LH

Luteinizing hormone (LH) is a glycoprotein hormone produced and secreted by gonadotroph cells of the anterior pituitary. Together with follicle-stimulating hormone (FSH), LH forms the gonadotropin component of the hypothalamic-pituitary-gonadal (HPG) axis.

LH has important but different functions in females and males. In females, it acts primarily on ovarian theca cells and, later in follicular development, on mature granulosa cells. It promotes steroid hormone production, participates in final follicular maturation, triggers ovulation through the midcycle LH surge, and supports formation and endocrine activity of the corpus luteum.

In males, LH acts primarily on Leydig cells located in the interstitial tissue of the testes. LH stimulation promotes testosterone synthesis, making it a central regulator of testicular androgen production.

Hormone Classification

LH is a glycoprotein hormone composed of two noncovalently associated subunits, an alpha subunit and a beta subunit.

LH belongs to the same glycoprotein hormone family as FSH, thyroid-stimulating hormone (TSH), and human chorionic gonadotropin (hCG).

LH Structure

The alpha subunit of LH is shared with FSH, TSH, and hCG. The beta subunit provides most of the biological specificity that distinguishes LH from the other glycoprotein hormones.

Both subunits are required to form biologically active LH.

Site of LH Production

LH is synthesized and secreted by gonadotrophs in the anterior pituitary, particularly within the pars distalis of the adenohypophysis.

Gonadotrophs also produce FSH, allowing the same pituitary endocrine cell lineage to coordinate two complementary gonadal signals.

Gonadotroph Cells

Gonadotrophs are specialized endocrine cells that respond to hypothalamic gonadotropin-releasing hormone.

They synthesize LH and FSH and release these hormones into the systemic circulation, from which they reach the ovaries or testes.

Hypothalamic Regulation

The principal hypothalamic regulator of LH secretion is gonadotropin-releasing hormone (GnRH).

GnRH is released from hypothalamic neurons into the primary capillary plexus at the median eminence and transported through hypophyseal portal vessels to anterior pituitary gonadotrophs.

Gonadotropin-Releasing Hormone

GnRH is a peptide hormone that controls secretion of both LH and FSH.

Normal gonadotropin secretion depends on the pulsatile release of GnRH rather than continuous physiological exposure.

GnRH Pulsatility

GnRH is normally released in discrete pulses. Changes in the frequency and amplitude of these pulses influence gonadotroph activity and the relative secretion of LH and FSH.

Relatively faster GnRH pulse frequencies tend to favor LH secretion, although gonadotropin regulation also depends on gonadal feedback and physiological state.

Continuous GnRH Exposure

Prolonged continuous stimulation of pituitary GnRH receptors causes receptor desensitization and suppression of gonadotropin secretion.

This response is therapeutically important because continuous-acting GnRH agonists can eventually suppress LH and FSH despite initially stimulating their release.

LH Secretory Pathway

Step	Event
1	Hypothalamic neurons synthesize GnRH
2	GnRH is released at the median eminence
3	Hypophyseal portal vessels carry GnRH to the anterior pituitary
4	GnRH binds receptors on gonadotroph cells
5	Gonadotrophs synthesize and release LH and FSH
6	LH enters the systemic circulation
7	LH acts on ovarian or testicular target cells

LH Receptor

LH acts through the luteinizing hormone/choriogonadotropin receptor (LHCGR), a G protein-coupled receptor.

The receptor is expressed prominently on ovarian theca cells and luteal cells in females and Leydig cells in males. It also recognizes hCG because of the close structural relationship between LH and hCG.

LH Receptor Signaling

The LH receptor primarily couples to Gs proteins. Receptor activation stimulates adenylyl cyclase, increases intracellular cyclic AMP, and activates protein kinase A.

These pathways regulate steroidogenic enzymes and other cellular processes required for gonadal endocrine function.

LH Signaling Sequence

Step	Event
1	LH binds the LH receptor
2	Gs signaling is activated
3	Adenylyl cyclase activity increases
4	Intracellular cAMP increases
5	Protein kinase A is activated
6	Steroidogenic and other target-cell processes are stimulated

LH in Females

In females, LH participates in ovarian steroidogenesis, follicular maturation, ovulation, corpus luteum formation, and progesterone production.

Its actions change during the ovarian cycle as follicles mature and ovarian cells alter their expression of gonadotropin receptors.

