LH Blood Test: What It Measures And Why It Matters

How Does Luteinizing Hormone Work in the Body?

LH is part of the hypothalamic-pituitary-gonadal (HPG) axis, a three-tier feedback system that regulates reproductive hormones.

In men, the hypothalamus sends GnRH to the pituitary, the pituitary releases LH, and LH tells Leydig cells in the testes to make testosterone. When testosterone rises high enough, the hypothalamus and pituitary detect the increase and reduce GnRH and LH output. That feedback loop keeps testosterone within a functional range.

In women, LH follows a monthly pattern. Levels stay relatively low in the first half of the menstrual cycle, then spike sharply around day 14. That mid-cycle LH surge triggers ovulation, the release of an egg from the ovary. After ovulation, LH supports the corpus luteum, a temporary structure that produces progesterone.

Because LH responds to signals from both directions (the hypothalamus above it and the gonads below it), an LH measurement tells a provider whether the brain-to-gonad communication line is working. A high LH means the pituitary is sending a strong signal but the gonads aren't responding. A low or flat LH means the pituitary itself may not be sending enough signal.

Why Would a Provider Order an LH Test?

The Endocrine Society guideline (Bhasin et al., 2018) recommends measuring LH and FSH after androgen deficiency is confirmed by two separate fasting morning testosterone draws. The purpose is to classify the cause as primary (testicular) or secondary (hypothalamic or pituitary) before any treatment decision is made.

The AUA guideline (Mulhall et al., 2018) recommends measuring serum LH in men with confirmed low testosterone. When LH comes back low or inappropriately normal alongside confirmed low testosterone, the AUA recommends measuring prolactin as the next step, because that pattern can point to a pituitary tumor or medication-related suppression.

Beyond the male hormone workup, providers order LH testing for:

• Women with irregular or absent menstrual periods, to evaluate ovarian function

• Couples experiencing infertility, in both partners

• Children showing signs of early or delayed puberty

• Adults with symptoms of pituitary dysfunction (fatigue, low sex drive, loss of muscle mass)

LH is almost always ordered alongside FSH, because the two hormones work in tandem. An LH and FSH panel provides more information than either marker alone.

How Is an LH Blood Test Done?

LH is measured through a standard blood draw, typically from a vein in the arm. The sample goes to a lab for immunoassay testing. No special preparation is required for men, though fasting morning blood draws are standard when LH is ordered alongside testosterone.

For women of reproductive age, the timing of the draw within the menstrual cycle matters. Providers may request the test on a specific cycle day, because LH fluctuates throughout the month.

Results usually arrive within a few days to 2 weeks depending on the lab. One thing that matters for interpreting results is that LH reference ranges vary between assay platforms. Even labs running the same equipment can report different lower and upper limits, and reference intervals for gonadotropins aren't standardized the way testosterone assays are through the CDC Hormone Standardization Program (Jayasena et al., 2023). A single LH value should be read against the reporting lab's own range, not against a number copied from a textbook or a different lab.

What Do LH Test Results Mean?

LH results only make sense when read alongside testosterone (in men), estrogen (in women), and FSH. The number alone doesn't tell the full story. Context, including the person's age, symptoms, and which other hormones were tested, determines what a given LH level means.

How Reference Ranges Work for LH

LH reference intervals vary by sex, age, menstrual cycle phase, and assay platform. In men, the normal range is relatively narrow and stable after puberty. In premenopausal women, it shifts throughout the monthly cycle, peaking sharply at mid-cycle before ovulation. After menopause, LH rises as ovarian hormone production declines.

Because different labs use different immunoassay systems, the same blood sample can produce slightly different numbers depending on where it's processed. The only reliable comparison is between a client's result and the reference range printed on that specific lab report. Numbers from other sources (a textbook, a website, or a prior lab using a different assay) may not apply.

High LH

In men, an elevated LH with low testosterone is consistent with primary hypogonadism. The pituitary is working hard, but the testes aren't responding. Causes include genetic conditions like Klinefelter syndrome, prior testicular injury, radiation exposure, or autoimmune damage.

In premenopausal women, chronically high LH may indicate primary ovarian insufficiency, polycystic ovary syndrome, or conditions like Turner syndrome. In women over 45, rising LH is a normal part of the transition toward menopause, as the ovaries produce less estrogen and the pituitary increases its signal.

Low or Inappropriately Normal LH

Low LH alongside low testosterone is consistent with secondary hypogonadism. The testes could produce more hormone, but they aren't getting the signal. The cause may be in the hypothalamus, the pituitary, or both.

