FSH Explained: What Your Fertility Labs Really Mean (And What They Don’t)

If you’ve ever looked at your fertility labs and felt confused by your FSH level, this episode is for you.

Today, we’re breaking down what FSH actually means, why timing is everything, and how estradiol plays a critical role in whether your results can even be interpreted accurately.

Because here’s the truth:

An FSH value on its own doesn’t tell the full story.

In this episode, you’ll learn:

• Why FSH must always be interpreted alongside estradiol

• The best time in your cycle to check these labs (and why it matters)

• What “baseline” hormones actually look like

• When an FSH level becomes concerning

• How elevated estradiol can falsely lower your FSH

• Why one “normal” result doesn’t always mean everything is okay

• How ovarian aging shows up in your labs over time

• What high FSH may indicate about ovarian response and stimulation

We also walk through real examples so you can understand how to think about your own numbers in a more nuanced, accurate way.

This is not about one snapshot in time.

Fertility is dynamic, and your labs should be interpreted that way.

If you’ve been told your labs are “normal” but something still feels off, or if you’ve seen fluctuations and don’t know what they mean, this episode will give you a clearer framework.

As always, this is for education and empowerment only and not medical advice.

If you’re ready for personalized guidance, we’d love to support you inside a private fertility consult.

You are not behind. You are not broken.

You just need the right lens.

Also, be sure to check out our website: loveandsciencefertility.com

Follow us on social media:

IG: www.instagram.com/loveandsciencefertility

FB: www.facebook.com/profile.php?id=61553692167183

Please don’t let infertility have the final word. We are here to take the burden from you so that you can achieve your goal of building your family with confidence and compassion. I’m rooting for you always.

In Gratitude,

Dr. Erica Bove

Transcript:

