LH & FSH
Pituitary gonadotropins. LH signals the testes to produce testosterone (or triggers ovulation); FSH drives sperm production (or follicle development).
These two separate primary from secondary hypogonadism — which completely changes the treatment path. They're also the first values to hit zero on exogenous testosterone, making them the fertility marker.
Check a LH & FSH result against this range →
What LH & FSH actually measures — the analyte, and the assay
These are two heterodimeric glycoprotein hormones, not steroids, and every practical fact about the test follows from that. Each is a pair of subunits: an alpha subunit that is identical in LH, FSH, TSH and hCG, and a beta subunit that gives each hormone its identity. An assay therefore has to detect the beta subunit or the intact dimer, because the half of the molecule with the most antibody surface is shared with three other hormones.
Method: a two-site sandwich immunometric assay. One antibody captures, a second labeled antibody reports, and signal rises with concentration — the opposite architecture to the competitive steroid assays elsewhere in this cohort, with the opposite failure modes. Sandwich assays are vulnerable to heterophile antibodies, which bridge capture and detection directly and produce a falsely HIGH gonadotropin, and to the hook effect at extreme concentrations Haddad 2019. Results are reported in IU/L or mIU/mL against WHO international standards, which are potency units rather than a molar count, so two platforms calibrated to different preparations do not produce interchangeable numbers Lorde 2023.
The measurement is honest. The sampling is the problem. Gonadotropin release is driven by pulses of hypothalamic GnRH, and the pituitary reads pulse frequency rather than a level; that is why pulsatile GnRH delivery treats anovulation and hypogonadotropic hypogonadism while continuous exposure does the reverse Filicori 2023. One venous sample is one point on a pulse train.
How big is that effect? It has a number. The European biological variation study drew weekly samples from 38 men across ten weeks and found within-subject variation of 8% for FSH and 22% for LH Itkonen 2024. Same men, same laboratory, same protocol: LH swings almost three times as much as FSH. The textbook explanation is the one to hold on to — FSH carries heavier glycosylation and clears more slowly, so its curve is a smoothed version of the same pulses. And when researchers want to know a man's actual LH output they do not take one tube: a 24-hour sampling study in 10 men and 10 matched controls reported total LH secretion of 212 (156–268) against 186 (130–242) U/L per 24 hours van der Spoel 2016.
LH & FSH: what changes the blood, and what only changes the reading
What changes the gonadotropins in your blood:
- Where in the pulse you were bled. The largest term for LH, quantified at a 22% within-subject coefficient of variation Itkonen 2024, and the reason a single low LH is a weaker finding than a single low FSH.
- Negative feedback from the gonad. Testosterone and estradiol suppress LH; inhibin B suppresses FSH selectively. This is why exogenous androgen drives both toward zero and why FSH rises first when spermatogenesis fails.
- Cycle day, in a woman. The midcycle LH surge is a severalfold excursion lasting a day or two, so a value drawn without a cycle day is not interpretable at all.
- Age and pubertal stage. Interpreting gonadotropins across the pubertal transition needs stage-specific expectations rather than adult intervals Howard 2021.
- Energy availability. In 135 women with well-characterized functional hypothalamic amenorrhea, LH was 2.0 mIU/mL or below in 39% and FSH in 13%, and 81.5% had an LH:FSH ratio of 1.0 or lower Boegl 2024. Underfuelling switches the pulse generator down, and it shows in LH before FSH.
- Opioids, hyperprolactinemia, glucocorticoids and pituitary disease, all of which suppress from above.
What changes only the reading:
- Heterophile antibodies, which in a sandwich format bridge the two antibodies and push the result up — the opposite direction to the steroid assays on the neighboring pages Haddad 2019.
- Biotin on streptavidin-capture platforms, which in a sandwich assay usually pushes the result DOWN, so a biotin-affected LH can imitate a suppressed axis Balzer 2023.
- Cross-reactivity with hCG in LH assays, through the shared alpha subunit and the close homology of the beta subunits — which matters directly for anyone using hCG on or after testosterone Ghazal 2022.
- Platform and standard. Between-assay and between-interval differences change who gets called abnormal Lorde 2023.
- Ordinary preanalytical variables — tube additive, hemolysis, delayed separation Caruso 2020.