Theca Interna Cells

Theca interna cells surround the granulosa cell compartment of developing ovarian follicles and are major LH-responsive cells during the follicular phase.

LH stimulates these cells to synthesize androgen precursors from cholesterol.

Ovarian Androgen Production

LH-dependent theca cells produce androgens, particularly androstenedione and testosterone, which can diffuse into the granulosa cell layer.

These androgen precursors provide substrate for estrogen synthesis.

Two-Cell, Two-Gonadotropin Model

Estrogen production by the developing ovarian follicle depends on coordinated actions of LH and FSH on different cell populations.

Cell Type	Major Gonadotropin	Principal Role
Theca interna cell	LH	Produces androgen precursors
Granulosa cell	FSH	Converts androgens to estrogens through aromatase

LH and FSH Cooperation

LH and FSH work together to support ovarian estrogen production. LH promotes androgen synthesis in theca cells, while FSH stimulates aromatase activity in granulosa cells.

Aromatase converts theca-derived androgen precursors into estrogens, particularly estradiol.

Estradiol Production

As the dominant follicle develops, granulosa cells produce increasing amounts of estradiol.

Estradiol has local ovarian actions and systemic effects on the reproductive tract, secondary sexual characteristics, bone, and hypothalamic-pituitary feedback.

Dominant Follicle

During the follicular phase, one follicle usually becomes dominant. Its granulosa cells become increasingly responsive to gonadotropins and produce progressively greater amounts of estradiol.

Late in follicular development, granulosa cells of the dominant follicle acquire substantial LH receptor expression, preparing the follicle to respond to the midcycle LH surge.

Positive Feedback Before Ovulation

For much of the menstrual cycle, ovarian steroids exert predominantly negative feedback on the hypothalamic-pituitary system. Near midcycle, however, sustained high estradiol concentrations produced by the mature dominant follicle produce positive feedback.

This changes hypothalamic and pituitary responsiveness and leads to a dramatic increase in LH secretion.

LH Surge

The LH surge is a rapid midcycle increase in circulating LH that occurs in response to sustained high estradiol concentrations from the dominant follicle.

The surge initiates the sequence of events leading to ovulation and luteinization of follicular cells.

Ovulation

Ovulation is the release of the secondary oocyte and surrounding cumulus cells from the mature ovarian follicle.

The LH surge triggers biochemical and structural changes in the follicular wall, including proteolytic activity, prostaglandin production, and changes in local vascular and cellular function that culminate in follicular rupture.

LH and Oocyte Maturation

The LH surge also promotes resumption of meiotic maturation in the oocyte.

The primary oocyte completes meiosis I, producing a secondary oocyte that enters meiosis II and normally arrests at metaphase II until fertilization.

Luteinization

After ovulation, LH promotes luteinization of the remaining follicular cells.

Granulosa and theca cells undergo structural and functional changes and contribute to formation of the corpus luteum.

Corpus Luteum

The corpus luteum is a temporary endocrine structure formed from the ruptured ovarian follicle after ovulation.

It secretes progesterone, estrogen, inhibin A, and other factors that support the luteal phase of the menstrual cycle.

LH and Progesterone

LH supports steroidogenesis within the corpus luteum and therefore contributes to progesterone production.

Progesterone transforms the estrogen-primed endometrium into a secretory endometrium capable of supporting early implantation.

Luteal Phase

The luteal phase begins after ovulation and is characterized by activity of the corpus luteum.

Progesterone and estrogen produced during this phase exert negative feedback on GnRH and gonadotropin secretion.

Corpus Luteum Regression

If pregnancy does not occur, the corpus luteum has a limited functional lifespan and eventually regresses.

Declining progesterone and estrogen concentrations contribute to breakdown of the functional endometrium and onset of menstruation.

LH During the Menstrual Cycle

Phase	LH Activity
Early follicular phase	Supports theca cell androgen production
Late follicular phase	Increasing estradiol prepares the hypothalamic-pituitary system for positive feedback
Midcycle	Large LH surge triggers ovulation and luteinization
Luteal phase	Supports corpus luteum steroidogenesis
Late luteal phase	LH support becomes insufficient to prevent corpus luteum regression in the absence of pregnancy

LH in Early Pregnancy

After implantation, the developing trophoblast produces human chorionic gonadotropin (hCG). hCG binds the same receptor as LH and maintains corpus luteum function during early pregnancy.

