Hi there,

In 1990, WHO researchers reported the results of a birth-control trial for men. Among the 157 men who entered the contraceptive phase, only one pregnancy occurred.

The drug was testosterone.

We do not use it today as birth control, but the experiment established a consequence that matters to many men: taking testosterone can suppress sperm production, even while it raises the level people associate with masculinity and fertility.

Most of my patients considering testosterone, women and men, want to support their health as they age. It's become quite popular in the longevity world, and so I'll review today some of why it's such an important hormone for healthspan and lifespan.

Photo: Kampus Production / Pexels, used for illustration.

TL;DR

• Testosterone helps maintain muscle, bones, sexual function and red blood cells. Taking more helps some people, but the benefits depend on what is wrong to begin with.
• Find out why a result is low. Sleep, weight, medicines and the proteins that carry testosterone can all affect the number.
• Discuss fertility before choosing a drug. Testosterone can suppress sperm production. Other treatments can stimulate the testes when they can still respond.
• Women need better products and better evidence. Treatment can help distressing low desire after menopause, but the US still has no approved female testosterone product.

Why testosterone matters

Testosterone helps maintain the muscle you need to get up from a low chair, carry a suitcase and keep lifting as you age. It contributes to sexual desire and helps your body make oxygen-carrying red blood cells. In men, some testosterone becomes estradiol, which helps keep bones strong. These are the very functions we want to preserve as we get older, which helps explain the interest in longevity medicine.

Normal hormone roles do not guarantee treatment benefits. Evidence for muscle, bone and red-cell effects is strongest in men; treatment evidence for women is discussed below.

Both men and women make testosterone and estradiol. The amounts can surprise people: an adult man can have more estradiol in his blood than a postmenopausal woman. A woman can have more testosterone than estradiol by weight. The concentration alone doesn't tell you how strongly a hormone acts.

Lab ranges from ARUP and Mayo, not averages or treatment targets. The panels use different units and scales. Estradiol varies across the menstrual cycle; hormone therapy can change the results.

Why levels may be falling

Testosterone has fallen faster than aging alone can explain in some groups of men. In the Massachusetts Male Aging Study, men of the same age had lower levels at later measurements. From 1987 to 2004, total testosterone fell by about 1% per calendar year, even after researchers accounted for several health and lifestyle differences. Finnish surveys found a similar pattern in later-born men.

It isn't universal. An NHANES comparison found no overall decline after adjustment. And the evidence for changes in women as they age doesn't establish the same decline across generations.

Obesity can lower testosterone in men by changing hormone signals and the proteins that carry it through the blood. It may explain part of the generational decline; these studies can't tell us how much. Metabolic health matters, but weight loss doesn't make every testosterone measurement rise.

In the randomized Look AHEAD study, men assigned an intensive lifestyle program had about 14% higher total testosterone than the control group at one year. Their calculated bioavailable testosterone did not rise significantly however. Among the postmenopausal women, calculated bioavailable testosterone was about 13% lower, which isn't easy to explain. All these participants had overweight or obesity and type 2 diabetes so it's also tough to extrapolate to a healthier population.

Testing testosterone levels

Much of the testosterone in your blood travels attached to proteins. Total testosterone counts all of it. Free testosterone is unbound; bioavailable testosterone usually means free hormone plus the amount loosely bound to albumin. SHBG holds it more tightly. When those proteins change, your total result can change without a matching change in the available hormone.

These tests count different forms of testosterone. None directly measures hormone activity in your tissues, and lowering SHBG is not a treatment goal.

What the steroid era taught us

Seven pounds of fat-free mass in ten weeks, without lifting weights. That was the average gain among healthy young men given high-dose testosterone in a 1996 trial. Men who combined testosterone with lifting gained about 13 pounds. Fat-free mass includes more than muscle, but muscle size and strength increased too. It's easy to see the appeal.

