Key takeaways
- The hypothalamic-pituitary-gonadal axis runs GnRH from the hypothalamus to LH and FSH from the pituitary to the gonads, with the sex hormones produced at the end feeding back to suppress the start.
- GnRH is released in pulses every 60 to 120 minutes because the rhythm is itself the signal — continuous exposure desensitises the pituitary and shuts the axis down.
- GnRH is a ten-amino-acid peptide released by a small, scattered population of hypothalamic neurons.
- In men, LH-stimulated Leydig cells produce around 95% of circulating testosterone, with a small amount aromatised locally to estradiol.
- Testosterone and estradiol both suppress GnRH at the hypothalamus and gonadotropin release at the pituitary.
Almost every argument about hormone therapy — why one option preserves fertility and another destroys it, why testicles shrink on treatment, why recovery takes three months for one man and eighteen for another — is an argument about one control loop. The hypothalamic-pituitary-gonadal axis is a three-stage cascade with feedback, and once you can see where an intervention plugs into it, the differences between treatments stop being opinion and become anatomy.
The three components
Three organs in series, each instructing the next, the final product reporting back to the first two:
- The hypothalamus releases GnRH, gonadotropin-releasing hormone
- GnRH instructs the anterior pituitary to release LH and FSH
- LH and FSH instruct the gonads — testes in men, ovaries in women — to make sex hormones and gametes
- Those sex hormones feed back and suppress GnRH, LH and FSH
The feedback is what makes it a control system rather than a chain: output rises, upstream signalling falls, output settles. It is a thermostat, and like any thermostat it can be fooled by supplying the output directly.
The pulse generator
GnRH is a ten-amino-acid peptide released by a small, scattered population of hypothalamic neurons. It never reaches the general circulation in meaningful quantity — it travels down a short dedicated portal system straight into the pituitary, which is why it cannot be usefully measured in a blood test.
Those neurons decide nothing on their own. Upstream sit kisspeptin neurons, the actual integrators, collecting signals about energy availability, sex hormones, stress, sleep and light and converting them into a pattern of GnRH release (Xie et al., Front Endocrinol 2022). This is the mechanism behind something clinicians see constantly: severe energy restriction, overtraining, chronic stress and poor sleep suppress reproductive hormones — here, at the top, before any gland is involved.
Why pulsatility is the whole design
GnRH is released in discrete pulses, roughly every 60 to 120 minutes, and the frequency and amplitude of those pulses are themselves information — they determine how much LH is released relative to FSH (Herbison, Endocrinology 2018).
This is not a quirk. Continuous GnRH exposure does the opposite of what intuition predicts: it desensitises the pituitary and shuts the axis down. The experiment that showed it is one of the cleaner results in endocrinology — the same hormone, delivered steadily instead of in pulses, suppressed rather than stimulated (Belchetz et al., Science 1978). That finding is used therapeutically: GnRH agonists given continuously are how testosterone is suppressed in prostate cancer treatment.
The lesson generalises. A receptor exposed to a constant signal downregulates, so rhythm as well as concentration carries endocrine information — a theme running through hormone pulsatility and the HPA axis.
LH and FSH: two signals, two targets
Gonadotrope cells in the anterior pituitary answer each GnRH pulse with two hormones that have different jobs:
- LH acts on the testicular Leydig cell to drive testosterone production, and on the ovarian theca cell to produce the androgens that granulosa cells convert to estrogen.
- FSH acts on the testicular Sertoli cell to support sperm production, and on ovarian granulosa cells to drive follicle development and aromatisation.
The separation matters clinically: testosterone production and sperm production run on different signals to different cells, which is why a normal testosterone level and impaired fertility can coexist, or the reverse.
Gonadal output
In men, LH-stimulated Leydig cells produce around 95% of circulating testosterone, with a small amount aromatised locally to estradiol. Critically, the testosterone concentration inside the testis is far higher than in blood, and spermatogenesis depends on that local concentration rather than the circulating one — which explains most of what happens on treatment.
In women the same two signals drive a cycle rather than a steady state: FSH develops the follicle, LH triggers ovulation, and the corpus luteum left behind produces progesterone through the second half of the cycle.