Reference interval or decision threshold — which kind of number LH & FSH is
Reference intervals, and for these two hormones that is a weaker instrument than usual. The adult ranges are the central 95% of a measured population on one platform; nothing about a number inside them is tied to an outcome.
The index of individuality is what makes this concrete. In the biological variation study every one of the five hormones examined had an index between 0.14 and 0.66, with FSH at the bottom of that span Itkonen 2024. An index of 0.14 means a man's own FSH occupies a very narrow band inside a wide population band — so a doubling of his FSH can happen entirely inside the printed normal range and be missed. For FSH in particular, a population interval is close to the wrong tool, and a previous result from the same person is worth more than the range beside it.
Where real decision thresholds do exist, they are structural rather than numeric. The one that changes management is the PATTERN: high gonadotropins with a low sex steroid means the gonad has failed, low or inappropriately normal gonadotropins with a low sex steroid means the problem is above it. A laboratory-medicine review of male hypogonadism makes the same point about the whole workup — the axis is read as a set, and the analytes still need harmonization before fixed action limits can be trusted Livingston 2023.
One number that is quoted as a threshold and is not one: the LH:FSH ratio. The current diagnostic framework for polycystic ovary syndrome requires two of three features — hyperandrogenism, irregular cycles, and polycystic ovarian morphology or an elevated AMH Joham 2025. No gonadotropin ratio appears among them. Where the ratio does discriminate is in the opposite direction: 81.5% of women with functional hypothalamic amenorrhea sat at 1.0 or below and only 2.2% reached 2.1 or above Boegl 2024, which makes a low ratio informative about underfuelling rather than a high one being diagnostic of PCOS.
How you would know your LH & FSH was wrong — and when to redraw
The retest window is set by the pulse, not by the calendar. If the question is whether a single low LH is real, the answer is not to wait three months — it is to draw again on a separate morning, because the thing you are trying to average out has a period measured in hours. If the question is recovery of the axis after exogenous androgen, the window is months, and the first thing to move is usually FSH.
How large a change has to be. Working from the published within-subject variation, arithmetic on a 22% coefficient puts the difference two LH results must show at roughly 60% before chance becomes an unlikely explanation, against about 22% for FSH at its 8% coefficient Itkonen 2024. An LH that moves from 4.1 to 5.2 mIU/mL has not moved.
What would have to change alongside them:
- Total testosterone and free testosterone, drawn at 7–10am, because the gonadotropins are only interpretable against the steroid they are regulating Livingston 2023.
- Prolactin, since hyperprolactinemia is a treatable cause of a suppressed axis and is missed when only testosterone is measured.
- Estradiol, which supplies part of the feedback in men and most of it in women.
- SHBG, without which a total testosterone cannot be read as exposure.
- AMH in a woman where ovarian reserve rather than pituitary function is the question.
- A semen analysis where fertility is the question, because no gonadotropin value substitutes for counting the cells.
The falsifying pattern is specific: a suppressed LH with a normal total testosterone, a normal prolactin and no exogenous androgen is more likely to be a pulse trough or a biotin-affected sandwich assay than a pituitary problem — redraw before anything else Balzer 2023.
What LH & FSH cannot tell you
A single draw cannot establish that your axis is suppressed. That is the central limitation and the measured variation says so: 22% within-subject variation for LH in men sampled weekly under controlled conditions Itkonen 2024. Suppression is a pattern across repeated draws and a matched sex steroid, not one low number.
They cannot measure fertility. FSH indexes the Sertoli-cell signal and LH the Leydig-cell signal; neither counts a sperm. A normal FSH is entirely compatible with azoospermia when the block is obstructive.
They cannot localize a pituitary lesion, which is imaging, and they cannot separate hypothalamic from pituitary failure, which needs dynamic testing or a GnRH-based approach Filicori 2023.
They cannot be read against adult intervals in an adolescent, where stage rather than age determines what is normal Howard 2021.
The wrong inference readers draw is that a low LH on TRT means the testes are damaged. It means the feedback loop is doing exactly what feedback loops do; what it does not tell you is whether the gonad will restart, and that question is answered by time off, a repeat draw and a semen analysis, not by the LH that reported the suppression.
