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

The hypothalamic pituitary gonadal axis is a three step hormone loop in which pulsatile GnRH from the hypothalamus triggers pituitary LH and FSH release, and LH then stimulates the testes to produce testosterone. That loop explains why high LH with low testosterone points to primary hypogonadism, while low or normal LH with low testosterone points to secondary hypogonadism. It also explains why Enclomiphene can restore natural production in the right men, while testosterone replacement shuts the loop down.

Key takeaways

• The HPG axis is the hypothalamus → pituitary → testes signaling loop, and normal testosterone production depends on pulsatile GnRH, pituitary LH and FSH release, and testicular response.
• High LH + low testosterone indicates primary hypogonadism, while low or normal LH + low testosterone indicates secondary hypogonadism, and that distinction changes treatment completely.
• Morning fasting blood work between 07:00-11:00 is essential because testosterone follows a circadian rhythm and standardized collection reduces misclassification.
• At Veedma, persistent symptoms plus total testosterone below 350 ng/dL or free testosterone below 100 pg/mL trigger clinical decision making, but a number alone is never the diagnosis.
• Secondary or functional hypogonadism with LH below 8 mIU/mL is often an Enclomiphene scenario, while primary hypogonadism requires testosterone replacement because damaged testes cannot respond to stimulation.
• EAU recommends pituitary MRI when secondary hypogonadism is present with elevated prolactin, headache or visual disturbance, or severe hypogonadism below 6 nmol/L with inadequate gonadotropins.

What the HPG axis does

The HPG axis is the body’s testosterone control loop, linking the hypothalamus, pituitary, and testes through pulsatile hormone signals.^{[1] [11]}

The hypothalamic pituitary gonadal axis coordinates hormone output from the brain and testes so that testosterone production and sperm production stay matched to the body’s needs. GnRH is gonadotropin releasing hormone, a brain signal released by the hypothalamus in pulses. The pituitary is a gland under the brain that converts those pulses into LH and FSH. LH is luteinizing hormone, the pituitary signal that drives testicular testosterone production. FSH is follicle stimulating hormone, the pituitary signal that supports sperm development. When clinicians refer to the brain to testes connection, this is the chain they mean.^{[1] [11]}

The chain in plain language

In plain language, the sequence is straightforward. The hypothalamus releases GnRH in pulses. The pituitary responds by releasing LH and FSH. LH reaches the Leydig cells in the testes and stimulates testosterone synthesis. Leydig cells are the testicular cells that make testosterone. FSH reaches the Sertoli cells and supports spermatogenesis. Sertoli cells are the support cells that help sperm mature. Testosterone then circulates back to the hypothalamus and pituitary to regulate the next round of signaling.^{[1] [11]}

Why this loop matters clinically

This loop is the reason male hypogonadism has different forms. If the brain is signaling appropriately but the testes cannot respond, the problem is primary. If the brain is not sending enough signal, the problem is secondary. The European Male Ageing Study showed that primary, secondary, and compensated patterns are biologically distinct states rather than one broad “low testosterone” category.^[2]

That distinction is not academic. It determines whether a man needs replacement, stimulation, or pituitary evaluation. For a deeper discussion of the distinction itself, see Primary vs secondary hypogonadism.

How testosterone is produced

Testosterone is produced mainly by Leydig cells in the testes after LH from the pituitary binds to its receptor and activates steroid synthesis.^{[1] [11]}

Many men ask how testosterone is produced, but the answer starts one step earlier in the brain. GnRH does not make testosterone directly. GnRH testosterone production is indirect, because GnRH first stimulates the pituitary, the pituitary releases LH, and LH then tells the Leydig cells to convert cholesterol into testosterone. According to the 2018 Endocrine Society guideline, this hypothalamic pituitary gonadal axis framework is the basis of all diagnostic and treatment decisions in male hypogonadism.^[1]

LH drives testicular testosterone production

LH is the main “make testosterone” signal. When LH reaches the testes, Leydig cells increase steroidogenesis and release testosterone into the circulation. Some testosterone enters the bloodstream, while a much higher concentration stays inside the testes, where it supports sperm production. This is why a low testosterone result can reflect either poor LH signaling or poor testicular response. The same low serum number can come from two different failures in the chain.^{[1] [2]}

FSH supports sperm production

FSH is the main pituitary signal that supports Sertoli cell function and spermatogenesis. FSH does not replace LH, and LH does not replace FSH. The LH FSH testosterone relationship is therefore not redundant. LH tells the testes to make testosterone. FSH helps the testes use the right environment for sperm development. When both signals stay active, fertility is preserved. When exogenous testosterone suppresses them, sperm production falls.^{[1] [10]}