Feedback, and the estradiol detail
Testosterone and estradiol both suppress GnRH at the hypothalamus and gonadotropin release at the pituitary. The detail worth knowing is that in men a substantial part of testosterone's negative feedback is not exerted by testosterone at all — it is exerted by the estradiol made from it, which is why blocking aromatase raises LH and testosterone (Hayes et al., J Clin Endocrinol Metab 2001).
One exception matters. In women, sustained high estradiol in late follicular phase flips the sign: feedback turns positive and triggers the LH surge that causes ovulation. The same hormone, at a different concentration and duration, produces the opposite response.
When the axis breaks, and how to tell where
This is the most useful clinical application of the model, and it takes two numbers.
If testosterone is low and LH and FSH are high, the pituitary is shouting and the testes are not answering — primary hypogonadism. Nothing that stimulates the axis harder will help, because the signal is already maximal.
If testosterone is low and LH and FSH are low or inappropriately normal, the failure is upstream at the pituitary or hypothalamus — secondary hypogonadism. The gonad is intact and unstimulated, which is exactly the situation where a treatment raising endogenous signalling can work.
Secondary patterns have common and often reversible causes: obesity, which raises aromatisation and suppresses the axis through estradiol feedback; opioids; sustained energy deficiency; poor sleep; severe illness; and raised prolactin, which suppresses GnRH directly and must be excluded rather than assumed. Diagnosis needs repeat morning testosterone alongside LH and FSH before any of it is interpreted (Bhasin et al., J Clin Endocrinol Metab 2018). The low testosterone checklist covers that first panel, and free versus total testosterone why one number is not enough.
Where each treatment plugs in
| Treatment | Acts on | Effect on the axis |
|---|---|---|
| Testosterone therapy | Supplies the end product | Suppresses the whole axis through feedback; intratesticular testosterone falls |
| hCG | Mimics LH at the testis | Stimulates Leydig cells directly, bypassing the pituitary |
| Enclomiphene | Blocks estradiol feedback at the hypothalamus | Raises LH, FSH and endogenous testosterone; requires a working axis |
| Clomiphene | Mixed estrogen agonist/antagonist | Similar effect, with more off-target activity |
| Aromatase inhibitor | Blocks conversion to estradiol | Less feedback, so LH and testosterone rise — at the cost of estradiol |
| Hormone therapy in women | Supplies estradiol and progesterone | Suppresses the axis in the same way |
Read down that table and the fertility question answers itself. Anything supplying the end product removes the pituitary signal, and without LH and FSH the intratesticular testosterone spermatogenesis requires collapses — which is why fertility falls on testosterone therapy. Low-dose hCG maintains intratesticular testosterone in men whose gonadotropins are otherwise suppressed (Coviello et al., J Clin Endocrinol Metab 2005), which is the mechanistic basis for using hCG alongside TRT.
Recovery after suppression
When exogenous hormone stops the axis restarts in order — hypothalamic pulses, pituitary responsiveness, gonadal output, then gamete production — and the last step takes longest. Recovery is usual but neither immediate nor guaranteed, and the spread between individuals is wide — pooled analysis after suppression of the axis shows most men returning to baseline sperm production, over months rather than weeks (Liu et al., Lancet 2006). Longer suppression, older age and lower baseline function all lengthen it, which is the honest reason fertility planning belongs before starting.
The clinical insight: LH and FSH alongside testosterone tell you whether the problem is the gland or the instruction, and that distinction determines which treatments are even capable of working. A panel measuring testosterone alone cannot answer the question it was ordered to answer.
Bottom line
The HPG axis runs hypothalamus to pituitary to gonad, with the sex hormones made at the end feeding back to restrain the start. GnRH arrives in pulses because pulsatility is itself the signal — continuous exposure shuts the system down. LH drives sex hormone production and FSH drives gamete production, which is why fertility and testosterone levels do not move together. Whether low testosterone is a failing gonad or a failing instruction is answered by LH and FSH, and every treatment here is defined by where in the loop it acts.
Educational content, not medical advice. Laboratory interpretation and any treatment decision are made by a licensed physician after individual evaluation. Individual results vary.
