LH (Luteinizing Hormone) Blood Test

An LH blood test measures luteinizing hormone, a pituitary hormone that drives key reproductive events including ovulation and testosterone production. It is often interpreted together with FSH and the sex hormones to understand fertility, the menstrual cycle, and puberty.

What the test measures

Luteinizing hormone (LH) is made by the pituitary gland, a small structure at the base of the brain, under the direction of signals from the hypothalamus. In women, a sharp mid-cycle surge of LH triggers ovulation, the release of an egg, and supports the structure left behind, the corpus luteum, which then produces progesterone. In men, LH stimulates specialized cells in the testes to produce testosterone. LH works closely with follicle-stimulating hormone (FSH), and because the two share a regulatory system, they are usually evaluated together.

LH release is pulsatile, arriving in bursts through the day, and in menstruating people it changes substantially across the cycle. As a result, both the time of day and the cycle day can affect how a single result is read.

The HPG axis

LH sits within the hypothalamic-pituitary-gonadal (HPG) axis. The hypothalamus releases pulses of gonadotropin-releasing hormone, the pituitary responds by releasing LH and FSH, and the ovaries or testes respond by producing sex hormones. Those sex hormones then feed back to the brain and pituitary to fine-tune the next round of signals. Comparing LH with the sex hormones therefore helps locate where in this loop a problem may lie.

Why a clinician might order it

LH is commonly tested when investigating fertility, irregular or absent periods, signs of early or delayed puberty, or low testosterone in men. By comparing LH with sex hormone levels, clinicians can often tell whether a problem lies in the ovaries or testes versus the pituitary or hypothalamus. LH is also part of evaluating conditions such as polycystic ovary syndrome (PCOS) and the menopausal transition, and it may be measured to help time other parts of a fertility assessment.

What happens during the test

An LH test uses a routine blood sample drawn from a vein in the arm. The skin is cleaned, a band may be applied, and a small tube of blood is collected in a minute or two. Brief tenderness or a small bruise afterward is common. For people who menstruate, the draw may be scheduled on a particular cycle day, and in some fertility evaluations LH is measured more than once because of its pulsatile release.

How to prepare

Fasting is generally not required. For people who menstruate, the test may be timed to a specific cycle day so the result can be matched to the right reference range. Because LH is released in pulses, a single value can vary, and clinicians keep this in mind, sometimes repeating the measurement. Hormonal medications, including hormonal contraception and fertility medicines, can affect LH, so it is helpful to share a current list.

What can affect results

• Cycle phase: LH changes across the menstrual cycle and surges at mid-cycle, so timing strongly affects the value.
• Pulsatile release: bursts of LH mean two draws taken close together can differ.
• Medications: hormonal contraception, fertility treatments, and some other medicines can raise or lower LH.
• Life stage: levels are low before puberty, rise through puberty, and increase after menopause as ovarian output falls.

How results are generally interpreted

LH is read together with FSH and sex hormones such as estradiol or testosterone, because the combination indicates where in the HPG axis a problem may sit:

• High LH with low sex hormones often suggests the ovaries or testes are not responding well, with the pituitary signaling harder. This pattern is common after menopause.
• Low LH with low sex hormones may point toward a pituitary or hypothalamic cause, since the pituitary is not raising its signal as expected.
• The LH-to-FSH relationship can be informative in certain conditions, including some cases of PCOS, where the balance between the two is considered alongside other findings.
• A mid-cycle LH surge is a normal event that signals approaching ovulation rather than a problem.

Qualitative meaning of high and low values

In broad terms, a higher-than-expected LH outside the mid-cycle surge can reflect reduced feedback from the ovaries or testes, as the pituitary works harder to stimulate them. A lower-than-expected LH can reflect reduced signaling from the pituitary or hypothalamus. Neither finding is meaningful on its own; both are interpreted with FSH, the sex hormones, age, and the clinical picture.

Reading results over time and in context

Because LH arrives in pulses and changes with the cycle, a single value is best understood as one snapshot rather than a fixed number. In a menstruating person, matching the result to the cycle day is essential, and in some evaluations the test is repeated or paired with other measurements taken on the same day. Across the lifespan, what counts as expected shifts: levels are low in childhood, rise during puberty, follow a cyclical pattern during the reproductive years, and rise again after menopause. Interpreting any one result therefore depends heavily on age, sex, cycle timing, and the reason for testing, which is why clinicians read LH together with FSH and the sex hormones rather than on its own.

Illustrative reference ranges

The values below are illustrative only and vary by laboratory, assay, sex, age, and cycle phase. Always use the range on your own report.

What the results may lead to

An LH result usually guides the next questions rather than providing a final answer. When LH is high with low sex hormones, attention often turns to the ovaries or testes and the reason they may be underperforming, which can differ greatly between a postmenopausal woman, a younger woman, and a man. When LH is low with low sex hormones, the focus shifts toward the pituitary and hypothalamus, and further pituitary testing or imaging may follow. In a fertility assessment, LH may be combined with FSH, the sex hormones, and cycle tracking to build a fuller picture.

Because LH is one signal within a connected system, a single value is interpreted cautiously. The same number can mean different things depending on cycle day, age, sex, and the reason for testing, and a result that looks unusual in isolation may be entirely expected once the context is known. For this reason a clinician may repeat the test, time it differently, or pair it with other measurements before drawing conclusions, and may also consider non-hormonal explanations for the symptoms that prompted testing.

Related tests

LH is almost always interpreted with the FSH test and with the sex hormones it helps regulate. When reproductive symptoms are present, prolactin may be added, since elevated prolactin can disturb the cycle, and thyroid tests such as the free T4 or free T3 may be considered when thyroid involvement is suspected. See the blood tests index, the hormones library, or read about related conditions.