What happens to LH and FSH on TRT?
LH and FSH on TRT usually fall, often to very low levels. The pituitary gland senses the testosterone you take and stops sending the signals that tell your testes to work. That’s how the body normally keeps testosterone in balance.
That fall is expected. It’s also why sperm production drops on treatment.
The two hormones matter most before you start. Measured at diagnosis, they show where low testosterone comes from. Guidelines use them for that, not for routine checks during treatment.
They’re among the less routine tests in our guide to what to monitor on TRT.
What do LH and FSH do in men?
Both are hormones made by the pituitary, a small gland under the brain:
- LH (luteinizing hormone) tells the testes to make testosterone.
- FSH (follicle-stimulating hormone) supports sperm production in the testes.
The pituitary releases them when it gets a signal called GnRH (gonadotropin-releasing hormone). GnRH comes from the hypothalamus, a control centre in the brain.
Testosterone feeds back to the brain. When there’s plenty, the brain turns GnRH, LH and FSH down; when there’s too little, it turns them up. Doctors call this loop the HPG axis.
Why do LH and FSH fall on TRT?
Because the brain can’t tell your own testosterone from the testosterone you take. Testosterone labels describe the effect. Through feedback on the pituitary’s LH, your own testosterone release is held back.
At large doses, sperm production may also be held back through feedback on FSH.
It can happen fast. In one study, healthy men got weekly testosterone injections. Within three weeks, their LH and FSH fell to 5% and 3% of their starting levels.
In the men who got only testosterone injections, the testosterone made inside the testes fell by 94%. That number explains a common mix-up.
Your blood testosterone can be normal or high on TRT while the testosterone inside the testes is very low. Sperm production depends on that inside level. The blood level comes from your injections or gel; the testes, without LH, make far less of their own.
Real-world treatment is less uniform. In a clinic study of 227 men on TRT, LH below 1.0 IU/mL counted as suppressed (pushed down). 73% had at least one suppressed result, but only 22% stayed suppressed throughout.
Men on injections were more likely to be suppressed than men on gels. So were men with higher estradiol or a lower LH before starting.
What do LH and FSH show before TRT?
Before treatment, LH and FSH point to where the problem is. The Endocrine Society measures both to sort low testosterone into two types.
In primary hypogonadism, the testes themselves don’t respond. In secondary hypogonadism, the pituitary or hypothalamus doesn’t send enough signal. The AUA measures LH in every man with low testosterone.
| Pattern | Testosterone | LH and FSH | What it suggests |
|---|---|---|---|
| Primary | Low | High | The testes aren’t responding to the signal |
| Secondary | Low | Low or normal | The pituitary or hypothalamus isn’t sending enough |
| Compensated | Normal | High | The testes need extra signal to keep up |
| On TRT | Normal or high | Low | The brain has turned its signals down |
A large European study looked at men aged 40 to 79. About 11.8% had the secondary pattern, 2.0% the primary pattern and 9.5% the compensated pattern. Secondary was linked with obesity, and primary mostly with age.
Why do baseline results matter so much?
Once you start TRT, LH and FSH reflect the treatment, not the original problem. Take two men: one whose low testosterone came from the pituitary, and one whose testes weren’t responding. Both will show low LH and FSH on treatment.
So the only chance to see the original pattern is before the first dose.
That pattern can change what happens next. If the secondary pattern has a cause that can be reversed, such as obesity, a medicine or a serious illness, your prescriber may look into that first.
If the primary pattern points to a condition like Klinefelter syndrome, that has its own effects on fertility and follow-up. Either way, it’s worth keeping a copy of these results.
What causes each pattern?
The most common reversible causes of the secondary pattern are obesity, severe illness and medicines. Those medicines include opioids and corticosteroids. Obesity, metabolic syndrome and type 2 diabetes lower testosterone without raising LH and FSH.
Klinefelter syndrome is a common cause of the primary pattern. It affects about 2 in 1,000 men and often goes undiagnosed.
When LH is low or normal with low testosterone, the AUA also measures prolactin, another pituitary hormone. A high level can point to a pituitary problem.
The label for one current testosterone product also asks prescribers to check prolactin before starting and 3 to 4 months after. Our guide to how low testosterone is diagnosed covers the full work-up.
What are normal LH and FSH levels for men?
Labs set their own ranges.
As examples, one US lab lists an adult male LH range of 1.2 to 8.6 mIU/mL. Another lists an adult male FSH range of 1.5 to 12.4 mIU/mL. MedlinePlus notes that normal ranges vary among labs.
