How the HPG axis works: The brain-testes connection explained

The hypothalamic pituitary gonadal axis is a three step signaling chain: pulsatile GnRH from the hypothalamus triggers pituitary LH and FSH, and LH then drives Leydig cells in the testes to make testosterone. This “brain to testes” pathway also explains why low testosterone is not one disease, but a set of different failure points with different treatments. Once you understand testosterone negative feedback, the logic behind morning testing, LH and FSH measurement, and fertility preserving treatment becomes much clearer.

Key takeaways

• The HPG axis controls testosterone through a fixed sequence: hypothalamic GnRH pulses, pituitary LH and FSH release, then testicular testosterone and sperm production.^{[1] [8]}
• High LH plus low testosterone points to primary hypogonadism, while low or inappropriately normal LH plus low testosterone points to secondary hypogonadism.^{[1] [8]}
• Testosterone should be measured in the morning, ideally between 07:00 and 11:00, because levels peak early and still show a daily rhythm even in older men.^{[1] [6]}
• In men with persistent symptoms, total testosterone below 350 ng/dL or free testosterone below 100 pg/mL can support hypogonadism, depending on binding protein effects and assay quality.^[8]
• Exogenous testosterone suppresses LH, FSH, and spermatogenesis through negative feedback, while Enclomiphene raises testosterone by stimulating the body’s own GnRH and LH signaling.^{[3] [7]}
• EAU guidance supports pituitary MRI in men with secondary hypogonadism when prolactin is elevated, when headache or visual symptoms suggest a mass, or when severe hypogonadism is present with inadequate gonadotropins, especially below 6 nmol/L.^{[1] [8]}

What the HPG axis does

The HPG axis is the hormone signaling chain that controls how testosterone is produced in men.^{[1] [8]}

The hypothalamic pituitary gonadal axis links the brain and the testes in a stepwise loop. The hypothalamus is the brain region that initiates reproductive hormone signaling. The pituitary is the small gland beneath the brain that relays that signal into the bloodstream. The testes are the organs that make testosterone and sperm.

GnRH starts the signal

GnRH, short for gonadotropin releasing hormone, is released by the hypothalamus in pulses rather than as a constant stream.^[1] According to the Endocrine Society guideline, this pulsatile pattern is essential because pituitary gonadotroph cells respond to rhythmic stimulation by releasing LH and FSH. If the GnRH signal is weak, irregular, or suppressed, the rest of the axis falls downstream with it.

In practical terms, testosterone production begins with GnRH signaling in the brain. This is why male hypogonadism can start above the testes, even when the testes themselves are still capable of responding.

LH, FSH, and testicular function

LH and FSH have different jobs inside the testes.^{[1] [8]}

LH, or luteinizing hormone, stimulates Leydig cells. Leydig cells are the testosterone producing cells in the testes. This is the core step in how testosterone is produced. Cholesterol is converted through a sequence of steroidogenic enzymes into testosterone inside these cells.

FSH, or follicle stimulating hormone, primarily acts on Sertoli cells. Sertoli cells are the support cells that nurture developing sperm. FSH does not make most testosterone directly, but it is essential to normal spermatogenesis and helps maintain the environment sperm cells need to mature.

This division of labor explains why “LH FSH testosterone” should always be interpreted together. A man can have low testosterone because the testes are failing, because the pituitary is under signaling, or because both problems coexist.