This preserves progesterone production until placental steroidogenesis becomes sufficient to maintain pregnancy.

LH and hCG

LH and hCG are structurally related glycoprotein hormones and act through the same LH/hCG receptor.

hCG has a longer circulating half-life and provides sustained luteotropic stimulation during early pregnancy.

LH Versus hCG

Feature	LH	hCG
Major source	Anterior pituitary	Placental trophoblast
Receptor	LH/hCG receptor	LH/hCG receptor
Major reproductive role	Ovulation and gonadal steroidogenesis	Maintenance of corpus luteum in early pregnancy
Circulating half-life	Shorter	Longer

LH in Males

In males, the principal target of LH is the Leydig cell within the interstitial tissue of the testis.

LH stimulates Leydig cells to synthesize testosterone from cholesterol.

Leydig Cells

Leydig cells are endocrine cells located in the connective tissue between seminiferous tubules.

They contain the steroidogenic machinery necessary for androgen synthesis and express LH receptors on their cell membranes.

Testosterone Production

LH receptor activation stimulates cholesterol transport and steroidogenic enzyme activity within Leydig cells.

The resulting testosterone enters the circulation and also diffuses locally into seminiferous tubules, where high intratesticular androgen concentrations are important for spermatogenesis.

Steroidogenesis

Gonadal steroid hormones are synthesized from cholesterol. LH promotes steroidogenesis by stimulating mechanisms that make cholesterol available to mitochondrial steroidogenic enzymes.

Subsequent enzymatic reactions ultimately produce testosterone and related steroid intermediates.

LH and Spermatogenesis

LH does not primarily act directly on developing germ cells. Instead, it supports spermatogenesis indirectly by stimulating Leydig cell testosterone production.

Testosterone then acts within the seminiferous tubules together with FSH-dependent Sertoli cell functions to maintain normal sperm production.

LH and FSH in the Testis

Hormone	Primary Target	Major Function
LH	Leydig cells	Stimulates testosterone production
FSH	Sertoli cells	Supports spermatogenesis and Sertoli cell function

Testosterone Feedback

Testosterone and its metabolites provide negative feedback to the hypothalamus and anterior pituitary.

This feedback reduces GnRH and LH secretion and helps maintain testosterone concentrations within an appropriate physiological range.

Male HPG Axis

Level	Hormone	Principal Effect
Hypothalamus	GnRH	Stimulates pituitary gonadotrophs
Anterior pituitary	LH	Stimulates Leydig cells
Testis	Testosterone	Supports reproductive function and provides feedback

Hypothalamic-Pituitary-Gonadal Axis

The HPG axis integrates hypothalamic GnRH secretion, pituitary LH and FSH release, and gonadal production of sex steroids and inhibins.

It regulates puberty, reproductive development, gametogenesis, menstrual cyclicity, ovulation, gonadal steroidogenesis, and fertility.

Negative Feedback Regulation

For most physiological states, gonadal steroid hormones provide negative feedback to the hypothalamus and anterior pituitary.

Testosterone, estradiol, and progesterone can alter GnRH and gonadotropin secretion according to sex, reproductive stage, and hormonal environment.

Positive Feedback Regulation

The major physiological example of positive feedback within the HPG axis occurs in females near ovulation.

Sustained high estradiol concentrations from the dominant follicle produce positive feedback that generates the midcycle LH surge.

LH and Puberty

At puberty, maturation of pulsatile GnRH secretion increases pituitary gonadotropin release.

LH contributes to ovarian steroidogenesis in females and stimulates increasing testicular testosterone production in males.

LH in Males During Puberty

Increasing LH stimulation of Leydig cells raises testosterone production during male puberty.

Testosterone contributes to maturation of the reproductive tract, development of secondary sexual characteristics, skeletal and muscular changes, and support of spermatogenesis.

LH in Females During Puberty

In females, maturation of the HPG axis allows coordinated cyclic secretion of LH and FSH.

These gonadotropins support ovarian follicular development, estrogen production, and eventually the establishment of ovulatory menstrual cycles.

LH and Menopause

During menopause, depletion of functional ovarian follicles reduces ovarian steroid and inhibin feedback.