Bodybuilders have spent decades experimenting with testosterone and related anabolic steroids. Controlled studies help separate what the drugs can do from what people hope they will do. In a dose-response experiment, higher doses produced more muscle and strength, higher hemoglobin and lower HDL cholesterol. Sexual function, mood and visual-spatial cognition did not change significantly at any dose.

The highest doses in these experiments pushed testosterone above the usual range. Longer-term steroid use has also exposed risks: researchers tracking steroid cycles documented changes in blood pressure, cholesterol and hematocrit—the share of blood made up of red cells. A Danish study found more serious cardiovascular disease among men sanctioned for anabolic-steroid use. Other drugs and behaviors may have contributed; that observational study can't tell us the risk of monitored treatment for a confirmed deficiency.

We can learn from that experience. We also need to ask more of a healthspan treatment than just about muscle gain - is it good for our goals in longevity?

Most of my patients aren't trying to become Arnold

Most people aren't aiming to be bodybuilders. You probably want to feel well, keep your strength, and stay independent. So should you try to increase your testosterone? For someone with a true deficiency, treatment can help. But no single testosterone level has been shown to give everyone the longest or healthiest life.

In the Testosterone Trials, older men with low levels had better sexual function and modest mood improvements with testosterone replacement. The main fatigue measure did not improve however. Another randomized trial found that treatment helped correct anemia in some men with hypogonadism, or testosterone deficiency.

Bone health shows why we have to look beyond a better test result. Some trials found improved bone density. Yet in the TRAVERSE fracture study, 3.5% of men on testosterone had a clinical fracture, compared with 2.5% on placebo, over a median 3.2 years. These were men aged 45–80 with symptoms prior to treatment, repeatedly low testosterone and cardiovascular disease or high risk. The study did not establish why more fractures occurred, or show that testosterone weakened their bones - we can only guess at what this means regarding testosterone and bone changes.

That all being said, getting testosterone levels a bit higher especially in men with symptoms is a common goal in my longevity clinic. But I'll often start with other options, since testosterone replacement can lead to low sperm production (which matters for men who still plan to have children), and requires indefinite (life-long) treatment in most practical cses.

How to raise testosterone

I start by asking about sleep, weight and the medicines you take. If you have a sleep disorder, take an opioid or have a problem in the testes or pituitary, I would want to address that before choosing a hormone drug. Women and men both benefit from better overall health, even when testosterone doesn't rise.

Men can lose testosterone when they lose sleep. In a small laboratory experiment, ten healthy young men had about 10–15% lower daytime testosterone after roughly a week of five-hour nights than after well-rested nights. Researchers tested sleep restriction; they did not test whether sleeping longer would treat a persistent deficiency. However, this is probably the lowest hanging fruit anyone can tackle!

If we consider medication, I want to know why your level is low, what you want to improve and whether you want children. Several drugs can raise the blood result substantially. They do that in different ways, and a larger rise doesn't necessarily mean you will feel better.

For men, fertility can change the prescription

Two men can have the same testosterone result and need different prescriptions. Before prescribing, I ask about symptoms and confirm the low level with repeat morning, fasting measurements. If total testosterone is borderline or your binding proteins make it hard to interpret, I may check free testosterone too. I also use LH and FSH, the pituitary hormones that signal to the testes, to help work out where the problem starts.

When you take testosterone, your brain and pituitary can turn down their signals to the testes through negative feedback. Your blood level rises, but your testes make less testosterone locally. With less LH and FSH signaling, you can make fewer sperm or stop making them altogether.

GnRH carries the brain's signal to the pituitary. Testosterone and the estradiol made from it help regulate this loop. Men respond differently, so testosterone therapy is not reliable birth control.

If you want children now or in the future, I would avoid prescribing testosterone itself, in line with the AUA/ASRM guideline. After stopping, you may need months or years to recover sperm production, and you may not recover fully.

If your testes can still respond to hormonal signals, you may be able to take a drug that stimulates your own testosterone production instead.

How much did testosterone rise in trials?