How negative feedback controls the axis

The testosterone negative feedback loop works largely through estradiol signaling back to the hypothalamus and pituitary.^{[6] [11]}

Negative feedback is the braking system of the hypothalamic pituitary gonadal axis. When enough testosterone has been produced, part of it is converted to estradiol by the aromatase enzyme. Aromatase is the enzyme that converts testosterone into estradiol. Estradiol then signals the hypothalamus and pituitary to reduce GnRH, LH, and FSH output. A 2013 New England Journal of Medicine study helped clarify how much estradiol contributes to hormonal feedback and androgen related physiology in men.^[6]

Estradiol is part of the brake

In men, estradiol is not a “female hormone.” It is a normal male hormone made from testosterone, and it is part of the feedback system that tells the brain when enough androgen output has been achieved. This matters because the brain does not respond only to testosterone itself. It also responds to the estradiol made from testosterone. That is why testosterone negative feedback often operates through aromatization and estrogen receptor signaling in the hypothalamus.^{[6] [11]}

Why TRT and Enclomiphene do opposite things

Exogenous testosterone suppresses natural production because the brain senses adequate androgen exposure and reduces GnRH, LH, and FSH output.^{[1] [10]}

That suppression is the central reason testosterone replacement therapy lowers spermatogenesis and is contraindicated in men pursuing fertility. A 2019 World Journal of Men’s Health review summarized the clinical point plainly: testosterone acts as a contraceptive because it suppresses gonadotropins.^[10]

Enclomiphene exploits the same biology in the opposite direction. Enclomiphene is the purified trans isomer used to avoid estrogenic effects from the cis isomer, zuclomiphene. It blocks estrogen receptors at the hypothalamus, so the brain cannot “see” the estradiol feedback signal normally. GnRH rises. LH rises. The testes are stimulated to produce testosterone through the body’s own pathway. Because LH and FSH remain active, spermatogenesis is preserved or may improve rather than shut down. Clinical trials have shown that Enclomiphene can increase testosterone while preserving sperm counts, unlike topical testosterone.^[9]

Why LH and FSH are essential for diagnosis

LH and FSH are mandatory because low testosterone without gonadotropin data cannot tell you whether the failure is in the testes or in brain signaling.^{[1] [2]}

Primary hypogonadism means the testes are failing. Secondary hypogonadism means the hypothalamus or pituitary is not sending enough signal. The defining lab pattern is simple. High LH + low testosterone means the brain is signaling hard and the testes are not responding. Low or normal LH + low testosterone means the brain is not signaling adequately, so the testes may still be able to respond if stimulated. The European Male Ageing Study and modern endocrine guidance both treat this distinction as essential, not optional.^{[2] [12]}

The lab patterns that change treatment

This is why any clinician who prescribes testosterone without first checking LH and FSH is guessing at treatment. It is also why a low number alone is not the diagnosis. Male hypogonadism remains a clinical syndrome requiring persistent symptoms plus biochemical evidence. For the complete lab framework, see The complete low testosterone testing guide.^{[1] [12]}

Why free testosterone still matters

Total testosterone can miss biologically important deficiency when binding proteins are abnormal.^[5]

That is why Veedma prioritizes direct free testosterone measurement by equilibrium dialysis with LC MS/MS rather than relying on an SHBG based estimate. Our clinical decision thresholds, when symptoms persist, are total testosterone below 350 ng/dL and free testosterone below 100 pg/mL. We also require LH and FSH to classify the problem, because treatment depends on where the axis is failing. Veedma does not order SHBG as a separate test, because direct free testosterone measurement answers the clinically relevant question more accurately.

How estradiol, SHBG, and prolactin alter the signal

Estradiol and prolactin can suppress HPG signaling. SHBG does not regulate the axis directly, but it changes how much testosterone remains free and how total testosterone should be interpreted.^{[5] [8]}

Estradiol and obesity

Obesity can suppress the HPG axis by increasing aromatase activity in adipose tissue, which converts more testosterone into estradiol and strengthens negative feedback on GnRH and LH.^{[6] [7]}

This is the core mechanism behind obesity driven functional hypogonadism. Adipose tissue is not hormonally silent. It actively changes steroid metabolism. More adipose tissue means more aromatase activity. More aromatase means more estradiol. More estradiol means stronger suppression of hypothalamic and pituitary output. LH falls, testicular stimulation falls, and testosterone falls further. Clinical reviews of functional hypogonadism describe this as an intact but suppressed axis and a typical setting for stimulation therapy rather than immediate replacement.^{[7] [11]}