A survey of 117 US labs found that the upper limits for FSH and LH varied a lot. Most labs simply used the test maker’s range.
A single result is a snapshot. Even in healthy men, LH varied by about 22% and FSH by about 8% within one person from week to week. That’s one reason prescribers read them next to testosterone from the same draw, not on their own.
Units are written as mIU/mL or IU/L, which give the same number. Read your result against the range printed on your own report, and keep using the same lab if you can. Our guide to reading TRT lab results explains reference ranges and flags.
Do you need LH and FSH tested while on TRT?
Usually not as a routine. The AUA’s follow-up table lists FSH as optional at baseline for men who care about fertility. It does not recommend FSH once treatment is under way.
The Endocrine Society’s plan for monitoring treatment centres on testosterone, hematocrit and prostate checks.
Some prescribers still check them, for example when fertility is on the table or when results don’t add up. If yours does, the result is read with the fact that you’re on treatment in mind.
Repeat tests are more common in two situations. One is fertility planning. There, FSH and a semen analysis help a prescriber or urologist judge how much sperm production is affected.
The other is stopping treatment. LH, FSH and testosterone measured over the following months show whether your own production is coming back. In both cases, results from before you started are the yardstick.
What if your LH is normal or high on TRT?
It isn’t always a sign of a problem. In the clinic study above, 78% of men had an LH above 1.0 IU/mL at some point on treatment. So suppression wasn’t constant.
Note when your last dose was before each draw, because your prescriber reads the result in that light.
The type of treatment matters too. In that study, men who used injections had about 2.4 times the odds of a suppressed LH compared with men who used gels.
Missed or late doses, how a gel is used, or a lab difference could also play a part. Those are worth noting before your next visit. Bring the timing of your last dose and the draw to your prescriber; they decide whether anything needs a closer look.
Does TRT shrink your testicles?
It can. With less LH and FSH, the testes have less reason to work. Testosterone labels list testicular atrophy (shrinking testicles) among possible side effects.
Small testes are also a common sign of low testosterone itself, before any treatment.
We found no good study measuring testicle size in men on prescribed TRT.
The only measured data come from healthy men in contraception trials. A 2023 review reports their testicle volume was still about 15% below baseline 18 weeks after stopping testosterone enanthate. After stopping testosterone undecanoate, it was about 4% below baseline at 12 months.
Those men were healthy, so the numbers may not match men treated for low testosterone. If you notice a change, or any lump or pain, tell your prescriber. A new lump should always be checked.
How do LH and FSH affect fertility on TRT?
FSH drives sperm production, so when it falls, sperm counts usually fall too. In a World Health Organization study, 271 healthy men got weekly testosterone injections. Of those, 157 stopped making sperm, a running total of 65% by six months.
That doesn’t make TRT reliable birth control. Not every man reaches zero, and some pregnancies still happen. The AUA says testosterone should not be prescribed to men who are trying to conceive now.
If children are in your plans, raise it before you start; a prescriber or urologist can discuss the options. Our guide to TRT and fertility covers this in depth.
How long do LH and FSH take to recover after stopping TRT?
Usually months, sometimes more than a year. In one trial, men without low testosterone stopped two years of testosterone injections. Their fully suppressed LH and FSH slowly climbed back toward each man’s own starting level over the next 12 to 15 months.
The authors said full recovery may take longer than 12 months. Those men didn’t have hypogonadism, so recovery after long-term TRT may differ. That’s more likely if the testes weren’t working fully before treatment.
Sperm counts follow their own, separate timeline. In pooled studies of healthy men, 90% had recovered to 20 million sperm per mL within 12 months, and all within 24 months. The TRT and fertility guide covers those numbers and their limits.
Stopping TRT is something to plan with your prescriber, not to do suddenly. Your pre-treatment LH and FSH are the yardstick for judging recovery.
What should you ask your prescriber about LH and FSH?
- Were my LH and FSH measured before I started, and what pattern did they show?
- Does that pattern change anything about my treatment or other tests?
- Do you need to check LH or FSH again while I’m on TRT?
- If I want children in the future, what should we do now?
- If I stop, how will we check my own production coming back?
How can you keep track of LH and FSH?
The most useful LH and FSH results are the ones taken before your first dose, because treatment hides the original pattern afterwards. Keep them with the lab name, units and range, next to your total testosterone from the same draw.
TRTracker stores each result against its lab range, so your pre-treatment baseline is always there when a question comes up later. Our overview of TRT blood work lists every marker that’s commonly checked.
The blood test timing planner shows when to book future draws. Our TRT lab schedule explains the usual timing. The level calculator shows how your schedule shapes testosterone between injections.