How The HPG Axis Controls Testosterone And Fertility

The HPG axis is the feedback loop between the brain and the gonads that regulates a man's testosterone and plays a central role in fertility. The hypothalamus sends a signal, the pituitary answers, and the testes respond by making testosterone and sperm. When the loop runs well, hormone levels hold steady. When one part stalls, testosterone drops and fertility can take a hit.

That loop is also the reason testosterone therapy is trickier than it looks. Most men who reach out to telehealth hormone care about low energy or low drive are really asking about this system, whether they know the name or not.

The hypothalamic-pituitary-gonadal (HPG) axis is the hormone pathway linking the hypothalamus, the pituitary gland, and the gonads. It runs reproduction and sex hormone production. The hypothalamus signals the pituitary, the pituitary signals the testes, and the testes release hormones that signal back to keep the whole system in balance.

What do gonadotropins do?

Gonadotropins are the two hormones the pituitary sends down to the gonads: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In men, they tell the testes to make testosterone and build sperm. The hypothalamus starts the chain with GnRH, and the pituitary releases LH and FSH into the bloodstream.

Think of LH and FSH as two messengers with different jobs. One handles hormone output. The other handles sperm.

Luteinizing hormone and testosterone

LH tells the Leydig cells in the testes to make testosterone, the main male sex hormone. Testosterone does a lot. It drives sex drive, builds muscle and bone, grows body and facial hair, deepens the voice, and helps start sperm production.

A portion of circulating testosterone converts into dihydrotestosterone (DHT), a stronger androgen. Some tissues, like the prostate, react more to DHT than to testosterone itself.

Here's the part that matters for therapy. When testosterone climbs high enough, it signals the hypothalamus and pituitary to ease off. GnRH and LH drop, and testosterone settles back down. That off-switch is negative feedback, and it sits at the center of how the whole axis behaves.

FSH and sperm production

FSH works on the Sertoli cells in the testes to drive sperm production. It also triggers proteins that keep that process running. Androgen-binding protein holds testosterone at high levels right where sperm are made. An enzyme called aromatase converts some testosterone into estradiol. Growth factors support sperm production.

There's a separate brake here too. The testes release a hormone called inhibin that dials back FSH only, without touching LH. So the body can fine-tune sperm output and testosterone output on different dials.

What controls the HPG axis?

GnRH sets the pace. The hypothalamus releases it in pulses, not a steady stream, and the rhythm of those pulses decides how much LH and FSH the pituitary makes. Change the pulse pattern and the whole downstream signal changes with it.

Those neurons don't work in isolation. They read the rest of the body and adjust. Light and dark cycles, body fat, and stress signaling all feed into GnRH release. This is why poor sleep, heavy stress, and excess body fat can drag testosterone down without any problem in the testes at all. The signal from the top just gets quieter.

How testosterone therapy affects the HPG axis

Taking testosterone from outside the body raises blood levels, and the brain reads that as plenty. So it cuts the GnRH, LH, and FSH it was sending. Natural production slows, and sperm output can fall with it. Most explainers of the HPG axis stop before this point. It's the part most men actually need.

Why TRT shuts down natural testosterone

Negative feedback again. Outside testosterone trips the same off-switch that high natural testosterone would. LH and FSH fall, the testes stop getting the signal, and they can shrink and slow sperm production. This is why fertility comes up before therapy starts, not after.

Most men never ask the one question that matters most here: what happens to their own supply. Some approaches used under medical supervision may help support the body's own signaling during testosterone replacement therapy rather than override it, though whether they fit is an individual decision made with a provider. The effects on fertility may not be fully reversible for everyone, which is why fertility goals belong in the conversation early.

Primary vs secondary low testosterone

Where the breakdown sits changes everything about the fix. In primary gonadal failure, the testes themselves can't keep up, so the brain keeps shouting and LH and FSH run high. In secondary low testosterone, the signal from the hypothalamus or pituitary is weak, so LH and FSH come back low. A simple blood test for LH and FSH tells the two apart.