SHBG shifts total vs free testosterone

SHBG is a liver made protein that binds testosterone tightly and changes how much hormone remains biologically available.^[5]

SHBG, or sex hormone binding globulin, is produced by the liver and binds testosterone strongly. When SHBG rises, free testosterone can fall even if total testosterone looks acceptable. When SHBG falls, total testosterone may look lower while free testosterone is less affected. A 2007 J Clin Endocrinol Metabolism position statement emphasized that abnormal binding protein states can make testosterone interpretation misleading if clinicians rely on total testosterone alone.^[5]

Factors that tend to increase SHBG include aging, liver disease, hyperthyroidism, anticonvulsants, and estrogen exposure. Factors that tend to decrease SHBG include obesity, insulin resistance, hypothyroidism, growth hormone, and androgens. These shifts are one reason total and free testosterone can tell different stories.

Prolactin can silence GnRH

Elevated prolactin suppresses GnRH and can cause secondary hypogonadism that is often treatable.^[8]

Prolactin is a pituitary hormone, and when it is elevated it can shut down hypothalamic signaling enough to reduce LH, FSH, and testosterone. Causes include pituitary adenomas and medications such as antipsychotics and metoclopramide. The Endocrine Society’s hyperprolactinemia guideline notes that prolactin driven hypogonadism should be identified before reflexively prescribing testosterone, because dopamine agonist treatment can correct the cause and restore the axis.^[8]

Why test timing and pituitary evaluation matter

Morning fasting testing is essential because testosterone follows a circadian rhythm, and standardized collection improves interpretation even in older men.^{[3] [4]}

Why morning fasting testing matters

Studies of serum testosterone in younger and older men show that total, free, and bioavailable testosterone are higher in the morning, with age flattening the rhythm rather than eliminating it.^[3]

That is why Veedma uses morning blood draws between 07:00-11:00 in standardized fasting conditions. Poor timing and normal biologic variability can change the result enough to misclassify a man. Studies also show meaningful intraindividual variability in testosterone, which reinforces the need for repeat, standardized collection rather than casual afternoon testing.^[4]

When the pituitary needs imaging

The pituitary is the critical checkpoint when testosterone is low but LH and FSH are not appropriately elevated.^{[1] [8]}

EAU recommends pituitary MRI when secondary hypogonadism is present with elevated prolactin, headache or visual disturbance, or severe hypogonadism below 6 nmol/L with inadequate gonadotropins. Pituitary tumors, traumatic brain injury, and infiltrative diseases can all disrupt this checkpoint. The point is not to image every man with a low testosterone result. The point is to image the right men when the lab pattern and clinical picture suggest the pituitary is the problem.

How the axis guides treatment

Treatment should follow the part of the HPG axis that is failing, not the testosterone number alone.^{[1] [9] [10]}

Functional secondary hypogonadism

Functional secondary hypogonadism means the axis is structurally intact but suppressed, most often by obesity, metabolic disease, or reversible physiologic stressors.^{[7] [11]}

This is the most common real world pattern, and it is the reason Enclomiphene is such an important option. When LH is below 8 mIU/mL and the testes are still capable of responding, Enclomiphene is the ideal first line therapy. It works with the body’s own regulation, raises testosterone through endogenous production, preserves spermatogenesis and fertility, maintains testicular size and function, and can potentially be discontinued if the underlying drivers are corrected. Compared with TRT, it also carries a lower hematocrit burden because testosterone is raised physiologically rather than imposed from outside the axis.

For the reversibility question in more detail, see Functional vs organic hypogonadism.

When the testes or pituitary cannot respond

If the testes are damaged, stimulation therapy cannot make them work. In primary hypogonadism, high LH already proves that the brain is signaling hard, so Enclomiphene will not solve the problem. TRT is the appropriate treatment. If the pituitary itself is damaged, gonadotropin therapy with hCG and FSH can bypass the pituitary and stimulate the testes directly, especially when fertility matters. The conservative, guideline aligned sequence is simple. Use stimulation therapy first in secondary and functional hypogonadism. Escalate to TRT only if needed. Use TRT up front when the testes cannot respond.^{[1] [9] [10]}

This treatment logic is the basis of Veedma’s approach. We use a thorough diagnostic workup with more than 40 biomarkers twice per year, or review outside results including Function Health labs, to determine whether the axis is intact, suppressed, or failing. From there, treatment is individualized. Enclomiphene is first line when the pattern is secondary or functional. Testosterone Cypionate is reserved for men who truly need replacement. Ongoing monitoring then confirms whether the chosen protocol is restoring physiology safely and effectively.

