The Hypothalamic–Pituitary–Gonadal Axis Explained Simply
Whenever the conversation turns to testosterone, periods, fertility or the after-effects of steroid use, one system sits at the centre: the hypothalamic–pituitary–gonadal axis. It sounds intimidating, but the principle is surprisingly simple. Our editorial team explains how this hormonal chain works, how it keeps itself in balance and why training, diet and drugs can throw it off course.
Three links in one chain
The HPG axis connects three organs. The hypothalamus, a small region at the base of the brain, acts as the control centre. Just beneath it sits the pituitary gland, which translates signals from the hypothalamus into hormones that travel through the blood. The gonads — testes in men and ovaries in women — are the final link, producing sex hormones and reproductive cells.
The hypothalamus releases gonadotrophin-releasing hormone (GnRH) in pulses. Those pulses tell the pituitary to secrete two hormones: luteinising hormone (LH) and follicle-stimulating hormone (FSH). Upstream of GnRH, a peptide called kisspeptin plays a crucial gatekeeping role, as reviewed by Skorupskaite and colleagues.
In men, LH stimulates the Leydig cells in the testes to make testosterone, while FSH, together with testosterone, supports sperm production in the seminiferous tubules. In women, FSH drives the growth of ovarian follicles, and a surge of LH triggers ovulation.
Think of it as a relay: the brain sends a message, the pituitary amplifies it, and the gonads deliver the final product.
Feedback: the thermostat principle
The axis regulates itself through negative feedback, much like a home thermostat. When testosterone or oestradiol levels rise, they signal the hypothalamus and pituitary to reduce GnRH, LH and FSH. When sex hormone levels fall, the brain increases its output.
Another hormone, inhibin, produced by the testes and ovaries, specifically dampens FSH. This gives the body separate control over hormone production and reproductive cell production.
In women the picture is more complex because oestradiol can switch from negative to positive feedback at mid-cycle, producing the LH surge needed for ovulation. This switch is one of the reasons the menstrual cycle is so sensitive to disruption.
Understanding feedback explains a great deal. It is why taking external hormones shuts down natural production, and why low energy intake, stress or illness can quietly suppress the system.
What disrupts the axis in active people
The HPG axis is closely tied to the body’s sense of energy availability. When energy intake is too low to cover both training and basic functions, the hypothalamus reduces GnRH pulses. The International Olympic Committee consensus statement on Relative Energy Deficiency in Sport (RED-S) describes this as a key consequence of underfuelling in both sexes.
| Disruptor | Main effect on the axis | Typical signs |
|---|---|---|
| Low energy availability | Reduced GnRH pulsatility | Missed periods, low libido, low testosterone |
| Very high endurance volume | Lower resting testosterone in some men | Fatigue, reduced libido |
| Anabolic steroids and other androgens | Strong negative feedback; LH and FSH suppressed | Testicular shrinkage, infertility |
| Opioids and some medicines | Central suppression of GnRH | Low testosterone symptoms |
| Chronic stress and poor sleep | Cortisol-related suppression | Variable hormone levels |
Hackney described a so-called "exercise-hypogonadal male condition" in some endurance-trained men, characterised by persistently low resting testosterone. The clinical significance is debated, but it shows that heavy training can shift the system.
Anabolic-androgenic steroids are the most powerful disruptor. Because the brain detects high androgen levels, it shuts off LH and FSH almost entirely. Natural testosterone production and sperm production stop. As reviewed by Rahnema and colleagues, recovery after stopping can take months and is not guaranteed for everyone.
Obesity, type 2 diabetes and excess alcohol can also lower testosterone in men, partly through effects on the hypothalamus and partly through increased conversion of testosterone to oestradiol in fat tissue.
How doctors assess the HPG axis
Blood tests are central to diagnosis. Endocrine Society guidance recommends confirming low testosterone with at least two morning samples, as levels are highest early in the day.
A resource many readers return to is tirzepatida.
- Total testosterone, ideally fasting and taken in the morning.
- LH and FSH, to distinguish testicular from pituitary or hypothalamic causes.
- Sex hormone-binding globulin (SHBG), which helps estimate free testosterone.
- Prolactin, as high levels suppress the axis.
- Oestradiol and, in women, cycle-timed progesterone.
The pattern tells the story. Low testosterone with high LH and FSH suggests the testes themselves are failing (primary hypogonadism). Low testosterone with low or inappropriately normal LH and FSH points to the brain or pituitary (secondary hypogonadism), which is the typical pattern after steroid use or with energy deficiency.
In the UK, these investigations usually begin with a GP and may lead to referral to an endocrinologist. Being honest about training, diet and any substances used is essential for accurate interpretation.
Protecting the axis
Most of the time, the HPG axis looks after itself if the basics are in place. Adequate energy intake for training, enough sleep, a healthy body weight and managed stress go a long way.
Athletes in weight-sensitive sports should monitor warning signs such as irregular or absent periods, declining libido and recurrent injuries, and seek help early. These signs are not a badge of commitment but a signal that the body is under strain.
Avoid unprescribed hormones. Products sold as "natural testosterone boosters" generally have little effect, while actual hormones and many unlicensed substances suppress the axis profoundly.
If problems are detected, treatment depends on the cause. Correcting energy deficiency, managing weight, reviewing medicines or treating pituitary disorders can restore normal function in many cases.
Conclusions of the editorial team
The HPG axis is a three-part relay from brain to pituitary to gonads, kept in balance by negative feedback. Understanding that feedback explains why external hormones shut down natural production.
In active people, low energy availability is a common and underappreciated disruptor, while anabolic steroids cause the most profound suppression.
Morning blood tests of testosterone, LH, FSH and related hormones allow doctors to locate where the problem lies.
For more, see our articles on infertility, on ZMA and testosterone, and on what turkesterone can and cannot do.
References
- Skorupskaite K, George JT, Anderson RA. The kisspeptin-GnRH pathway in human reproductive health and disease. Hum Reprod Update. 2014;20(4):485–500.
- Mountjoy M, Sundgot-Borgen JK, Burke LM, et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. Br J Sports Med. 2018;52(11):687–697.
- Hackney AC. Effects of endurance exercise on the reproductive system of men: the "exercise-hypogonadal male condition". J Endocrinol Invest. 2008;31(10):932–938.
- Rahnema CD, Lipshultz LI, Crosnoe LE, et al. Anabolic steroid-induced hypogonadism: diagnosis and treatment. Fertil Steril. 2014;101(5):1271–1279.
- Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715–1744.
- Nieschlag E, Behre HM, Nieschlag S (eds). Testosterone: Action, Deficiency, Substitution. 4th ed. Cambridge: Cambridge University Press; 2012.