• Clomiphene: Men taking it in a 12-week randomized trial went from an average 226 to 688 ng/dL; men on placebo stayed near 220. Researchers enrolled 78 men with obesity and low testosterone. Despite the hormone rise, the men did not report significantly better overall symptom or sexual-function scores than those on placebo.
• Enclomiphene and testosterone gel: In one of two randomized 16-week trials, men taking enclomiphene went from average morning levels of 203 to 446 ng/dL. Men using gel went from 209 to 351. The men had low testosterone and low or normal LH; they maintained sperm concentrations on average with enclomiphene, while sperm concentrations fell with gel.
• hCG: Men receiving injections in a three-month randomized trial raised testosterone by about 150%. Researchers enrolled 40 men over 60 with partial androgen deficiency. The men gained lean mass but did not improve their measured strength or physical function.
• Anastrozole: Men over 60 taking it in a year-long randomized trial went from 319 to 524 ng/dL at three months, then averaged 474 at one year. The men also had lower estradiol and lost spine bone density relative to placebo.

Researchers studied different men and used different doses, so I wouldn't rank the drugs or predict your response from these averages. I would check follow-up labs and ask how you feel.

As an important longevity side note, I don't typically use aromatase inhibitors. Your body converts some testosterone into estradiol; these drugs reduce that conversion. Men also need estradiol to help maintain their bones. Instead, if a patient's hormone treatment pushes estradiol too high or causes estrogen-related side effects, I usually lower the dose or change the drug or delivery method first. I prefer to adjust the medication causing the problem before adding another one. I can often keep treatment simpler and avoid suppressing estradiol more than necessary.

Clomiphene has a longer clinical history than enclomiphene. It contains two forms of the molecule: enclomiphene and zuclomiphene. I often choose enclomiphene because it leaves out zuclomiphene, which lasts longer in the body and can have more estrogen-like activity.

Researchers reviewed records from 66 men who switched from clomiphene to enclomiphene. They recorded fewer side effects after the switch, but didn't randomize the treatments. Enclomiphene can still raise estradiol as testosterone rises.

Men with diagnosed low LH/FSH signaling have also fathered children after many months of treatment in small FSH-with-hCG studies. Preserving sperm counts in a hormone trial is a different finding from recording pregnancies and live births.

If you choose replacement, you also have to choose how to take it. You apply gel daily and need to avoid transferring it to someone else. With some injections, your level can rise sharply and fall before the next dose. With oral products, you need to follow the food instructions and monitor blood pressure. With pellets, you undergo a procedure and can't adjust or stop the dose as quickly.

And there of course are some potential adverse effects to be aware of. Clomiphene can cause mood changes, breast tenderness and visual problems that sometimes persist; new visual symptoms need prompt review. People taking enclomiphene have also reported visual symptoms, and European regulators raised concern about clots. hCG can cause breast tenderness, acne or fluid retention.

Women need a product of their own

Testosterone can help postmenopausal women whose low sexual desire causes distress. In a 2019 review of 36 randomized trials involving 8,480 women, women receiving testosterone reported better desire, less sexual distress and roughly one more satisfying sexual event per month than women in the comparison groups.

Yet US clinicians still have no approved female testosterone product to prescribe.

For a woman who may benefit, a clinician can prescribe a gel made for men off-label at a much smaller dose. Under the global consensus and ISSWSH guideline, clinicians use treatment through the skin and monitor levels to stay within the usual premenopausal range. For a standard 1% gel, ISSWSH describes a clinician-managed starting dose of roughly one-tenth the standard male dose.

Women shouldn't need to adapt a men's product to get an appropriate dose. Nor can we assume a compounded product will be more consistent or better studied. If a pellet or injection pushes your level too high, your clinician can't quickly undo the dose. You can develop acne, unwanted hair growth or potentially lasting voice changes. I'm particularly wary of pellets for this reason, as it takes a very experienced operator to use.

Australia has registered a female-specific cream for postmenopausal HSDD. Women in the US need reliable female-dose products. Researchers also need to follow women long enough to assess heart and breast safety alongside the benefits women want. FDA held a workshop in September to examine those gaps.