"Inappropriately normal" is a term both the AUA and the Endocrine Society use. It means LH hasn't risen in response to a clearly low testosterone, which it should do in a working feedback loop. This pattern may warrant prolactin testing because it can indicate a prolactinoma, medication effects (opioids, glucocorticoids), severe obesity-related suppression, or other pituitary conditions (Mulhall et al., 2018).

How Does LH Help Classify Hypogonadism?

LH separates two conditions that look the same on a testosterone report but require different workups. It's a step that gets skipped more often than it should.

The Endocrine Society guideline (Bhasin et al., 2018) uses LH and FSH to sort hypogonadism into two categories, each with a different cause and a different workup.

Without LH, a provider has a low testosterone number but no map of where the breakdown is happening. That matters because the treatment path, the monitoring plan, and the prognosis differ between the two categories.

About 30% of single low testosterone values normalize on repeat testing, according to the Endocrine Society's own summary of the 2018 guideline. Starting a hormone protocol based on one afternoon draw, without LH and without a second morning confirmation, risks treating a number that was never abnormal in the first place.

What Happens to LH During Testosterone Replacement?

Exogenous testosterone suppresses LH. That's expected physiology, not a side effect.

When a man receives testosterone from an outside source (injection, gel, or other route), the hypothalamus detects the rising androgen level and reduces GnRH output. The pituitary, receiving less GnRH, lowers its LH and FSH secretion. Within weeks, LH typically drops to near-undetectable levels.

This suppression is why exogenous testosterone reduces sperm production. Without adequate LH and FSH, the testes lose their signal to maintain spermatogenesis. The AUA recommends discussing fertility before any testosterone prescription, because the effect on sperm is common and recovery after stopping is variable, not guaranteed (Mulhall et al., 2018). That conversation is part of shared decision-making between the client and their provider.

Once someone is on testosterone replacement, LH is not part of routine on-treatment monitoring in most guideline-based protocols. Suppression is the expected state. A later LH draw makes sense if a provider suspects nonadherence, if the person is transitioning off therapy, or if fertility recovery is the question.

Clients working through a hormone evaluation can order individual biomarker tests or bundled lab panels that include LH, FSH, total testosterone, free testosterone, and other markers used in the full diagnostic workup. Testing before starting any hormone protocol captures the baseline LH that providers need for classification.

For those preparing to test, following proper test preparation steps (morning draw, fasting, same lab each time) matters more than most people realize, since even small procedural differences can shift results enough to change a clinical decision.

Disclaimer: This blog post is intended for informational purposes only and should not be considered medical advice. Always consult a healthcare professional before making changes to your health routine.

FAQs

Does a normal LH level mean testosterone is fine? 

Not necessarily. LH is interpreted alongside testosterone, not in isolation. Low testosterone paired with a low or inappropriately normal LH is consistent with secondary hypogonadism in both the AUA and Endocrine Society frameworks. That pattern may call for prolactin testing and sometimes pituitary imaging, even though LH itself fell within the lab's reference range (Mulhall et al., 2018).

Should LH be tested before starting testosterone therapy? 

Yes. Both the AUA and Endocrine Society guidelines recommend measuring LH after confirmed low testosterone. The reason is classification: LH separates primary hypogonadism (testicular) from secondary hypogonadism (hypothalamic or pituitary), and each category has a different cause workup (Bhasin et al., 2018). Skipping this step means starting therapy without knowing why testosterone is low.

Why is LH near zero on testosterone replacement? 

Exogenous testosterone suppresses pituitary LH output through negative feedback on the hypothalamus. Near-zero LH is the expected response, not a sign that the protocol is wrong. Providers don't typically recheck LH at every follow-up once suppression is documented.

How many low testosterone draws are needed before LH testing? 

Two, drawn on separate fasting mornings. The Endocrine Society notes that roughly 30% of single low values normalize on a second draw. Once two values confirm deficiency, LH and FSH are measured to classify the cause.

What LH result leads to a prolactin test? 

Low testosterone combined with a low or inappropriately normal LH. The AUA guideline recommends prolactin measurement in this scenario because it can indicate a prolactinoma or other pituitary pathology (Mulhall et al., 2018).

Can LH testing be ordered without a doctor visit? 

Direct-access lab services like Marek Diagnostics allow clients to order an LH and FSH test or broader lab panels without a traditional office visit. A healthcare provider reviews and approves the order, and results are delivered by email. Interpreting LH in context (alongside testosterone, symptoms, and medical history) should still involve a qualified clinician.

References

• J Clin Endocrinol Metab. 2018;103(5):1715-1744

• J Urol. 2018;200(2):423-432

• Ann Clin Biochem. 2023