Loss of negative feedback causes circulating LH and FSH concentrations to increase, although the rise in FSH is typically more pronounced.

LH and Primary Ovarian Insufficiency

When ovarian function becomes impaired, reduced sex steroid feedback can cause increased LH and FSH secretion.

Elevated gonadotropins in this setting indicate that the hypothalamic-pituitary system is attempting to stimulate poorly responsive ovarian tissue.

LH and Primary Testicular Failure

Primary testicular failure involving impaired Leydig cell function can reduce testosterone production and therefore diminish negative feedback.

As a result, LH concentrations may become elevated.

Hypogonadotropic Hypogonadism

Hypogonadotropic hypogonadism results from insufficient hypothalamic GnRH secretion or impaired pituitary gonadotropin secretion.

LH and FSH are low or inappropriately normal despite reduced gonadal function.

Hypergonadotropic Hypogonadism

Hypergonadotropic hypogonadism results from primary gonadal dysfunction.

Reduced gonadal hormone production and feedback cause increased secretion of LH and FSH.

Primary Versus Central Hypogonadism

Feature	Primary Hypogonadism	Central Hypogonadism
Primary defect	Ovary or testis	Hypothalamus or pituitary
Sex steroid production	Reduced	Reduced
LH and FSH	Usually elevated	Low or inappropriately normal
Feedback state	Reduced gonadal feedback	Insufficient central stimulation

Pituitary Disease and LH

Structural or functional disorders affecting the anterior pituitary can reduce gonadotropin secretion and cause secondary hypogonadism.

Large pituitary lesions may impair LH secretion by compressing normal gonadotroph tissue or disrupting communication between the hypothalamus and pituitary.

Hypothalamic Disease and LH

Disruption of pulsatile GnRH secretion can reduce LH even when the anterior pituitary is structurally intact.

Hypothalamic disorders can therefore produce reproductive dysfunction through insufficient stimulation of otherwise functional gonadotrophs.

LH Measurement

Serum LH measurement is used in evaluation of gonadal function, menstrual disorders, infertility, pubertal abnormalities, menopause, and suspected hypothalamic or pituitary dysfunction.

LH values are interpreted together with sex, age, reproductive stage, menstrual-cycle phase, sex steroid concentrations, FSH, and other relevant clinical findings.

LH in Female Endocrine Evaluation

In females, LH concentrations vary substantially across the menstrual cycle. Interpretation therefore depends strongly on cycle timing and the clinical question.

LH may be considered together with FSH, estradiol, progesterone, prolactin, and other reproductive endocrine measurements.

LH in Male Endocrine Evaluation

In males, LH is commonly interpreted together with testosterone and FSH.

Low testosterone accompanied by elevated LH suggests a different anatomical level of dysfunction from low testosterone accompanied by low or inappropriately normal LH.

LH and Infertility

Abnormal LH secretion can contribute to infertility in both sexes. In females, insufficient LH signaling can interfere with steroidogenesis and ovulation. In males, inadequate LH can reduce Leydig cell testosterone production and impair the hormonal environment required for spermatogenesis.

Excessive or dysregulated gonadotropin patterns can also accompany disorders of gonadal or hypothalamic-pituitary function.

LH and Polycystic Ovary Syndrome

Some individuals with polycystic ovary syndrome (PCOS) demonstrate increased LH secretion or an elevated LH-to-FSH relationship, although this finding is neither universal nor required for diagnosis.

Altered GnRH pulsatility and ovarian androgen production can contribute to reproductive endocrine abnormalities in PCOS.

LH and Ovulation Testing

The urinary LH surge can be detected using ovulation predictor tests. Detection of rising urinary LH can help identify the period preceding ovulation.

These tests detect a hormonal signal associated with impending ovulation rather than directly confirming that follicular rupture has occurred.

Exogenous LH Activity

LH activity can be provided therapeutically in selected reproductive treatments, either through preparations containing LH activity or through hCG acting at the same receptor.

Such approaches can be used to support steroidogenesis or induce final follicular maturation under controlled clinical conditions.