How negative feedback controls testosterone

Testosterone negative feedback keeps the HPG axis in balance by reducing GnRH, LH, and FSH output once androgen signaling is sufficient.^{[1] [3]}

Negative feedback is the key to understanding natural hormone regulation. When testosterone rises, some of it is converted to estradiol by the aromatase enzyme. Estradiol then signals the hypothalamus and pituitary that enough sex steroid is present. The result is a reduction in GnRH pulses and lower LH and FSH release.^{[1] [8]}

This is not a side detail. It is the mechanism that explains why exogenous testosterone suppresses the body’s own production. When outside testosterone raises circulating androgen levels, the brain detects adequate sex steroid activity, largely through estradiol mediated feedback, and turns down the upstream signal. LH falls. FSH falls. Intratesticular testosterone falls. Sperm production declines.^{[1] [7]}

A 2020 review in Translational Andrology and Urology described the clinical consequence clearly: testosterone replacement increases serum testosterone, but it suppresses gonadotropins and can impair fertility.^[7]

Why Enclomiphene works differently

Enclomiphene raises testosterone by blocking estrogen feedback at the hypothalamus, not by replacing testosterone from the outside.^{[3] [7]}

Enclomiphene is the purified trans isomer, separated from the estrogenic cis isomer zuclomiphene. By selectively blocking estrogen receptors at the hypothalamus, Enclomiphene prevents the brain from “seeing” the estradiol brake signal. GnRH output rises. Pituitary LH and FSH rise. The testes are stimulated to produce testosterone through the body’s own pathway.

The downstream biology is the exact opposite of TRT. LH and FSH stay active rather than being suppressed. Testicular size and function are maintained rather than shut down. Spermatogenesis is preserved, and in some men may improve, because the axis remains switched on.^[7]

Why LH and FSH are essential

LH and FSH are mandatory diagnostic tests because they show whether low testosterone comes from testicular failure or failed brain signaling.^{[1] [8]}

This is the single most important classification step in male hypogonadism. A low testosterone value alone does not tell you where the defect is. According to the EAU guideline, gonadotropin measurement is required to distinguish primary from secondary hypogonadism and to direct further evaluation.^[8]

High LH plus low testosterone indicates primary hypogonadism. The plain language meaning is simple. The brain is signaling hard, but the testes cannot respond. Low or inappropriately normal LH plus low testosterone indicates secondary hypogonadism. In that pattern, the testes may still be capable of working, but the hypothalamus or pituitary is not sending enough signal.

That distinction determines treatment. In primary hypogonadism, increasing pituitary signaling will not restore testosterone because the testes are not responding adequately. Secondary and functional hypogonadism often can respond to stimulation therapy, especially when the axis is suppressed rather than structurally destroyed. For a deeper diagnostic framework, see Primary vs secondary hypogonadism.

Why free testosterone still matters

Free testosterone is the fraction not tightly bound and therefore most available to tissues.^[2]

Most laboratories still lean heavily on total testosterone, but that can miss men whose biologically active testosterone is low despite a “normal” total value. The Endocrine Society measurement statement emphasized that assay choice matters, and that free testosterone testing should use accurate methods rather than unreliable direct analog assays.^[2] Veedma prioritizes direct free testosterone measurement by equilibrium dialysis with LC MS/MS because it avoids hidden testosterone deficiency caused by altered binding proteins.

If you want a fuller explanation of why one lab number can be misleading, see Why your testosterone test came back “normal” and why that might be wrong.

How estradiol, SHBG, and prolactin shape the axis

Estradiol, SHBG, and prolactin each alter HPG axis behavior in ways that can change both diagnosis and treatment selection.^{[2] [5] [8]}

Estradiol and obesity driven suppression

Estradiol is produced in men by aromatization of testosterone, especially in adipose tissue.^[8]

This matters because estradiol provides part of the negative feedback that slows GnRH and LH release. In obesity, aromatase activity rises, more testosterone is converted to estradiol, and the hypothalamus experiences a stronger “stop” signal. LH output falls. Testosterone production falls. This is a central mechanism behind obesity driven functional hypogonadism and one reason functional suppression is so common in real world practice.^[8]

The clinical implication is straightforward. If the axis is intact but suppressed by excess estradiol signaling from adipose tissue, stimulation therapy can make sense before lifelong replacement. For a broader discussion of reversibility, see Functional vs organic hypogonadism.

