The signi
cance of insulin-like factor 3 as
a marker of intratesticular testosterone
Male factor infertility is a complex clinical problem caused by
multiple contributory factors. Clinicians currently rely on
semen analyses as a tool for diagnosis, an indicator of
spermatogenesis, a barometer of treatment success, and an
indirect predictor of normal intratesticular testosterone
(ITT). The inherent variabilities and
uctuations in semen
analyses, as well as in ITT, has driven researchers to seek
novel biomarkers that may act as more accurate surrogates
of testicular function during treatment for testosterone-
related infertility.
The randomized trial by Roth et al.
seeks to identify
a serum marker of ITT, which is typically obtained through
an invasive
ne-needle aspiration biopsy. Although the in-
vestigators conclude that insulin-like factor 3 (INSL3) can
be useful in monitoring the ef
cacy of human chorionic
gonadotropin (hCG) stimulation in the hypogonadotropic
infertile male
, the possibilities for further, more diverse
applications exist. Indeed, given that a normal ITT is
necessary for functional spermatogenesis
to follow ITT levels with a serum marker would allow cli-
nicians to track testicular function in a minimally invasive
manner throughout the course of male infertility treat-
ments. Furthermore, in patients with hypogonadism and
testosterone-stimulating supplementation, the values of
ITT and testicular function could be monitored in a mini-
mally invasive manner and adjusted to preserve fertility
when desired. These applications are currently limited by
the unknown normal range of ITT and the complicated
physiologic mechanisms that control spermatogenesis;
however, future work will no doubt begin to clarify many
of these questions.
As published in this issue of
Fertility and Sterility,
investigators present the results of a prospective, randomized
clinical trial on 37 healthy men (ages 18 to 50 years)
More speci
cally, this current report is a subanalysis of a pre-
viously reported clinical trial
. Luteinizing hormone (LH)
and follicle-stimulating hormone (FSH) were antagonized
centrally with acyline (a GnRH antagonist) followed by
randomization to supplementation with either testosterone
gel or various low concentrations of hCG
. Testicular
ne-needle aspiration for ITT and serum values of testosterone
were obtained before acyline administration and again 10 days
investigators then examined the effects of their experimental
protocol on several potential serum biomarkers of ITT
including INSL3, 17
-hydroxyprogesterone (17-OHP), anti-
ullerian hormone, and inhibin B, with the intent of determin-
ing correlation and statistical signi
A testicular protein produced by the Leydig cells,
INSL3 is the primary focus of this study and the only
serum marker that the investigators found to increase lin-
early with low-dose hCG therapy
increases during puberty, decreases with age, and is pri-
marily regulated by LH
(1, 2)
. In the clinical context,
they theorized that INSL3 re
ected ITT levels, thus
allowing clinicians to monitor the status of testicular
function during androgen suppression or by serving as
Unfortunately, it is dif
cult to conclude that INSL3 can
serve as a biomarker for spermatogenesis because,
although INSL3 is involved in cryptorchidism, the only
known role for INSL3 in adults is in bone metabolism.
Therefore, a Leydig cell origin for INSL3 is accepted and
a relationship to ITT proven
, but the mechanisms and
reasons for the interaction between INSL3 and the LH/
Indeed, in men with normal hormone serum levels, INSL3
is unaffected by hCG-evoked gonadotropic stimulation
is only in a chemically induced state of gonadotropin depriva-
tion that INSL3 signi
cantly decreases, and then recovers, in
response to hCG
: mimicking the changes seen with ITT
In this experimental perturbation, serum levels of INSL3 were
statistically signi
cantly correlated with serum testosterone
and ITT while the serum levels of inhibin B, antim
ullerian hor-
mone, and 17-OHP exhibited no statistically signi
cant cor-
With respect to the relationship between ITT and INSL3
being statistically signi
cant, it is known that ITT concentra-
tions are quite variable among fertile men and that they typ-
ically re
ect LH pulsatility
(2, 3)
. Indeed, ITT values in
infertile men are completely unknown, and the participants
in their study only exhibited a parallel response when
placed in an arti
cial state of hypogonadotropic gonadism.
This suggests that although INSL3 correlated with ITT in
certain situations, employing INSL3 as a spermatogenic
biomarker in the general population would be premature.
Indeed, neither the normal levels nor the absolute value of
ITT controlling spermatogenesis is known
. Further
spermatogenesis is that knockout mice models of the LH
receptor still exhibit spermatogenesis, and very low levels
of ITT (
20.2 ng/g) can support spermatogenesis in
humans with a mutated LH gene
. Moreover, 1% to 2%
of ITT is converted to estradiol by aromatase, which, along
with the interaction of estradiol and FSH
, yields a much
more complicated description of ITT physiology.
The article by Roth et al.
presents a subset analysis of
a well-conducted, randomized, placebo-controlled clinical
trial. It discusses a physiologic proof of concept: in patients
with an inhibited gonadotropic axis, INSL3 can serve as
a marker of ITT. Although the
ndings of this study seem
to speci
cally highlight patients with Kallmann
s syndrome,
it would be interesting to compare the ITT in patients with
normal spermatogenesis with those who have testicular fail-
ure. Nevertheless, it is tempting to speculate that the rela-
tionship between ITT and, by extension, spermatogenesis
can be quanti
ed by a serum biomarker. Whether or not
INSL3 is such a marker obviously requires further investiga-
tion, and the work by Roth et al.
paves the way toward
this discussion.
Jason R. Kovac, M.D., Ph.D.
Larry I. Lipshultz, M.D.
Scott Department of Urology, Baylor College of Medicine,
Houston, Texas
VOL. 99 NO. 1 / JANUARY 2013