What happened to the heart warnings?

In 2015, FDA required warnings about possible heart attack and stroke risk with testosterone. A decade later in 2025, it asked manufacturers to remove that cardiovascular-risk language from the boxed warning.

In TRAVERSE, researchers assigned 5,246 men aged 45–80 to testosterone gel or placebo. The men had to have symptoms, two fasting testosterone results below 300 ng/dL, and cardiovascular disease or high risk. Over about 33 months of follow-up, 7.0% of men on testosterone gel and 7.3% on placebo died from cardiovascular disease or had a nonfatal heart attack or stroke. On average, the men used their assigned treatment for about 22 months.

I find that reassuring for men whose health resembles the participants'. Notably, the researchers didn't test lifelong use or bodybuilding doses, and they certainly didn't show that testosterone prevents heart disease. More men on testosterone did develop atrial fibrillation, acute kidney injury and pulmonary embolism, though researchers had less statistical certainty about those findings than the main result. Testosterone also raised blood pressure in separate studies, so we do still need to be careful when using these medications.

Researchers still need to study women separately. In an October 20 conference report, they described more cardiovascular events among testosterone users in a matched postmenopausal cohort. They compared existing health records, so they couldn't establish that testosterone caused the events - this kind of research is notoriously hard to interpret. In the public report, they also left important questions about dose, route and blood levels unresolved. We can't assume women will respond like the men in TRAVERSE either.

Before prescribing, I would ask about a recent heart attack or stroke, high hematocrit, untreated severe sleep apnea, a history of clots and poorly controlled blood pressure. I would not start TRT within six months of a heart attack or stroke, in line with clinical guidelines and just in case. If you've had prostate cancer, I would involve a specialist; TRAVERSE's prostate researchers excluded men with that history and it takes deeper consideration of your risk/benefit balance.

If you're considering hormone therapy

Testosterone has been vilified, and now parts of the wellness world oversell it. I think many people could benefit from improving low testosterone levels. Before I recommend a treatment, I ask how you're feeling, what you want to improve and why your levels are low.

I want hormone care to help people keep their strength, sexual health and independence as they age. If you start treatment, you and your clinician should check whether you feel and function better.

If you'd like help exploring your options, these pages explain more about the treatments and how to get started:

• Enclomiphene evaluation for men: baseline labs and a doctor's review to see whether enclomiphene makes sense for you.

• Women's menopausal hormone care: evaluation for estradiol and progesterone options (testosterone not yet available).

All care on Curator is conducted by licensed MDs and includes full oversight and support. I'm a co-founder of Curator. Purchases through my page may support my work and this Letter.

Hillary Lin, MD

From The Longevity Show

In my Lp(a) episode, I explain why a promising lab result isn't enough to prove that a drug prevents heart attacks.

Watch on YouTube | Subscribe to The Longevity Show

⚡ Longevity quick hits

• Loading the tibia extended lifespan by 9.4% in old mice.
• Does HRT change the response to GLP-1s? Researchers reported greater sustained semaglutide weight loss among women using hormone therapy in an October 20 conference report.
• A cancer vaccine triggered immunity but missed its main clinical goal. In a 144-patient pancreatic-cancer trial, 89 of 90 evaluable vaccine recipients developed immune responses, but researchers did not show a significant improvement in survival without recurrence.

Where to find me

• October 28, New York: Join me, Lindy Esposito, and Emily Hu at VOSK for Deadlifts Are for Everyone!.
• October 29, online: I'm joining Offcall for a LinkedIn Live on AI tools for independent physicians. Updates on LinkedIn.
• February 2027, Scottsdale: I'll join the faculty at the Science of Skin Longevity Summit for AI in diagnostics and the peptides panel.
• March 5–6, 2027, San Francisco: I'll be back at the Livelong Women's Health Summit, with Boston and New York stops later in the year.

Advanced diagnostics

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