys
(47).
Adulthood
In the adult, FSH stimulates the production of inhibin B
in the testis and inhibin B inhibits the secretion of FSH.
Clinical ®ndings show a strong inverse correlation
between inhibin B and FSH in healthy men and in men
with testicular disorders (8, 11, 54±56). However,
unlike the LH±testosterone axis, the production of
inhibin B is not only dependent on FSH. After puberty,
in the presence of FSH, the prime regulator of inhibin B
levels is the spermatogenic status and inhibin B
production is directly proportional to the `amount' of
spermatogenesis, as shown by the direct correlation
between serum inhibin B and sperm count (57). When
spermatogenesis is damaged, as in SCO syndrome or
after testicular irradiation, inhibin B falls and FSH
increases. Under these circumstances, Sertoli cell
function might be altered as well, at least because, in
the absence of germ cells, Sertoli cells cannot ful®l their
primary function. In any case, although FSH stimulates
inhibin B secretion in normal men (11), inhibin B
production is resistant to the endogenous, elevated FSH
levels in infertile men. Therefore, FSH stimulation of
inhibin B secretion in the adult is mediated by and
necessitates the presence of germ cells. It is possible
that, in the adult, FSH stimulates the a subunit and
thereby the secretion of inhibin B, provided that
enough b
B
subunit is available: the availability of b
B
subunits for assembly into the dimeric, bioactive form
would be the limiting factor under the control of the
germ cells. In fact, the transcription of the a and b
B
subunit genes is regulated by different mechanisms
(58) and FSH stimulates the a subunit mRNA
expression but not that of the b
B
subunit (59).
Moreover, FSH stimulates pro-a-C secretion (11, 26)
and pro-a-C is elevated in infertile men while inhibin B
is low (6, 10). Expression of the gene encoding inhibin
a is upregulated through a cAMP mechansim (58, 60)
whereas the inhibin b
B
gene does not have a classical
564
S J Meachem and others EUROPEAN JOURNAL OF ENDOCRINOLOGY (2001) 145
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cAMP response element (61) and is not markedly
in¯uenced by hypophysectomy or FSH stimulation (62)
in the rat. Thus a decrease in the inhibin B feedback
signal results in a sustained increase in FSH secretion
which, in turn, directly stimulates inhibin a. This
hypothesis explains the lack of correlation between FSH
and inhibin originally described using the Monash
assay (6) which crossreacted with inhibin precursors
containing the a subunit.
Which germ cell type is responsible for the regulation
of inhibin B? Studies in the rat report that inhibin
secretion from Sertoli cells is regulated by interaction
with pachytene spermatocytes and early spermatids
(63, 64). However, in the primate, there is evidence to
suggest that spermatogonia may also modulate inhibin
B. Arrest of spermatogonia proliferation in irradiated
monkeys was associated with a rapid and marked
decrease of inhibin B in the presence of later germ cell
types (65) and normal inhibin B levels have been found
in a patient presenting with Reifenstein's syndrome in
whom spermatogonia were the only germ cell type
present (55). As a general rule, inhibin B concentra-
tions are progressively lower in groups of men with
increasing spermatogenic damage with undetectable
inhibin B in azoospermic men with SCO syndrome (8,
55, 66). However, in many individual cases it is not
possible to show any clear-cut relationship between
spermatogenic cell type, inhibin B and FSH (55, 66). It
might be that other, still unrecognised factor(s) take
part in the complex interplay between FSH, inhibin B
and spermatogenesis, which makes inhibin B a rather
unpredictable clinical marker for spermatogenesis in
individual patients (see below).
Clinical signi®cance of inhibin B
measurement
The demonstration that inhibin B is the relevant
inhibin form in man prompted a number of clinical
investigations aimed at the analysis of this `new'
endocrine parameter in several pathophysiological
settings. To what extent is inhibin B a reliable and
clinically useful biomarker of spermatogenesis? Does it
have any prognostic value?