SHBG and the gap between total and free testosterone

SHBG, or sex hormone binding globulin, is a liver produced protein that binds testosterone tightly and reduces the free fraction available to tissues.^[2]

Rosner and colleagues noted that changes in SHBG can substantially change the relationship between total and free testosterone.^[2] Factors that increase SHBG include aging, liver disease, hyperthyroidism, anticonvulsants, and estrogens. Factors that decrease SHBG include obesity, insulin resistance, hypothyroidism, growth hormone, and androgens. The practical effect is that two men with the same total testosterone can have very different biologic exposure at the tissue level.

This is why Veedma does not rely on SHBG as a separate workaround test. Equilibrium dialysis measures free testosterone directly rather than estimating it indirectly.

Prolactin as a central brake

Elevated prolactin suppresses GnRH and can cause secondary hypogonadism that is often treatable.^[5]

The Endocrine Society hyperprolactinemia guideline identifies pituitary adenomas and medication effects as major causes of prolactin excess.^[5] Antipsychotics and metoclopramide are classic examples. In men, high prolactin can reduce GnRH secretion, lower LH and FSH, and cause low testosterone. This matters because the treatment target may be the prolactin disorder itself, often with a dopamine agonist, rather than immediate hormone replacement.

Why timing and pituitary evaluation matter

Testosterone testing is most informative when it is done fasting in the morning and interpreted in the context of pituitary warning signs.^{[1] [6] [8]}

Testosterone secretion follows a circadian rhythm. Circadian rhythm means a repeating 24 hour biological cycle. Diver and colleagues showed that serum testosterone peaks in the early morning, and although the rhythm flattens with age, it does not disappear entirely in older men.^[6] That is why testing between 07:00 and 11:00 matters. Food intake can also lower measured testosterone, so fasting conditions improve consistency.^[1]

Brambilla and colleagues found substantial within person variation in testosterone and related hormones over time.^[4] In clinical practice, that means one random afternoon draw is a weak basis for diagnosis. Morning confirmation is better. Morning confirmation with the right assay is better still.

The pituitary is the critical checkpoint

The pituitary is the relay station that converts GnRH into LH and FSH, so damage there can silence the entire axis.^{[1] [8]}

According to the Endocrine Society guideline and the EAU guideline, pituitary imaging becomes important when secondary hypogonadism is accompanied by elevated prolactin, headache, visual disturbance, or severe testosterone deficiency with inadequate gonadotropins.^{[1] [8]} The EAU specifically highlights severe hypogonadism below 6 nmol/L in this context. Pituitary tumors, traumatic brain injury, and infiltrative diseases can all interfere with this checkpoint.

That is why a low testosterone result plus low LH should never be treated as a simple “just take testosterone” scenario. It may represent functional suppression. It may also represent a pituitary disorder that needs targeted investigation.

How the axis guides treatment

The correct treatment for low testosterone depends on whether the HPG axis is intact but suppressed, whether the testes are damaged, or whether pituitary signaling is organically impaired.^{[3] [8]}

Functional secondary hypogonadism is the most common real world pattern. In that scenario, the axis is present but under signaling, often because obesity, metabolic disease, medication effects, or elevated estradiol are pushing too hard on the negative feedback brake. This is the setting where Enclomiphene is first-line for secondary or functional hypogonadism when LH is below 8 mIU/mL and the axis is intact. It works with the existing axis, preserves fertility, maintains testicular function, and may be discontinued if the underlying suppressive factors improve.^{[3] [8]}

Primary hypogonadism is different. If LH is high and testosterone is low, the testes are already receiving the message and cannot respond adequately. Enclomiphene cannot rescue unresponsive testes. In that setting, testosterone replacement is the only effective hormonal strategy.^{[1] [8]}

Organic secondary hypogonadism creates a third pathway. If pituitary output is damaged, gonadotropin therapy with hCG and, when fertility is the goal, FSH can bypass the pituitary and stimulate the testes more directly.^[8] The conservative, guideline aligned sequence is to use stimulation therapy first in secondary and functional hypogonadism, then escalate only if needed.