Hello, my loves. Welcome back to the Love and Science podcast. Today, we're going to talk about FSH timing and meaning. Perhaps you've gotten your labs back before and maybe you're a cardiologist or maybe you're an engineer or something very far out on the spokes of the medical subspecialties. And you're like, how do I interpret whether these values are normal? Let me give you a framework for menstrual cycle physiology, as well as how to interpret your values. Okay. So we were taught in medical school that the menstrual cycle is a diagram and it's linear and there's a follicular phase and there's a luteal phase. And in the middle of that is an LH surge where the egg is ovulated. And then that sort of causes the transition from the follicular phase to the luteal phase. That is only partially true. If I could actually redesign the menstrual cycle diagram, it would be a cylinder, which is a little tricky because we have to do it in a three dimensional way or maybe make it a circle and some things would be upside down. I think about this
all the time because my mentor, Dr. Randolph, taught me that it's not like the follicular phase happens out of the blue. The follicular phase sets the stage for the luteal phase,
which then sets the stage for the next follicular phase. and so on and so forth. And that concept is going to be really important, not just for this episode, but as we explore menstrual cycle physiology, because truly everything we do in REI goes back to the menstrual cycle and an ideal menstrual cycle. Okay. So what is happening? So when we have a period,
what's happening is that we have just had the fall of the luteal phase hormones,
the estradiol and the progesterone from the corpus luteum. And when that happens,
it causes the breakdown of the uterine lining, which has shed. So that thickened lining is breaking
down. It starts to bleed off. And typically when this happens, our hormones are quote unquote at
baseline. How do we interpret this? I know that a person is at baseline if their estradiol is less
than 50. And in terms of units, that is picograms per ml.
In the US, in Canada, the units are slightly different. And the progesterone at this time is
usually low as well. It is typically less than one nanogram per ml,
which, you know, if it's less than 1.5, that is also considered low for most people.
It just depends on how quickly that corpus lydium is breaking down and the particular assay. of
the, you know, where the blood is being tested because there's some variability there as well. But
typically, you know, a progesterone over three suggests that somebody has ovulated. And once it
sort of drops below 1.5 to one, then we can say that person's at baseline.
Okay. So when we're thinking about the fact that the estradiol is suppressed,
then we look at the FSH because we know that estradiol and FSH are like a seesaw,
right? So when one goes up, the other goes down. When the estradiol is low, as it is naturally in
the beginning of the menstrual cycle, that's when the FSH should be unsuppressed,
if that makes sense. Otherwise, it can get really confusing. For instance, if we check an FSH in
the luteal phase, for instance. then oftentimes the elevated estradiol from the corpus luteum is
suppressing the follicle stimulating hormone. So we really can't interpret the values at that point
in the cycle. So typically we say that a cycle day two through four estradiol with FSH is the best
way to interpret these values. Now, you know, do we sometimes check on cycle day five?
Sure. But that increases the chances of a follicle being selected, the estrogen coming from that
follicle. and then suppressing the FSH. And like I said, if we're trying to see if FSH is too high,
we don't want it being falsely suppressed by the estradiol. So if the estradiol is less than 60 or
65, that's still considered acceptable to interpret the values. But once the estradiol starts to be
over 70 to 80, That's really when we start to see it suppressing the FSH. So drives me bonkers when
I will have labs that were ordered by another practitioner where we just have an FSH value.
I cannot interpret that value because unless I also have the corresponding estradiol, I don't know
if it's being falsely suppressed. Similarly, the same thing with an estradiol. If somebody just
checks an estradiol, it's kind of meaningless in a sense, because if I don't have the FSH at the
same time, that's really what I'm trying to get is like, how hard is the brain working to keep
cycles going? And we know that when the FSH is above 10,
and in terms of we're talking about international units per liter or milli-international units per
milliliter, that's the units for FSH. If the FSH is above 10 and definitely above 15,
but 10 is when I started to get concerned, that means that the brain, i.e. the pituitary, that's
where the FSH is coming from, the anterior pituitary, that that part of the body is working really
hard to keep the cycles going, which is telling us inherently that the ovaries are likely
struggling to do their job. So when we have an estradiol that's less than 50 and an FSH that's
maybe like seven or eight, everything is swimming along. But when we have, say an estradiol is
under 50 and the FSH is 13 or 17 or 25, that's really concerning because we know that there's
probably some FSH resistance in the body already going on because the body is used to seeing so
much FSH around. And it's probably going to be harder to induce ovulation if that's a problem in
the fertility clinic. It's also going to be harder to develop a large cohort of follicles,
especially when the FSH is already so high. So, you know, say I check somebody's day three
estradiol and for reference, cycle day one is the first day of flow. So where you need a pad or a
tampon or whatever you use to collect the menstrual blood, say I get somebody's day three estradiol
and their estradiol is 80 or 85 and their FSH is 13.
I need to rewind my brain back to say a couple of days ago before that estrogen started to rise,
right? Because we know the estrogen is probably coming from a developing follicle. So say we
checked it on like cycle day one, or maybe even the previous luteal phase before men's he started
when the estradiol was actually less than 50, that FSH might've been 18, 20,
22, something like that. And so it's this dynamic process. We can't just look at one snapshot in