Response to FSH stimulation
Is inhibin B a pharmacodynamic parameter of FSH
action, i.e. a marker of FSH-dependent Sertoli cell
function? If this is the case, it should be possible to
identify prospectively a subgroup of infertile patients
who might bene®t from FSH treatment. It has been
shown that FSH (3000 IU) induces a signi®cant
increase of serum FSH within 24 h from injection in
normal men (11). Such an increase is partially dose
dependent, and 1000 IU FSH are suf®cient to stimulate
inhibin B secretion signi®cantly (32). However, in
normal men, 225 IU recombinant FSH, i.e. a dose
capable of increasing inhibin B concentrations in
normal women (67), did not result in any appreciable
rise in serum inhibin B values (26). No acute FSH
stimulation test has been performed yet in men with
idiopathic infertility and normal or slightly elevated
FSH levels and the chronic treatment of infertile
patients yielded con¯icting results. In a placebo-
controlled study, FSH (150 IU daily for 12 weeks)
administration could not signi®cantly increase inhibin
B production in infertile men and no improvement in
semen parameters or pregnancy rate was observed
(68). On the other hand, when 11 patients with
oligozoospermia, moderate hypospermatogenesis and
normal FSH and inhibin levels were treated with FSH
(75 IU every second day for 3 months), inhibin B
increased signi®cantly in serum but sperm concentra-
tion improved only in six men (69). Therefore, in
infertile men as a group, serum inhibin B does not seem
to be of much predictive value.
Hypogonadotropic hypogonadism
A few studies have been dedicated to the study of the
effects of gonadotrophin-releasing hormone (GnRH) or
gonadotrophin treatment on inhibin B secretion in
hypogonadotropic hypogonadism. Serum inhibin B
levels are in the prepubertal range in men with isolated
GnRH de®ciency and increase signi®cantly into the
normal adult range during pulsatile treatment (56,
70). Similar to that observed during physiological
puberty, the negative feedback relationship between
inhibin B and FSH is evident during treatment as
demonstrated by the establishment of a negative
correlation between these two parameters (56). Long-
term GnRH treatment does not further increase inhibin
B levels, again demonstrating that, beside FSH, local
testicular factors are involved in the regulation of
inhibin B secretion (70). A recent study demonstrated
that the gonadotrophin effect on inhibin B secretion is
entirely due to FSH and that Leydig cells do not
contribute to the pool of circulating hormone. Recom-
binant FSH administration (150 IU/day for 1 month)
to patients with acquired hypogonadotropic hypogo-
nadism was able to increase serum inhibin B levels
progressively, while recombinant LH had no effect (25).
Together with the data on FSH stimulation in normal
men (11, 32), these results show that FSH and not LH
stimulates inhibin B production. However, when
spermatogenesis is normal, large supraphysiological
doses of FSH are necessary in order to raise the inhibin
B values further (11, 26, 32).
Male infertility
To date the only predictors of the state of spermato-
genesis are semen analysis, the measurement of FSH in
blood and testicular histology. The diagnostic accuracy
of FSH is limited by the fact that spermatogenic arrest
Inhibin B and male reproduction 565EUROPEAN JOURNAL OF ENDOCRINOLOGY (2001) 145
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at late stages does not lead to changes in FSH secretion
and that FSH may be normal even in patients with focal
SCO or hypospermatogenesis (71). In fact, serum FSH is
not an absolute parameter for the selection of
azoospermic men as candidates for testicular sperm
extraction (TESE) in assisted reproduction (72, 73).
Testis biopsy is an invasive procedure associated with
potential complications (74). In addition, a biopsy may
not be representative for the whole testis (75). Multiple
biopsies which are performed for TESE often show a
large variation in the completeness of spermatogenesis.
This heterogeneity is even more conspicuous in
patients with impaired spermatogenesis, where sections
with complete spermatogenesis may be found along
with others with focal spermatogenesis (71). Given
these limitations, any demonstration that inhibin B can
discriminate between complete absence of germ cells in
the testis and less severe disturbances of sperm
production would be of considerable clinical value.
Indeed, it has been shown that inhibin B serum levels
represent a good marker of spermatogenesis which
correlates with FSH, testis volume, sperm counts and
the presence of SCO tubules (30, 41, 54±57, 69, 76±
78). In particular, when used in combination with FSH,
inhibin B is a more sensitive marker than FSH alone
(55, 77). However, inhibin B, alone or in combination
with FSH, cannot predict the presence of sperm in
individual testicular tissue samples (55). Moreover,
inhibin B cannot predict accurately the type of
spermatogenic damage. For instance, in cases of late
spermatogenic arrest, inhibin B concentrations may
remain normal, exactly like FSH (66). In addition, in a
considerable number of cases of non-obstructive
azoospermia with focal SCO, both inhibin B and FSH
are in the normal range (55, 66). Therefore, despite the
fact that the combination of inhibin B and FSH
represent a more sensitive predictor of the spermato-
genic state than either of them alone, inhibin B has not
dramatically improved the diagnostic and prognostic
value of FSH for individual patients.
This has important implications for TESE. We have
investigated the diagnostic sensitivity of inhibin B in
successful retrieval of sperm in 52 TESE patients.
