Testosterone replacement therapy (TRT) works by supplying your body with testosterone from an outside source, which raises blood levels back into the normal range and relieves symptoms of low testosterone. A diagnosis typically requires a total testosterone level below 300 ng/dL, confirmed on two separate early morning blood draws. Once treatment begins, the external testosterone binds to the same receptors throughout your body that natural testosterone would, restoring functions like sex drive, energy, mood regulation, and muscle maintenance.
What Happens Inside Your Body on TRT
Your body normally produces testosterone through a feedback loop between your brain and your testes. The hypothalamus in your brain releases a signaling hormone, which tells the pituitary gland to release two other hormones (LH and FSH) that travel to the testes and trigger testosterone production. When testosterone levels are adequate, the brain detects this and dials back the signals.
When you introduce testosterone from an outside source, your brain reads those higher blood levels and assumes the testes are producing plenty. It shuts down the signaling chain. In studies using injectable testosterone, LH and FSH became undetectable within two to six weeks depending on the dose. This is why TRT causes the testes to shrink over time and why it suppresses sperm production. The testes simply aren’t receiving the “make testosterone” signal anymore.
The good news: this suppression reverses after stopping treatment. The feedback loop restarts, though full recovery can take months.
Delivery Methods
Testosterone can enter your body through several routes, and the method you use affects how stable your levels stay day to day.
- Injections are the most common form. Testosterone cypionate, the standard injectable in the U.S., has a half-life of about 8 days. Prescribing guidelines suggest 50 to 400 mg every two to four weeks, though many clinicians now prescribe smaller doses once or twice weekly to keep levels more stable and reduce the peaks and valleys that come with less frequent injections.
- Topical gels are applied daily, typically to the shoulders or upper arms. They deliver a steady, low dose that absorbs through the skin throughout the day. The tradeoff is strict consistency: you need to apply them at roughly the same time each day, and there’s a risk of transferring testosterone to partners or children through skin contact.
- Subcutaneous pellets are small crystalline implants placed under the skin, usually in the hip area, during a brief office procedure. A typical implant uses 10 to 14 pellets to reach peak levels of 500 to 800 ng/dL, and they last three to six months before needing replacement. Pellets offer the most hands-off experience but require a minor procedure each time.
When You’ll Notice Changes
TRT doesn’t produce overnight results. Different systems in your body respond on different timelines, and knowing what to expect helps set realistic expectations.
Sexual desire is one of the first things to change, typically appearing around three weeks and plateauing by six weeks. Mood improvements follow a slower arc: relief from depressive symptoms often begins between three and six weeks but doesn’t peak until roughly four to seven months in. Insulin sensitivity can start shifting within days, though meaningful changes in blood sugar control take three to twelve months to show up on lab work.
Body composition changes are the slowest. Shifts in fat mass, lean muscle, and strength start becoming measurable around 12 to 16 weeks and continue stabilizing over six to twelve months, with marginal gains possible even beyond that. This is why patience matters early on. If you’re three weeks in and don’t feel dramatically different physically, that’s completely normal.
The Fertility Tradeoff
One of the most important things to understand about TRT is that it suppresses sperm production, sometimes severely. Because the brain stops sending signals to the testes, the internal testosterone concentration inside the testes drops dramatically. Sperm production depends on testosterone levels within the testes being far higher than what circulates in your blood, and when those local levels fall by more than about 80%, sperm counts drop exponentially.
For men who want to preserve fertility while on TRT, doctors sometimes prescribe HCG (human chorionic gonadotropin) alongside testosterone. HCG mimics the LH signal that the brain has stopped sending, keeping the testes active. Research has shown that doses of 250 to 500 IU given every other day can maintain testicular testosterone levels even during TRT. One study demonstrated that 500 IU of HCG preserved semen quality in men on testosterone therapy. If future fertility matters to you, this is a conversation to have before starting treatment, not after.
Blood Count Monitoring
The most common side effect of TRT is a rise in red blood cell production. Testosterone stimulates the bone marrow to make more red blood cells, which thickens the blood. This is measured by hematocrit, the percentage of your blood volume occupied by red cells. Multiple medical societies agree that hematocrit above 54% requires intervention because of increased risk for blood clots and cardiovascular events.
If your hematocrit climbs too high, the typical first step is reducing your testosterone dose or changing how frequently you inject. Switching to a daily gel from weekly injections, for example, can sometimes smooth out the spikes that drive red blood cell production. In more stubborn cases, a therapeutic blood draw (essentially donating blood) can bring levels down quickly. This is why regular blood work, usually every three to six months in the first year, is a standard part of TRT management.
Prostate Monitoring
TRT does not cause prostate cancer based on current evidence, but testosterone does stimulate prostate tissue growth. For men between 55 and 69, guidelines recommend discussing prostate cancer risk before starting therapy and checking PSA (a protein marker the prostate produces) within the first 3 to 12 months. A PSA increase greater than 1.4 ng/mL above your baseline within the first year, or a total PSA above 4.0 ng/mL, warrants a urology referral. After the first year, screening follows the same schedule recommended for the general population based on your age.
Why Levels Fluctuate on TRT
Even on a stable dose, your testosterone levels won’t be a flat line. Injections create a peak within a day or two and a gradual decline until the next dose. Gels produce smaller daily fluctuations. Pellets create a slow decline over months. How you feel can shift with these patterns. Some men notice more energy and drive in the days after an injection and a slight dip before the next one. This is normal and one reason why injection frequency has trended toward more frequent, smaller doses. Splitting a biweekly dose into two weekly injections, or even smaller doses every other day, narrows the gap between peak and trough levels and often smooths out the subjective experience.
Your doctor will use blood work drawn at the trough (right before your next dose) to assess whether your levels are in range. If trough levels are adequate, you’re getting consistent coverage. If they’re low, adjusting the dose or frequency is straightforward.