Here is the HPG axis explained in one pass: a three-level circuit. The hypothalamus releases GnRH in pulses, the pituitary answers with LH and FSH, and the gonads produce sex hormones. Every level reports back to the one above it, so acting on the bottom step has predictable consequences for the steps above.
This article describes how the circuit works. It does not describe protocols, does not propose interventions and does not replace assessment by an endocrinologist.
The three levels of the circuit
Level 1: the hypothalamus. A small group of neurons releases GnRH (gonadotropin-releasing hormone) into the hypophyseal portal system. Not in a continuous stream, but in pulses.
Level 2: the anterior pituitary. It answers those pulses by releasing two gonadotropins:
- LH (luteinizing hormone): acts on the steroid-producing cells, Leydig cells in the testis and theca cells in the ovary.
- FSH (follicle-stimulating hormone): acts on the supporting cells, Sertoli cells in the testis and granulosa cells in the ovary.
Level 3: the gonads. They produce sex hormones and, in males, sustain spermatogenesis. They also produce inhibin, a signal aimed specifically upstream.
GnRH pulsatility: why the rhythm is the signal
This is the concept that makes everything else legible. Most quick versions of the HPG axis explained stop at the three levels, but the rhythm matters as much as the wiring.
GnRH is not released continuously. It comes out in pulses separated by intervals, and the frequency of those pulses is information, not a detail of the delivery system.
The demonstration is concrete and well established: continuous administration of a GnRH agonist does not stimulate the axis, it suppresses it. Pituitary receptors desensitize to a constant signal and stop responding.
That fact carries two implications.
The system reads patterns, not concentrations. Different pulse frequencies favor the release of LH or FSH differentially.
A sustained signal switches the circuit off. It is the same principle that shows up in the growth hormone axis: pulsatility is not an ornament of the physiology, it is how the physiology works.
Negative feedback and what triggers it
The circuit regulates itself. Hormones produced at level 3 report back to levels 1 and 2:
| Signal | Source | Effect |
|---|---|---|
| Testosterone | Leydig cells | Brakes GnRH and gonadotropins |
| Estradiol | Aromatization and ovary | Brakes GnRH and gonadotropins |
| Inhibin B | Sertoli / granulosa cells | Preferentially brakes FSH |
This is where the most important practical consequence in the article comes from:
Supplying sex hormone from outside is, as far as the circuit is concerned, indistinguishable from producing it. The axis detects elevated levels, triggers negative feedback, and reduces GnRH along with LH and FSH. Endogenous production falls.
This is not an unexpected adverse effect: it is the circuit doing exactly what it is built to do.
Where each compound acts on this axis
| Compound | Level where it acts | What it does |
|---|---|---|
| Kisspeptin-10 | Above level 1 | Activates GnRH neurons through the KISS1R receptor |
| HCG | Level 3 | Mimics the LH signal on Leydig cells |
| Exogenous sex hormone | Level 3 (direct supply) | Raises levels and triggers negative feedback |
The difference between the first two rows is conceptual and substantial.
Acting upstream (kisspeptin) stimulates the circuit through its natural entry point, preserving pulsatility and feedback.
Acting downstream (HCG) substitutes for one of the circuit's own signals, keeping the gonadal response but without restoring the hypothalamic signal.
Each has its own article: kisspeptin-10 and HCG. The HRT compounds in the catalog are in their own section.
Differences between the male and the female axis
The architecture is the same; the operating mode is not. The HPG axis explained as a single generic circuit hides an asymmetry worth stating.
In men, the axis runs in a relatively steady regime, with testosterone exerting negative feedback continuously.
In women, the axis is cyclical. And it contains a striking quirk: at one specific point in the cycle, elevated estradiol stops exerting negative feedback and starts exerting positive feedback, producing the LH surge that triggers ovulation.
It is one of the few cases in physiology where the same signal changes sign depending on context. And it explains why extrapolating between the sexes on this axis is especially risky.
Why no single lab test describes the state of the axis
One number does not tell you what state the circuit is in, for three reasons.
The hormones are pulsatile. A blood draw captures an instant. LH in particular varies sharply over the course of the day.
There is a circadian rhythm. Testosterone follows a daily pattern with higher values in the morning. The time of the draw changes the result.
The level does not say where the problem is. Low testosterone can come from gonadal failure (with high gonadotropins, because the axis is trying to compensate) or from a hypothalamic-pituitary failure (with low gonadotropins). These are different situations, and they can only be told apart by looking at the whole picture.
That is why assessing the axis is a clinical task that requires interpreting several parameters in context, and not something a stray lab test settles.
What the evidence does not settle
Recovery after suppression is variable and imperfectly predictable. It depends on duration, magnitude, age and baseline status, and no model anticipates it reliably.
Reference ranges vary between labs and populations, and the relationship between a number and symptoms is looser than the number suggests.
Interventions on the axis are far better characterized in defined clinical settings than outside them.
Frequently asked questions
Why does continuous administration of a GnRH agonist suppress the axis?
Because pituitary receptors desensitize to a constant signal. The system is built to read pulses; a continuous signal stops being information, and the receptor stops responding.
Does supplying testosterone from outside shut down endogenous production?
The circuit detects elevated levels and triggers negative feedback, reducing GnRH, LH and FSH. This is the normal functioning of the system, not an anomalous reaction.
Is a testosterone test enough to assess the axis?
No. The hormones are pulsatile, they have a circadian rhythm, and an isolated value does not distinguish whether the problem originates in the gonad or above it. Several parameters have to be interpreted together.
Is the axis the same in men and women?
The architecture is; the operating mode is not. The female axis is cyclical and includes a change of sign in estradiol feedback that has no male equivalent.
References
- Belchetz PE, et al. Hypophysial responses to continuous and intermittent delivery of hypothalamic gonadotropin-releasing hormone. Science, 1978;202(4368):631–633. DOI: 10.1126/science.100883
- Plant TM. 60 years of neuroendocrinology: The hypothalamo-pituitary-gonadal axis. Journal of Endocrinology, 2015;226(2):T41–T54. DOI: 10.1530/JOE-15-0113
- Marques P, et al. Physiology of GnRH and Gonadotropin Secretion. Endotext, NCBI Bookshelf. NBK279070
- Hayes FJ, et al. Differential regulation of gonadotropin secretion by testosterone in the human male. Journal of Clinical Endocrinology & Metabolism, 2001;86(1):53–58. DOI: 10.1210/jcem.86.1.7101
Written by the Bionic Editorial Team. Last reviewed: August 2026.
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This content is strictly educational and does not constitute medical advice, diagnosis or a therapeutic recommendation. The compounds mentioned are research products (Research Use Only) and are not approved by INVIMA, FDA, EMA or ANSM for therapeutic use in humans. Any health-related decision should be made with a licensed medical professional.