Sources read for these sections
- Itkonen O, et al. The European biological variation study (EuBIVAS): Biological variation data for testosterone, follicle stimulating hormone, prolactin, luteinizing hormone and dehydroepiandrosterone sulfate in men. Clinica Chimica Acta 2024 · PMID 38341016
- Filicori M. Pulsatile gonadotropin-releasing hormone: clinical applications of a physiologic paradigm. F&S Reports 2023 · PMID 37223766
- van der Spoel E, et al. Familial Longevity Is Not Associated with Major Differences in the Hypothalamic-Pituitary-Gonadal Axis in Healthy Middle-Aged Men. Frontiers in Endocrinology 2016 · PMID 27881971
- Howard SR. Interpretation of reproductive hormones before, during and after the pubertal transition-Identifying health and disordered puberty. Clinical Endocrinology (Oxford) 2021 · PMID 34368982
- Boegl M, et al. The LH:FSH Ratio in Functional Hypothalamic Amenorrhea: An Observational Study. Journal of Clinical Medicine 2024 · PMID 38592037
- Livingston M, et al. Adult Male Hypogonadism: A Laboratory Medicine Perspective on Its Diagnosis and Management. Diagnostics (Basel) 2023 · PMID 38132234
- Joham AE, et al. Approach to the Patient: Diagnostic Challenges in the Work Up for Polycystic Ovary Syndrome. Journal of Clinical Endocrinology and Metabolism 2025 · PMID 39836632
The plan of attack
In this order. Most people start at step four, which is why they change five things at once and learn nothing.
- Confirm the number is real
Pulsatile secretion. LH is released in pulses every 60–90 minutes, so a single draw catches a random point on a wave. Two samples an hour apart can differ substantially. For an important decision, pooled or repeated sampling is more reliable than one value. - Read it with its partner
Anyone on TRT who wants children later should have baseline LH/FSH and a semen analysis before starting. Recovery post-TRT is not guaranteed and takes months. Draw it alongside: Total Testosterone, Prolactin, Estradiol, Standard (ECLIA). - Work out which direction is yours
If it's high — In women, elevated FSH signals diminishing ovarian reserve/perimenopause. In men, it means the testes aren't responding to signal.
If it's low — Suppressed axis — most commonly exogenous testosterone, but also opioids, high prolactin, or severe under-eating (relative energy deficiency). - Fix it in this order
Nutrition. Restore energy availability — relative energy deficiency (RED-S) suppresses the entire HPG axis in both sexes and is dramatically under-recognized in lean, hard-training people.
Lifestyle. Reduce training volume if over-trained, restore body fat to a healthy range, sleep, and taper off opioids where applicable.
Supplements. No supplement meaningfully raises LH/FSH. Correct vitamin D and zinc as general support.
Hormones. Enclomiphene or clomiphene (SERMs) raise endogenous LH/FSH and testosterone while preserving fertility. hCG substitutes for LH directly. Both under medical supervision.
Compounds. hCG mimics LH; gonadorelin pulses stimulate pituitary LH/FSH release — used on TRT to maintain testicular volume and fertility. Kisspeptin is an emerging upstream option in research settings.
Work down the list, not across it. Adding a compound on top of an unfixed diet is why generic protocols fail. - Retest
Baseline before TRT; when investigating low T; when planning fertility. Change one thing at a time, or the retest can't tell you which thing worked.
How to fix it
📚 Endocrine Society CPG — Testosterone Therapy in Men with Hypogonadism (diagnostic algorithm).
This page can tell you what could have made your LH & FSH wrong. It cannot tell you whether it did.
Everything above is free and stays free — the assay, what changes the reading rather than the blood, the retest window and the sources. What no page can do is look at your draw: which laboratory ran it, at what hour, what you were taking that week, and what else was flagged beside it. Every one of those changes the answer, and none of them is on any page. Bringing a real result to people who know that list is what the members' area is for.
Bring your result — $10/mo →🩸 Test your LH & FSH
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One marker is a data point. These panels add the markers that make LH & FSH interpretable, name why each is on the list, and load the set into your cart at 10% off.
includes this + 10 more markers · built for men — Fatigue, low libido, poor recovery, mood or body composition changes — and you're considering TRT. Read this before you start.
includes this + 8 more markers · built for men — Coming off a cycle, running PCT, or checking whether your natural production actually recovered.
includes this + 9 more markers · built for women — Planning a pregnancy, actively trying, or wanting to understand your ovarian reserve before making decisions about timing.
includes this + 10 more markers · built for women — Anywhere from late 30s onward with cycle changes, night sweats, sleep disruption, mood shifts, joint aches, brain fog, or a libido that fell off a cliff.