Myth vs fact

Myth: LH and FSH are optional if testosterone is low

Fact: LH and FSH are required to classify primary versus secondary hypogonadism. Without them, you cannot know whether the testes are failing or whether the brain is under signaling, and you cannot choose correctly between TRT, Enclomiphene, or pituitary evaluation.^{[1] [2]}

Myth: Exogenous testosterone restarts natural production

Fact: Exogenous testosterone suppresses GnRH, LH, and FSH through negative feedback, which is why it lowers sperm production and should not be used in men seeking fertility.^{[9] [10]}

Myth: Obesity lowers testosterone only because of body weight

Fact: Obesity changes hormone signaling. More adipose tissue means more aromatase activity, more testosterone to estradiol conversion, stronger hypothalamic suppression, lower LH, and lower testosterone. That is a direct HPG axis mechanism, not just a body size association.^{[6] [7]}

Myth: A normal total testosterone always means the axis is fine

Fact: SHBG can raise or lower total testosterone independently of free testosterone, so total testosterone may look acceptable while biologically active free testosterone is low. That is why direct free testosterone measurement by equilibrium dialysis with LC MS/MS is so valuable.^[5]

Myth: Secondary hypogonadism always means lifelong TRT

Fact: In functional secondary hypogonadism, the axis is often intact but suppressed. In that setting, Enclomiphene can restore natural testosterone production while preserving fertility and testicular function, and some men may not need lifelong therapy if underlying drivers are corrected.^{[9] [11]}

Bottom line

The HPG axis is the hypothalamic pituitary gonadal axis, a closed loop in which GnRH pulses drive pituitary LH and FSH, LH stimulates Leydig cells to make testosterone, FSH supports sperm production, and testosterone plus estradiol feed back to slow the system. That loop is why high LH with low testosterone means testicular failure, while low or normal LH with low testosterone means a signaling problem that may respond to Enclomiphene instead of immediate TRT. For the full diagnostic and treatment roadmap, see the Low Testosterone hub.

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. Tajar A, Forti G, O’Neill TW, et al. Characteristics of secondary, primary, and compensated hypogonadism in aging men: evidence from the European Male Ageing Study. The Journal of clinical endocrinology and metabolism. 2010;95:1810-8. PMID: 20173018
3. Diver MJ, Imtiaz KE, Ahmad AM, Vora JP, Fraser WD. Diurnal rhythm of serum total, free, and bioavailable testosterone and of SHBG in middle-aged men compared with those in young men. Clinical endocrinology. 2003;58:710-717. PubMed
4. Brambilla DJ, O’Donnell AB, Matsumoto AM, McKinlay JB. Intraindividual variation in levels of serum testosterone and other reproductive and adrenal hormones in men. Clinical endocrinology. 2007;67:853-862. PubMed
5. Rosner W, Auchus RJ, Azziz R, et al. Position statement: Utility, limitations, and pitfalls in measuring testosterone: an Endocrine Society position statement. The Journal of clinical endocrinology and metabolism. 2007;92:405-13. PMID: 17090633
6. Finkelstein JS, Lee H, Burnett-Bowie SA, et al. Gonadal steroids and body composition, strength, and sexual function in men. The New England journal of medicine. 2013;369:1011-22. PMID: 24024838
7. Grossmann M. Male obesity-related secondary hypogonadism: pathophysiology, clinical implications, and management. Asian journal of andrology. 2019;21:261-266. PubMed
8. 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
9. Wiehle RD, Fontenot GK, Wike J, et al. Enclomiphene citrate stimulates testosterone production while preventing oligospermia: a randomized phase II clinical trial comparing topical testosterone. BJU international. 2014. PubMed
10. Patel AS, Leong JY, Ramos L, Ramasamy R. Testosterone is a contraceptive and should not be used in men who desire fertility. World journal of men’s health. 2019;37:45-54. PubMed
11. Huhtaniemi I. Late-onset hypogonadism: current concepts and controversies of pathogenesis, diagnosis, and treatment. Asian journal of andrology. 2014;16:192-202. PubMed
12. Wu FC, Tajar A, Beynon JM, et al. Identification of late-onset hypogonadism in middle-aged and elderly men. The New England journal of medicine. 2010;363:123-35. PMID: 20554979