Although the combination of serum FSH and inhibin B
measurements showed high diagnostic sensitivity
(75%) and speci®city (73%) in identifying patients in
whom sperm could be possibly retrieved, no sperm
could be retrieved in 25% of the cases with normal FSH
and normal inhibin B. On the other hand, sperm could
be extracted in 38% of the cases with inhibin B values
,20 pg/ml. Therefore, inhibin B alone or in combina-
tion with FSH cannot be used to decide if a patient
should undergo TESE (55). At odds with our observa-
tion, a recent study has suggested a cut-off of 40 pg/ml
as fully predictive of the presence of sperm upon TESE
(79). However, the low number of subjects analysed (17
vs 52 in our study) and the high variability of the
current inhibin B assay, especially at such low levels,
recommend caution. Many reports have suggested that
recovery of testicular spermatozoa may be possible in
.50% of cases of non-obstructive azoospermia regard-
less of clinical parameters such as testicular size or
serum FSH (73, 75, 79, 80) and that the success rate is
increased by collecting more tissue (81). Even if inhibin
B is useful, the clinical decision as to whether TESE
should be performed or not cannot be based on inhibin
B, either alone, or in combination with FSH and other
parameters.
Radiation and antineoplastic therapy
Elimination of the seminiferous epithelium through
radiation is followed by an increase in serum FSH
levels. An experimental study in cynomolgus monkeys
has recently shown that radiation is immediately
followed by a sharp drop in serum inhibin B values
which occurs well before the decrease in testis volume
and semen count and the increase in serum FSH (65).
Therefore, in this experimental model, inhibin B is an
early marker of testicular injury, most probably an
index of premeiotic germ cell proliferation. This
experimental observation in the monkey, however,
has not yet been followed by similar investigations in
the human. It is well known that even before
orchiectomy patients with testicular cancer have
impaired spermatogenesis (82) which deteriorates
even further after surgery (83). In this clinical setting,
inhibin B levels also correlate quite well with the
spermatogenic status and undetectable inhibin B levels
are associated with an absence of spermatogenic
activity (84). Similarly, treatment with radioiodine for
thyroid carcinoma (85) or chemotherapy (12, 86) are
followed by a decrease in spermatogenesis which is
re¯ected by decreased inhibin B and increased FSH
levels. Whether inhibin B is a more sensitive and
precocious parameter of the damage of the semini-
ferous epithelium than FSH has not been investigated.
However, on the basis of the results obtained in
monkeys after irradiation (65) or hemiorchiectomy
(87) it would seem that inhibin B could be useful as an
early marker of testicular toxicants.
Ageing
Few data are available on spermatogenesis in older
men, particulary in men over 60. Controlled studies
suggest that semen parameters do not necessarily
deteriorate as an effect of ageing in healthy men (88).
Recent studies have shown only a moderate decrease in
serum inhibin B in old men (30, 89) but, in the absence
of semen analysis, this decrease cannot be attributed to
the ageing process per se. Rather, similar to all other
clinical conditions examined until now, a decrease in
serum inhibin B could re¯ect a decrease in spermato-
genesis for whatever reason (90).
566
S J Meachem and others EUROPEAN JOURNAL OF ENDOCRINOLOGY (2001) 145
www.eje.org
Hormonal male contraception
Current regimens of hormonal male contraception are
based on administration of testosterone esters possibly
combined with a gestagen. This treatment leads to
rapid suppression of serum gonadotrophins and vari-
able suppression of spermatogenesis with azoospermia
being induced in about 60±90% of the Caucasian
population, mainly depending on the type of gestagen.
Some studies have investigated the concentrations of
inhibin B in serum (11, 91±94) and in seminal plasma
(94) in men treated with hormonal contraceptives.