Myth vs fact

Myth: LH and FSH are optional if testosterone is already low

Fact: LH and FSH are required to classify primary vs secondary hypogonadism, and that classification determines whether stimulation therapy is possible or futile.^{[1] [8]}

Myth: TRT and Enclomiphene work the same way

Fact: TRT raises testosterone from outside the body and suppresses gonadotropins through negative feedback, while Enclomiphene increases GnRH, LH, and FSH by blocking estrogen feedback at the hypothalamus.^{[3] [7]}

Myth: Obesity only affects testosterone by lowering self esteem

Fact: Excess adipose tissue increases aromatase activity, raises estradiol, strengthens negative feedback on the hypothalamus, lowers LH, and suppresses testicular testosterone production.^[8]

Myth: An afternoon testosterone test is good enough

Fact: Testosterone follows a morning dominant circadian rhythm and also varies within the same man over time, which is why guidelines recommend morning testing under standardized conditions.^{[1] [4] [6]}

Bottom line

The HPG axis works through a clear chain: GnRH from the hypothalamus stimulates pituitary LH and FSH, LH drives Leydig cell testosterone production, FSH supports Sertoli cell sperm production, and testosterone plus estradiol feed back to slow the system when enough hormone is present. That loop is why LH and FSH are indispensable in diagnosis, why obesity can suppress testosterone through estradiol signaling, and why Enclomiphene can restore natural production in many men with secondary or functional hypogonadism when LH is below 8 mIU/mL and the axis is intact while TRT cannot. For the full diagnostic and treatment roadmap, see the Low Testosterone hub.

Veedma offers a thorough diagnostic workup with an advanced lab panel measured by LC MS/MS, or a review of existing lab results including uploaded outside testing, followed by individualized treatment plans, Enclomiphene as the first option for appropriate secondary and functional cases when LH is below 8 mIU/mL and the axis is intact, the Enclomiphene plus Tadalafil combination tablet when erection or urinary symptoms are also present, and ongoing monitoring by licensed providers with protocol adjustments after the first month and then every 6 months.

References

1. Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. The Journal of clinical endocrinology and metabolism. 2018;103:1715-1744. PMID: 29562364
2. Bhasin S, Cunningham GR, Hayes FJ, et al. Testosterone therapy in men with androgen deficiency syndromes: an Endocrine Society clinical practice guideline. The Journal of clinical endocrinology and metabolism. 2010;95:2536-59. PMID: 20525905
3. Oduwole OO, Huhtaniemi IT, Misrahi M. The Roles of Luteinizing Hormone, Follicle-Stimulating Hormone and Testosterone in Spermatogenesis and Folliculogenesis Revisited. International journal of molecular sciences. 2021;22. PMID: 34884539
4. Kanakis GA, Tsametis CP, Goulis DG. Measuring testosterone in women and men. Maturitas. 2019;125:41-44. PMID: 31133215
5. Melmed S, Casanueva FF, Hoffman AR, et al. Diagnosis and treatment of hyperprolactinemia: an Endocrine Society clinical practice guideline. The Journal of clinical endocrinology and metabolism. 2011;96:273-88. PMID: 21296991
6. Bhasin S, Cunningham GR, Hayes FJ, et al. Testosterone therapy in men with androgen deficiency syndromes: an Endocrine Society clinical practice guideline. The Journal of clinical endocrinology and metabolism. 2010;95:2536-59. PMID: 20525905
7. Auriemma RS, Pirchio R, Pivonello C, et al. Approach to the Patient With Prolactinoma. The Journal of clinical endocrinology and metabolism. 2023;108:2400-2423. PMID: 36974474
8. Salonia A, Capogrosso P, Boeri L, et al. European Association of Urology Guidelines on Male Sexual and Reproductive Health: 2025 Update on Male Hypogonadism, Erectile Dysfunction, Premature Ejaculation, and Peyronie’s Disease. European urology. 2025;88:76-102. PMID: 40340108