LH Versus FSH

Feature	LH	FSH
Source	Anterior pituitary gonadotrophs	Anterior pituitary gonadotrophs
Hypothalamic regulator	GnRH	GnRH
Female major target	Theca cells and luteal cells	Granulosa cells
Female major function	Androgen production, ovulation and luteal function	Follicular development and aromatase activity
Male major target	Leydig cells	Sertoli cells
Male major function	Testosterone production	Support of spermatogenesis

LH Versus GnRH

Feature	LH	GnRH
Source	Anterior pituitary	Hypothalamus
Hormone class	Glycoprotein	Peptide
Primary target	Ovarian and testicular endocrine cells	Anterior pituitary gonadotrophs
Route	Systemic circulation	Hypophyseal portal circulation
Major role	Controls gonadal steroidogenesis and reproductive events	Controls LH and FSH secretion

LH Versus Testosterone

Feature	LH	Testosterone
Major source	Anterior pituitary	Testicular Leydig cells
Hormone class	Glycoprotein	Steroid
Major male target	Leydig cells	Multiple reproductive and somatic tissues
Relationship	Stimulates testosterone synthesis	Provides negative feedback on the HPG axis

LH Versus ACTH

Feature	LH	ACTH
Pituitary cell	Gonadotroph	Corticotroph
Major hypothalamic regulator	GnRH	CRH
Peripheral target	Gonads	Adrenal cortex
Major endocrine axis	HPG axis	HPA axis

LH Across the Reproductive Axis

Level	Structure	Function
Hypothalamus	GnRH neurons	Provide pulsatile stimulation of gonadotrophs
Anterior pituitary	Gonadotrophs	Produce LH
Female gonad	Theca, mature granulosa and luteal cells	Steroidogenesis, ovulation and corpus luteum function
Male gonad	Leydig cells	Testosterone production
Feedback	Gonadal steroids	Regulate hypothalamic and pituitary activity

Key Features of LH

Feature	Key Point
Full name	Luteinizing hormone
Hormone class	Glycoprotein gonadotropin
Source	Anterior pituitary gonadotrophs
Major hypothalamic regulator	GnRH
Normal regulatory pattern	Pulsatile GnRH stimulation
Receptor	LH/hCG receptor
Major signaling pathway	Gs, cAMP, protein kinase A
Female major target	Theca and luteal cells
Female major functions	Steroidogenesis, ovulation and corpus luteum support
Male major target	Leydig cells
Male major function	Testosterone production
Major endocrine axis	Hypothalamic-pituitary-gonadal axis

Anatomical and Physiological Importance

LH forms a major endocrine connection between the anterior pituitary and the gonads. Hypothalamic GnRH neurons release GnRH into the median eminence, and the hypophyseal portal circulation transports it to gonadotroph cells within the adenohypophysis. Pulsatile GnRH stimulation then promotes LH synthesis and secretion.

In the ovary, LH initially acts prominently on theca interna cells. These cells synthesize androgen precursors that pass into neighboring granulosa cells, where FSH-stimulated aromatase converts them to estrogens. This anatomical and functional cooperation between theca and granulosa cells forms the basis of the two-cell, two-gonadotropin model of ovarian estrogen production.

As the dominant follicle matures, sustained high estradiol concentrations temporarily convert the usual negative feedback relationship into positive feedback. The resulting LH surge initiates final oocyte maturation, follicular rupture, ovulation, and luteinization. The remaining follicular cells form the corpus luteum, whose progesterone and estrogen secretion is supported by LH during the luteal phase.

In the testis, LH acts on Leydig cells situated in the interstitial tissue between seminiferous tubules. These cells synthesize testosterone, which acts systemically and locally. Within the testis, high testosterone concentrations cooperate with FSH-responsive Sertoli cells to maintain the environment necessary for normal spermatogenesis.

LH secretion is regulated through gonadal feedback. Testosterone, estradiol, and progesterone generally exert negative feedback on the hypothalamic-pituitary system, while sustained high estradiol produces the specialized positive feedback response responsible for the preovulatory LH surge.

Because LH occupies the pituitary level of the HPG axis, its concentration can help localize reproductive endocrine dysfunction. Primary gonadal failure typically reduces sex steroid feedback and increases LH, whereas hypothalamic or pituitary dysfunction can produce low or inappropriately normal LH despite reduced gonadal function.

Through its secretion from anterior pituitary gonadotrophs, regulation by pulsatile GnRH, actions on ovarian theca and luteal cells, induction of ovulation, stimulation of testicular Leydig cells, and regulation by gonadal feedback, LH is a central hormonal regulator of reproductive anatomy and physiology.