However, the contradictory results of such studies are
puzzling. The administration of testosterone enanthate
(TE) for 65 weeks was followed by a rapid and
progressive decline in serum inhibin B concentrations
which continued until the end of the treatment (92). A
similar, signi®cant decline was already observed in
another study of 19±24 weeks of TE administration
(91). The treatment with TE plus levonorgestrel for 6
months was followed by a signi®cant decline of serum
inhibin B in one study (11), while, in our experience,
transdermal testosterone plus levonorgesterel for 22
weeks was not followed by a signi®cant decline in inhibin
B (93). Finally, treatment with testosterone pellets plus
different doses of desogestrel for 8 weeks did not result in
any signi®cant change of inhibin B in serum, while the
hormone was dose-dependently suppressed down to
undetectable levels in seminal plasma (94). Interestingly,
in the latter study, serum pro-a-C levels were signi®-
cantly decreased, again supporting the gonadotrophin
dependency of this parameter. In clinical trials of male
contraception based on testosterone undecanoate and
levonorgestrel or norethitestosterone enanthate (95,
96), we could not ®nd any signi®cant decrease in serum
inhibin B concentrations, despite a profound suppression
of gonadotrophins and spermatogenesis (Fig. 2). These
contradictory results are dif®cult to reconcile, but
differences in the duration of treatment, in the type of
gestagens and in their mode of administration might
play some role. Treatment with TE alone has been
reported to affect mainly type B spermatogonia which
stop proliferating as a result of gonadotrophin suppres-
sion (91). If spermatogonial proliferation is the main
determinant of inhibin b
B
production, a decline in
inhibin B has to be expected. The situation might be
different if a gestagen is added, which could have some
direct effect on inhibin b
B
transcription and/or might
lead to a spermatogenic arrest at later stages, not directly
in¯uencing inhibin B production. The effects of long-
term treatment (beyond 1 year) are not known. In any
case, the original hope of distinguishing between
responders and non-responders to hormonal contra-
ceptive regimens based on inhibin B determination
remains unful®lled and the dissociation between serum
inhibin B concentrations and sperm count in many
studies remains an enigma.
Conclusion
The development of a speci®c and sensitive assay for the
measurement of inhibin B has led to signi®cant
breakthroughs in our understanding of inhibin B
biology and control of FSH secretion (97). Yet its
clinical usefulness is rather disappointing and the
measurement of this hormone in clinical samples does
not signi®cantly reinforce the diagnostic arsenal of the
clinical andrologist. Although FSH and inhibin B
together are more sensitive than either one alone in
Figure 2 Inhibin B levels in the serum of male volunteers undergoing
trials of hormonal male contraception. Upper panel: volunteers
n 6 were treated with the GnRH antagonist Cetrorelix (dose
10 mg/day for 5 days, followed by 2 mg/day up to week 12) for 12
weeks and 19-nortestosterone (400 mg in week 1, followed by
200 mg every 3 weeks) for 28 weeks (95). Lower panel: volunteers
received either testosterone undecanaote (1000 mg every 6 weeks)
and placebo (B) or testosterone undecanaote plus norethisterone
enanthate (200 mg every 6 weeks) (A) (96). The arrows indicate the
duration of the treatment. Results are expressed as means^
S.D. No
signi®cant change in serum inhibin B levels could be detected over
the treatment and observation periods.
Inhibin B and male reproduction
567EUROPEAN JOURNAL OF ENDOCRINOLOGY (2001) 145
www.eje.org
predicting the histological status of the testis and/or the
presence of sperm in bioptic tissue, this parameter gives
no absolute certainty. Concerning diagnostic and
therapeutic recommendations for the individual
patient, inhibin B measurement does not really add
much to FSH values, semen analysis and clinical
evaluation of the testis. On the contrary, inhibin B
values are sometimes at odds with the other parameters
and can generate interpretative uncertainty. Moreover,
it is still unclear whether inhibin B can be used as a
functional parameter of Sertoli cells. Finally, inhibin B
is not predictive of FSH responsiveness in idiopathic
infertility and cannot discriminate between responders
and non-responders in trials for hormonal male
contraception. Thus, inhibin B is not an indispensable
diagnostic tool in clinical practice.
On the other hand, the measurement of inhibin B is
very useful in experimental studies and can still add
important information to our knowledge of testicular
function and regulation of the pituitary±gonadal axis
in several pathophysiological conditions. For instance,
inhibin B seems to be a very fast marker of testicular
damage and could become important for rapid
identi®cation of spermatogenic disorders in populations
exposed to testicular toxicants. Moreover, many enig-
matic aspects of inhibin B physiology remain to be
clari®ed and deserve further investigation. Finally,
methodological aspects should not be disregarded. We
should keep in mind that the totality of the data on
which our current concept is based has been generated
using only one and the same assay kit. Several decades
of experience with other protein hormone assays
suggest that standardisation and availability and
marketing of new, competing assay kits could hold
some surprises in store.
Acknowledgements
S J M was a recipient of a Wellcome Trust Fellowship. Our
own work reported here was in part supported by the
Deutsche Forschungsgemeinschaft Confocal Research
Group: `The Male Gamete: Production, Maturation,
Function' (Ni 130/15). The language editing of the
manuscript by S Nieschlag is gratefully acknowledged.
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Received 27 March 2001
Accepted 19 July 2001
Inhibin B and male reproduction
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