Now the part that surprises people. With age, testosterone usually falls, and you'd expect LH to climb to push the testes harder. In many older men it doesn't. That points to a signaling problem higher up, in the brain, not just tired testes. So a lot of low testosterone symptoms tied to aging trace back to the control center, not the factory. Testosterone tends to decline gradually with age, with the free portion often falling faster than total as the binding protein SHBG rises (Harman et al., 2001).

How do providers evaluate and monitor low testosterone?

Two things have to line up: real symptoms and a low blood level. That low level should be confirmed on more than one early-morning test rather than a single reading (Mulhall et al., 2018). A result off a direct-to-consumer panel is not enough on its own. Confirming with repeat testing helps make sure therapy is appropriate and avoids treating men who may not need it.

Monitoring matters just as much as starting. Testosterone can raise red blood cell concentration, so providers monitor blood counts and may lower the dose or pause therapy if they climb too high (Bhasin et al., 2018; Mulhall et al., 2018). The picture keeps shifting too. The FDA updated testosterone labeling in February 2025, adding a blood pressure warning and removing the older cardiovascular boxed warning after the TRAVERSE trial (FDA, 2025; Lincoff et al., 2023). That trial found testosterone was not inferior to placebo for major cardiac events, though it flagged other potential risks, so cardiovascular suitability is weighed for each person. Pairing symptoms and reference ranges with a full hormone panel gives a far clearer read than one testosterone number ever could. Whether to start therapy, and how to protect fertility along the way, is a shared decision made with a provider, based on your symptoms, history, and goals.

The HPG axis is the system every testosterone decision runs through. Read it well and lab results start to make sense. Ignore it and therapy can quietly switch off the body's own supply. The men who do best treat the axis as a full loop, not a single number, and they lean on an experienced team that reads the whole picture instead of one line on a lab sheet.

Disclaimer: This blog post is intended for informational purposes only and should not be considered medical advice. Always consult a healthcare professional before making changes to your health routine.

FAQs

What is the HPG axis? 

The HPG axis, or hypothalamic-pituitary-gonadal axis, is the hormone pathway that links the hypothalamus, the pituitary gland, and the gonads. It controls testosterone, sperm, and fertility. The hypothalamus signals the pituitary, the pituitary signals the testes, and the testes send hormones back to keep the loop balanced.

Does testosterone therapy shut down the HPG axis? 

Usually, yes. When testosterone comes from outside the body, the brain often reads levels as high enough and cuts back GnRH, LH, and FSH. Natural production can slow and sperm output can fall. This is why fertility planning usually comes up before therapy starts rather than after.

How is low testosterone confirmed? 

Providers look for real symptoms paired with a low blood level, confirmed on more than one early-morning test rather than a single reading. A result from a direct-to-consumer panel is not enough on its own.

Can the HPG axis recover after stopping testosterone? 

The system may recover over time for many men, but recovery is not guaranteed and can be incomplete, especially after long-term use. Some men use treatments under medical supervision to support the body's own signaling during or after therapy. Recovery varies from person to person, so it should be tracked with a licensed provider.

How does stress affect the HPG axis? 

Stress and poor sleep can quiet the pulses of GnRH from the hypothalamus, which lowers LH and FSH and, in turn, testosterone. The testes may be perfectly healthy while the signal from the brain stays weak. Sleep, body fat, and stress all feed into this control system.

Can testosterone therapy thicken the blood? 

Yes. Testosterone can raise red blood cell concentration, so providers monitor blood counts during therapy. If levels climb too high, they may lower the dose or pause treatment. Regular blood work is how this is caught early.

How does aging affect testosterone? 

Testosterone tends to decline slowly with age, and the free portion often drops faster than total as the binding protein SHBG rises. In many older men, LH does not climb the way it should when testosterone falls, which points to a signaling issue in the brain rather than the testes alone.

References

• FDA. Class-wide labeling changes for testosterone products. 2025

• N Engl J Med. 2023;389(2):107–117

• J Clin Endocrinol Metab. 2018;103(5):1715–1744

• J Urol. 2018;200(2):423–432

• J Clin Endocrinol Metab. 2001;86(2):724–731