Ask a guy at your gym what testosterone replacement therapy does and he'll probably tell you it makes him feel younger, leaner, and stronger. Ask him what it does to his ability to have kids and you'll likely get a shrug.

I had the same blind spot until we sat down with Dr. Peter Kolettis for the Our TIES podcast. Dr. Kolettis is a urologist who did his fellowship in male fertility at the Cleveland Clinic, has served on the American Urological Association's Male Infertility Best Practice Policy Update Panel, and is a past president of the Society for the Study of Male Reproduction. He's spent his career on this one thing. So when we asked him about TRT, I paid attention to the answer.

"Testosterone is like birth control, really," he told us. "It decreases sperm production."

Why a drug that raises testosterone lowers sperm

The logic surprised me at first. If testosterone is the male hormone, why would taking more of it shut fertility down?

Dr. Kolettis explained it the way I think every man on TRT should hear it. The body regulates its own testosterone through what's called the HPG axis, the communication loop between the hypothalamus, the pituitary gland, and the testes. When you introduce testosterone as a drug, that loop reads the high level in your blood and decides the body doesn't need to produce any more. It shuts down natural production. The same signals that drive sperm production get shut down with it.

"A lot of men that are on testosterone will be sterile," he said, "depending on the dose and the duration of the treatment."

He compared it directly to the way birth control pills work in women. Estrogen feeds back to the glands, disrupts the hormones, and suppresses fertility. Testosterone, taken as a drug, does the same thing in men.*

It's been studied as a contraceptive. That's not a metaphor. "It's been studied as a contraceptive before," Dr. Kolettis said.

The reversal is real, and Dr. Kolettis says it's harder than most men expect

The version most guys picture is simple. Stop the testosterone, bounce back, get on with your life. Dr. Kolettis described something different.

When you stop taking exogenous testosterone, your own production doesn't come back right away. It's been suppressed. There's a window where, in his words, you have "very, very low levels, like almost no testosterone." The body does recover, but it takes time.

"Most of the time it's reversible," he said, "but it can take time. And then going through the time when men have to be essentially weaned off the testosterone, if you will, that can be really difficult."

He named the symptoms men can experience during that recovery window: fatigue, decreased sex drive, the cluster of symptoms that come with low testosterone levels. "That's tough on men," he said. He mentioned that there are off-label medications sometimes used to help with the transition, clomiphene citrate and HCG among them, but he was clear that the process itself is hard.*

The guidance Dr. Kolettis gave for men of reproductive age

His recommendation was direct: men of reproductive age who want to have children shouldn't use testosterone.*

He added that if a man has genuinely low testosterone, there are other ways to treat it that don't jeopardize fertility.* That's a conversation to have with your doctor.*

The part that stayed with me is the simple logic underneath his answer. A man on TRT in his twenties or early thirties is likely going to want to have children at some point. When he does, he'll be in that recovery window Dr. Kolettis described, on a timeline that doesn't feel good and doesn't come with guarantees.

That's a real cost. It deserves to be in the conversation before a man starts TRT, not after.

The takeaway

If you're thinking about fatherhood in the next few years, the one thing to carry out of this is that testosterone as a drug suppresses sperm production, and coming off it is not a switch you flip. This is a conversation to have with your doctor.*

We've got more conversations like this one coming. If you want them in your inbox, subscribe to the TIES newsletter. More real answers from real experts, built the hard way, on purpose.

*This post reflects conversations from the Our TIES podcast and is intended for general informational purposes only. It is not medical advice, a diagnosis, or a treatment recommendation, and it should not be used as a substitute for professional medical guidance. Always consult a qualified healthcare provider about any medication, supplement, or health condition, including testosterone therapy and decisions related to fertility. Quotes from Dr. Peter Kolettis have been lightly edited for readability. The meaning remains the same.

Community notes

• Exogenous testosterone means testosterone taken as a drug (injection, gel, pellet, etc.), as opposed to the testosterone the body produces on its own.

• Off-label means a medication prescribed for a use the FDA didn't specifically approve it for. Off-label prescribing is legal and common, but it signals that the drug's effectiveness for that specific purpose may be less formally studied.

• Clomiphene citrate is a medication originally developed for female fertility treatment. It's used off-label in men to stimulate the body's own testosterone and sperm production. HCG (human chorionic gonadotropin) is a hormone used off-label to help restart testicular function after TRT.

• Recovery timelines after stopping TRT vary significantly between individuals. Factors include dose, duration of use, age, and baseline testicular function. Some men recover within months, others take a year or more, and in some cases recovery is incomplete. Dr. Kolettis spoke to the general pattern; an individual's experience may differ.

• The relationship between TRT and fertility is well established in the medical literature. Less settled: the optimal protocols for restoring fertility after TRT (clomiphene alone, HCG alone, combination, dosing), and long-term outcomes after extended TRT use.

• TRT is a legitimate medical treatment for men with clinically diagnosed hypogonadism, where the body isn't producing adequate testosterone on its own. Dr. Kolettis noted that men of reproductive age with genuinely low testosterone have other treatment options that don't compromise fertility.