Walking On Volume 6, Issue 9, September 2019 | Page 8

For the Health of It
Using Progesterone as a
Diagnostic Tool during Equine Pregnancy
Reprinted with permission from Equine Disease Quarterly, Volume 25, Number 3
Progestogens are a class of
steroid hormones largely responsi-
ble for sustaining the embryo and
maintaining uterine quiescence. In
horses, at least 10 known progesto-
gens are present in maternal cir-
culation during gestation. To date,
only a few of them are known to be
biologically active. Progesterone,
the most renowned of this class of
steroid hormones, is the only one
with clinical diagnostic application.
During early pregnancy, progester-
one is produced in the equine ovary
by the corpus luteum (CL), and
its concentrations remain elevated
and peak between 60 and 120 days
of gestation. From that point on,
progesterone slowly decreases until
it becomes nearly undetectable
around 180 to 200 days of gesta-
tion. During late gestation, other
progestogens produced by the fe-
to-placental unit are responsible for
maintaining the pregnancy. These
are first detectable by day 60 of ges-
tation and are completely capable
of maintaining pregnancy from
around 120 to 140 days of gestation
until term.
Circulating progesterone has
been used diagnostically to eval-
uate luteal function during early
8 • Walking On
pregnancy. When the circulating
progesterone (P4) concentration
is above 1 ng/mL, this is consid-
ered consistent with the presence
of luteal tissue, indicating that a
follicle has ovulated, luteinized and
is producing progesterone. When
the circulating progesterone con-
centration is above 4 ng/mL, this is
considered adequate for the main-
tenance of pregnancy. There are a
number of reasons for monitoring
and supplementing endogenous
progesterone with progestins (syn-
thetic progesterones) during preg-
nancy, such as uterine infections,
history of pregnancy loss, and luteal
insufficiency.
A few important issues regarding
laboratory techniques and pro-
gestogens require clarification. To
date, all clinical veterinary diagnos-
tic laboratories use immunoassays
to measure circulating progester-
one. The specificity of these tests is
limited by the antibodies used in
these assays. Due to the structural
similarities among different pro-
gestogens present in late gestation,
after day 120 of gestation, anti-
bodies are unable to differentiate
between those different molecules
and therefore can give false or in-
accurate results. In addition, dif-
ferent progesterone antibodies will
result in disparate amounts of cross
reactivity; therefore, each proges-
terone assay will measure different
amounts of progesterone, produc-
ing varying results between labora-
tories. It is important to emphasize
that the best clinical interpretation
for any progesterone result is the
one provided by the clinical labora-
tory that measured the progester-
one, as they have reference ranges
for their specific equine progester-
one assay. The specificity lacking in
immunoassays and the inter-labo-
ratory variations can be overcome
with the use of liquid chromatogra-
phy-mass spectrometry (LC-MS).
LC-MS has allowed researchers
to evaluate changes in different
progestogens during late gestation
and further elucidate links between
placental compromise during late
gestation and the changes associ-
ated with specific progestogens. It
would be advantageous for clinical
laboratories to switch to LC-MS to
provide diagnostic panels of greater
specificity and wider array of quan-
tifiable progestogens.
In summary, current tests for
progesterone in the mare are useful
to evaluate the presence of luteal
tissue (P4>1ng/mL) and to ensure
that levels of circulating progester-
one are adequate for maintenance
of early pregnancy (P4>4ng/mL)
until about 120 days of gestation.
From that point until term, current
clinical tests are somewhat unreli-
able due to the variety of progesto-
gens present in maternal circu-
lation. These limitations can be
overcome with the use of LC-MS.
CONTACT:
Alejandro Esteller-Vico, DVM, PhD
aestellervico@uky .edu • [phone]
Gluck Equine Research Center
University of Kentucky • Lexington KY