time. Obviously, if the, if the estradiol is less than 50, it's a lot easier to interpret the FSH,
but if the, a lot of people get falsely reassured, if the day two, three or four, estradiol is
elevated, we know that there's a suppression happening. So even an FSH of eight or nine on that day
in the context of an estradiol of 80 or 90, that's going to also make me worried that somebody is
starting their follicular recruitment, perhaps even at the end of the luteal phase, which that's
one of the reasons that women get shorter cycles, because that maybe somebody ovulates on day eight
or day 10. And then if their luteal phase is conserved, that's going to be, you know, a 22 to 24
day cycle if we do the math. And that's that short. And that's one of the signs of ovarian aging,
especially if you listen to previous podcast episodes, we talk about this in detail. And so, you
know, fertility and ovarian reserve, it's not like a light switch. It's not like all of a sudden we
wake up at 35 or 40 or whatever age we throw out and say, okay, the lights are off.
This is not happening. it's typically a progression over time and there is some cycle to cycle
variability. So you could even check, you know, an FSH and an estradiol every single month and you
might get different values as well. And so, you know, there is a maxim in our field that is,
you know, your, your fertility or your, your ovarian resort values are only as good as the worst
day of FSH. And that's what I teach all my trainees because sure, if you're,
you know, FSH and your estradiol come back three months after your initial one is. 45 is your
estradiol and your FSH is, you know, 8.5, you know, we'll cheer. Okay. This is good. You know, so
it's swimming along, you know, there's no problems, but if a previous FSH was like 19 or 25,
that's really important data because that means that, you know, there is some compensation
happening. There's some ovarian age happening, um, aging happening, and it's probably going to show
up when we try to stimulate somebody's ovaries and the ovaries are already used to seeing so much
FSH. that whether we use clomiphene or letrozole, which worked by elevating the endogenous FSH,
or if we use exogenous FSH, either in our gonadotropin or IVF cycles, then our delta,
right? Like where the FSH is starting to where we're actually going is not gonna be as great. And
we may only recruit one or two follicles. And so estrogen priming for a treatment corollary,
that's actually really useful in this scenario because if we can drive the FSH down in the luteal
phase, then we can maybe get a coordinated follicular phase. And if the antral follicle count is
say five, sometimes we can actually get to four or five follicles, whereas the resistance might've
pushed that down before and we might only get one or two dominant follicles. And so, you know,
looking at it, you have to look at, you know, number one, where in my cycle were these labs
obtained? And if you have an app, it's pretty easy. You just calculate the first day of flow and
then, you know, extrapolate. Okay. Maybe it was day two, maybe it was day three. So it was really
important to tell your doctor as they're interpreting your labs, like what cycle day you had your
blood drawn on. And then also was my estradiol less than 50, you know,
in the US units. And if it was fantastic, cause the FSH is a lot easier to interpret. We want it to
be less than 10 under those circumstances. But if you had an elevated day two, three or four
estradiol, that is not great because that means that the FSH value that you have is suppressed.
And, you know, and that it was probably higher earlier in your cycle. And so if you do the math,
so you can never really extrapolate, but if you do the math, say your FSH is like nine with an S
trial of 80, you're probably talking about an FSH greater than 10, which by definition is an
elevated FSH. And obviously we need to correlate this with the antimullarian hormone, which
correlates to response to medications. Also the androfollicle count, because that suggests how many
follicles we might get to best case scenario in a well-timed and stimulated cycle. But I think
that we forget that this is a dynamic process. And that just because we get labs checked on day
two, three, or four of the cycle doesn't mean that the ovaries don't have different plans. So we
have to look at both the estradiol and the FSH at the same time. We have to know where we're at in
the cycle. And we have to understand, importantly, if the estradiol is over 50, that maybe we check
in a different cycle. Maybe that's not representative of the most accurate interpretation of the
FSH. And if the FSH is on the higher end, there's a very good chance that we're dealing with some
elevated FSH and some, you know, ovarian resistance going on, which can be really tough to treat
sometimes. So let me know what other questions you have about FSH interpretation. But I think,
you know, acknowledging that, you know, one snapshot is not enough. Really, if we get another
reassuring value later in time that that's really not. going to reassure us overall,
but we have to keep that highest FSH in mind. Um, now the caveat with that is that occasionally we
have a, we do, if you look at the menstrual cycle diagram, we do have a small bump in the FSH right
around the time of ovulation, but usually it's not going to be like if the FSH is like 40 or 60,
like it's usually not up that high. So, you know, if we have an isolated elevated FSH value and
that's all we have, we really do need to get curious about getting all the labs together at the
appropriate timing. But I'm never going to feel really good about seeing an isolated FSH value
that's super elevated because that probably means that something deeper is going on. If we can add
on labs in real time to an isolated value, that's really wonderful. But if we can't,
then sometimes we just got to do our best to recheck the labs at the appropriate timing, which is
annoying, but it's really important. Hopefully this has elucidated why we always need to check the
labs together. And why the timing is so critical because, you know, beyond that window of the
menstrual cycle, the estrogen is going to be elevated and it's going to be really hard to interpret
the other labs. So you know how much I love you until the next time. Bye.