FERTILITY & HPG-AXIS RESEARCH · DEEP DIVE
The HPG Axis Explained: Testosterone, LH and FSH
The hypothalamic–pituitary–gonadal axis is the feedback loop behind testosterone and fertility. Understanding it explains why hormone therapy, sperm production and blood tests behave the way they do.
In 60 seconds
The brain reads testosterone levels and adjusts LH and FSH to match. Add testosterone from outside and the brain turns its own signal down — which lowers sperm production. That one feedback loop explains most of hormone research.
- GnRH → LH/FSH → testosterone and sperm, with feedback at every level.
- External testosterone suppresses LH and FSH and lowers sperm production.
- Primary vs secondary deficiency decides the clinical approach.
What it is
A three-level feedback loop: hypothalamus (GnRH), pituitary (LH, FSH) and gonads (testosterone, oestradiol, sperm or eggs).
What researchers looked at
How external testosterone suppresses LH and FSH, and how LH-like and FSH-like signals restore gonadal function.
What’s still unknown
What "optimal" means for any individual — guidelines define deficiency by symptoms plus repeated morning blood tests.
Three levels, one loop
The hypothalamus releases GnRH in pulses. The pituitary answers with LH (testosterone production) and FSH (sperm or follicle development). The gonads make testosterone and oestradiol, which feed back to the brain and dial the signal down when levels are high.
What hormone therapy does to the loop
External testosterone is read as "enough", so GnRH, LH and FSH fall. In a controlled study, LH and FSH dropped to about 5% and 3% of baseline and testosterone inside the testes fell 94%. A 2025 review links this directly to impaired sperm production in men of reproductive age.
That is why fertility questions belong at the start of any hormone discussion. The Endocrine Society’s 2018 guideline sets out how deficiency is diagnosed and monitored, and why fertility plans change the choices.
Primary versus secondary
Primary hypogonadism means the testes are not responding (LH and FSH are high). Secondary means the brain is not signalling (LH and FSH low or normal). The difference decides the clinical approach: in secondary cases, LH-like and FSH-like signals (hCG, then FSH activity) can restart the testes, as clinical cohorts show.
The clinical route
If you are trying to conceive or think you may have a hormone problem, the right route is a fertility clinic or an endocrinologist. They test properly (semen analysis, LH, FSH, testosterone, oestradiol) and prescribe registered medicines under monitoring. Nothing here replaces that.
KEY TAKEAWAYS
- GnRH → LH/FSH → testosterone and sperm, with feedback at every level.
- External testosterone suppresses LH and FSH and lowers sperm production.
- Primary vs secondary deficiency decides the clinical approach.
- Diagnosis needs symptoms plus repeated morning blood tests.
Sources · PubMed
- Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline · J Clin Endocrinol Metab 2018 · PMID 29562364
- Low-dose human chorionic gonadotropin maintains intratesticular testosterone in normal men with testosterone-induced gonadotropin suppression · J Clin Endocrinol Metab 2005 · PMID 15713727
- Testosterone replacement therapy and spermatogenesis in reproductive age men · Nat Rev Urol 2025 · PMID 40346275
- A combined analysis of data to identify predictive factors for spermatogenesis in men with hypogonadotropic hypogonadism treated with recombinant human FSH and hCG · Fertil Steril 2009 · PMID 18930225
