Testosterone replacement therapy (TRT) works by delivering testosterone from an outside source to restore blood levels that have fallen below normal. The American Urological Association uses a total testosterone level below 300 ng/dL as the diagnostic cutoff for low testosterone, and the goal of treatment is to bring levels into the 450 to 600 ng/dL range, the middle of the normal reference range for most labs. Once testosterone enters your bloodstream, it acts on cells throughout your body, triggering changes in muscle, fat, bone, mood, and sexual function over a timeline that spans weeks to months.
What Testosterone Does Inside Your Cells
Testosterone doesn’t just float through your blood doing its job. It has to enter individual cells and flip specific genetic switches to produce effects. Inside cells, testosterone binds to androgen receptors, which are proteins sitting in the cell’s cytoplasm, held inactive by chaperone proteins. When testosterone locks onto the receptor, those chaperone proteins fall away, the receptor changes shape, and the whole complex moves into the cell’s nucleus. There, it attaches to specific segments of DNA and turns genes on or off, changing the cell’s behavior.
This is why TRT affects so many different systems. Androgen receptors exist in muscle tissue, bone, the brain, fat cells, skin, and reproductive organs. The same basic lock-and-key process plays out in each location, but the genes being activated differ by tissue type, which is why testosterone can simultaneously build muscle, reduce fat, improve mood, and increase sex drive.
How Testosterone Converts to Other Hormones
Testosterone isn’t the only active hormone in this process. Your body converts some of it into two other compounds that carry out distinct functions. An enzyme called 5-alpha reductase converts testosterone into a more potent form called dihydrotestosterone (DHT), which drives effects in the skin, hair follicles, and prostate. A second enzyme, aromatase, converts testosterone into estradiol, the primary estrogen in men.
That estrogen conversion isn’t a flaw. Men need some estradiol for bone health, brain function, and sexual function. Research published in the New England Journal of Medicine found that muscle size and strength are driven by androgens, fat distribution is regulated by estrogen, and sexual function depends on both. The balance matters: too much aromatase activity from high testosterone doses can push estradiol levels too high, potentially causing water retention, breast tissue growth, or mood changes. Too little estradiol creates its own problems, including bone loss.
What Happens to Your Natural Production
This is one of the most important things to understand about TRT. Your brain constantly monitors hormone levels through a feedback loop involving the hypothalamus and pituitary gland. The hypothalamus releases a signaling hormone (GnRH) in pulses, which tells the pituitary to release LH and FSH. LH tells the testes to produce testosterone, and FSH drives sperm production.
When you add testosterone from an outside source, the brain detects the higher levels and dials back its own signals. Exogenous testosterone directly suppresses GnRH release, which causes LH and FSH to drop and eventually stop. The excess estradiol created by aromatization of that exogenous testosterone adds a second layer of suppression on LH. The result: your testes stop producing testosterone on their own and also stop making sperm. This suppression is essentially universal while you’re on TRT, and it’s the primary reason therapy is typically considered a long-term commitment.
The Fertility Trade-Off
Because TRT shuts down the signals that drive sperm production, it can cause very low or zero sperm counts. In a study of 66 men who became infertile after testosterone use, 70% were able to recover a meaningful sperm count after stopping TRT and starting fertility-specific medications. But recovery isn’t guaranteed or quick. Among men who had reached zero sperm counts, only about 65% recovered adequate levels within 12 months of treatment. Both older age and longer duration of testosterone use significantly reduced the chances of recovery at the 6- and 12-month marks. If you’re planning to have children, this is a conversation to have before starting therapy, not after.
Delivery Methods and How They Differ
TRT comes in several forms, and the choice affects how often you dose, how stable your levels stay, and what side effects you’re more likely to encounter.
- Injections (cypionate or enanthate): The most common forms, typically injected every one to two weeks. They have a half-life of about 173 hours (roughly 7 days). Levels peak 18 to 36 hours after injection and then gradually decline, creating a peak-and-trough pattern. Some men split doses into more frequent injections to smooth out these swings.
- Long-acting injections (undecanoate): These reach peak levels about 7 days after injection and are dosed roughly four times per year, usually at weeks 0 and 4, then every 10 weeks. The longer interval is convenient but requires in-office administration.
- Topical gels: Applied daily to the skin. They produce more stable day-to-day testosterone levels but require consistent daily application and carry a risk of transferring testosterone to others through skin contact.
- Pellets: Small implants placed under the skin, typically replaced every three to four months. They provide steady levels without daily or weekly dosing but require a minor in-office procedure for each insertion.
When You’ll Notice Changes
TRT doesn’t produce overnight results. Different systems in the body respond on different timelines, and knowing what to expect helps set realistic expectations.
Sexual interest and desire are among the first changes, typically noticeable within 3 weeks and plateauing around 6 weeks. Mood improvements follow a similar early timeline, with initial shifts in 3 to 6 weeks, but significant improvement in depressive symptoms can take 18 to 30 weeks to fully develop. Body composition changes are slower. Shifts in fat mass and lean muscle become measurable at 12 to 16 weeks and stabilize between 6 and 12 months, with marginal gains continuing beyond that. Muscle strength follows a similar pattern, becoming demonstrable after 12 to 20 weeks.
These timelines explain why clinicians typically reassess treatment at the 3-month and 6-month marks rather than after just a few weeks.
Monitoring and Blood Work
TRT requires ongoing blood monitoring because of one key side effect: it stimulates red blood cell production. As your red blood cell count rises, your blood becomes thicker, measured by a value called hematocrit. Current guidelines recommend not starting TRT if hematocrit already exceeds 50%, and reducing the dose or pausing therapy if it reaches 54%. At that level, the risk of blood clots rises. Regular blood draws, typically every 6 to 12 months once stable, catch this before it becomes dangerous. Some men need periodic blood donation or therapeutic phlebotomy to keep hematocrit in a safe range.
Cardiovascular Safety
For years, the biggest concern around TRT was heart risk. The TRAVERSE trial, published in the New England Journal of Medicine, was specifically designed to answer this question. It enrolled men with low testosterone who either had existing cardiovascular disease or were at high risk for it. Over the study period, 7.0% of men on testosterone experienced a major cardiac event (heart attack, stroke, or cardiovascular death) compared to 7.3% on placebo. The difference was not statistically meaningful, and the trial concluded that TRT was noninferior to placebo for major cardiac events. This doesn’t mean TRT is protective for the heart, but it resolved the concern that it poses an added danger in the population most likely to be affected.