What people use LH & FSH to decide
Nobody orders a test for its own sake. LH & FSH is on the test list for these pathways — each one links to what the pathway claims, and what its test list is read for before you spend anything on it.
Total testosterone alone tells you very little. Free T is the bioavailable fraction, SHBG explains why it might be low with a normal total, and LH/FSH tell you whether the problem is the testes or the pituitary — which decides the entire treatment.
LH and FSH decide the entire treatment. Low testosterone with HIGH LH means the testes have failed and no upstream stimulus will help; low testosterone with LOW LH is a pituitary signal problem, and that is the one this pathway fixes.
Recovery is tracked on LH and FSH, not on how you feel. They move first, and testosterone follows weeks later — which is why people abandon a working restart too early.
The most complete workup on this page, and it earns it. Raised AMH with an LH:FSH ratio above 2 and low SHBG is the classic picture; 17-OH-progesterone is there to rule out congenital adrenal hyperplasia, which mimics PCOS and is treated completely differently.
LH & FSH is also on the test list for these, where it narrows the picture rather than settling it:
What moves your LH & FSH
8 compounds and 2 supplements in the Vault have a documented effect on this marker, or are a reason to have measured it first:
Browse all 278 compounds & 371 supplements →
Would you feel it? Symptoms LH & FSH helps explain
People search for how they feel, not for a marker. These are the complaints where this one is worth checking, and whether it is first-line or a follow-up.
Why your LH & FSH might be wrong
Most abnormal results are interference, not disease. Check these before you change anything. Each says whether the number is wrong (repeat it), badly timed (redraw it), or real with a cause.
LH is released in pulses every 60–90 minutes, so a single draw catches a random point on a wave. Two samples an hour apart can differ substantially.
For an important decision, pooled or repeated sampling is more reliable than one value.
Varies severalfold across a normal cycle, so a result without a cycle day attached is close to uninterpretable.
Note the cycle day. Day 3 for the follicular baseline; seven days after ovulation for the luteal phase.
All suppress LH and FSH through negative feedback — often to undetectable. That is expected, not a pituitary problem.
A suppressed LH on TRT is the drug. It tells you nothing about your own axis until you stop.
What LH & FSH means in combination
One marker tells you a little; combinations tell you the story. These are the named patterns this one takes part in.
Three completely different diagnoses that present identically, distinguished by one pairing. Low-with-low means the axis was switched off centrally — almost always energy availability. High FSH means the ovary is failing and the pituitary is shouting. A raised LH:FSH ratio with androgens points at PCOS.
Exclude pregnancy first, always. Low-with-low goes to the RED-S protocol; high FSH needs proper assessment; PCOS has its own panel. Prolactin belongs in all three workups because it is treatable.
A high early-follicular estradiol suppresses FSH through negative feedback, so a diminishing ovarian reserve can hide behind a normal-looking FSH. This is the specific reason both are drawn together on day three rather than FSH alone.
Never interpret a day-3 FSH without the estradiol beside it. If estradiol is raised, the FSH is not interpretable and the draw should be repeated in a subsequent cycle.
What to test next
These put LH & FSH in context — each with its own full breakdown.
Frequently asked questions
LH 1.7–8.6 mIU/mL · FSH 1.5–12.4 mIU/mL. Ranges vary by laboratory and assay — always compare to the range printed on your own report.
Within range alongside healthy testosterone = intact axis. The pattern matters more than the absolute number.
In women, elevated FSH signals diminishing ovarian reserve/perimenopause. In men, it means the testes aren't responding to signal.
Suppressed axis — most commonly exogenous testosterone, but also opioids, high prolactin, or severe under-eating (relative energy deficiency).
You can order LH & FSH directly through Marek Diagnostics without a doctor's visit — drawn at any Quest Diagnostics location in the US. Code CAMERON applies 10% off automatically.
Where this goes next
This page is the free framework, and it splits by sex. The protocol itself — the dosing, the order to correct things in, the week-by-week schedule and what to retest — is a lesson inside Skool.