Testosterone replacement therapy (TRT) is appropriate when you have both persistent symptoms of low testosterone and blood tests that confirm consistently low levels. Neither piece alone is enough. Feeling tired doesn’t qualify you, and a single low lab result in an otherwise healthy, symptom-free man doesn’t either. The clinical threshold most commonly used is a total testosterone below roughly 300 ng/dL, though the formal lower limit of the normal range for healthy young men is 264 ng/dL.
Symptoms That Point to Low Testosterone
The earliest signs tend to be reduced sex drive, low energy, and depressed mood. These are vague enough to overlap with dozens of other conditions, which is why blood work matters. Over time, more specific changes develop: difficulty getting or maintaining erections, loss of muscle mass, increased body fat, reduced facial and body hair growth, and breast tissue enlargement. Some men experience hot flashes, trouble concentrating, and bone density loss that can progress to osteoporosis.
Not every symptom carries the same diagnostic weight. Low libido and unexplained fatigue are the complaints most strongly tied to measurably low testosterone. Erectile dysfunction, on the other hand, has many possible causes, so it alone isn’t a reliable signal without supporting lab values.
How Testosterone Is Tested
Testosterone levels peak in the early morning and drop throughout the day, so blood draws should happen between 7 and 10 a.m. You’ll typically get a total testosterone test first, which measures both the testosterone bound to proteins in your blood and the small fraction circulating freely. If results are borderline or don’t match your symptoms, your doctor may order a free testosterone test, which looks only at the unbound, active form.
One test isn’t enough for a diagnosis. The Endocrine Society recommends confirming low levels with a second morning fasting blood draw on a separate day. Testosterone fluctuates based on sleep, stress, illness, and meals, so a single result can be misleading. Both samples need to come back unequivocally low before treatment is considered.
What “Low” Actually Means by the Numbers
A large harmonized study across U.S. and European cohorts set the normal range for healthy, non-obese men aged 19 to 39 at 264 to 916 ng/dL, with a median of 531 ng/dL. Values below the 2.5th percentile (264 ng/dL) are considered definitively low. Many clinicians use 300 ng/dL as a practical cutoff, since that’s where symptoms tend to become noticeable for most men.
Testosterone does decline with age, but the drop is gradual, not a cliff. National survey data from healthy, lean, non-smoking men shows median total testosterone falling from about 620 ng/dL in the 20-to-39 age group to roughly 540 ng/dL in the 40-to-59 range and around 420 to 460 ng/dL after age 60. That natural decline doesn’t automatically mean you need treatment. TRT is for men whose levels fall well below these age-adjusted medians and who have symptoms that affect their quality of life.
Men Over 65 Face a Different Calculation
For men 65 and older, the Endocrine Society takes a more cautious approach. If you have symptoms like low libido or unexplained anemia alongside consistently low morning testosterone, treatment can be offered on an individualized basis after a frank discussion about risks and benefits. The evidence supporting TRT in older men is less robust, and the potential for side effects rises with age, so the decision requires more careful weighing.
Who Should Not Start TRT
Certain conditions rule out testosterone therapy entirely. TRT is contraindicated in men with untreated prostate cancer or breast cancer. Men considered high-risk for prostate cancer, including those with a first-degree relative who had the disease and African American men with a PSA above 3 ng/mL, are also generally excluded. If your red blood cell concentration (hematocrit) is already elevated above 54%, therapy must be paused or avoided until levels normalize, because testosterone further thickens the blood and raises the risk of clotting events.
Fertility Is a Major Consideration
If you’re planning to have children, this is one of the most important factors in deciding whether and when to start TRT. Exogenous testosterone tells your brain to stop signaling your testes to produce sperm. The effect is dose-dependent and can be dramatic: in one study, 100 mg per week of injectable testosterone reduced sperm counts to less than 1% of baseline values. At higher doses, complete absence of sperm (azoospermia) occurred within about 12 weeks.
There are workarounds. Adding HCG (a hormone that mimics the brain’s natural signal to the testes) alongside testosterone can help maintain sperm production. In one study of 26 men on TRT who also received HCG every other day, none became azoospermic, and nine achieved a pregnancy with their partner during the follow-up period. Selective estrogen receptor modulators like clomiphene can also help preserve fertility or restore sperm production after stopping TRT, with one study showing 96% of men recovering sperm counts after HCG therapy following testosterone discontinuation.
Still, the safest path for men actively trying to conceive is to explore alternatives to TRT first. Clomiphene or HCG monotherapy can raise your body’s own testosterone production without suppressing sperm. Your doctor can help you decide whether to delay TRT, use it with a fertility-preserving add-on, or take a different route entirely.
How TRT Is Delivered
The three main options are intramuscular injections, topical gels or patches, and subcutaneous pellets. Each has trade-offs in convenience, consistency, and effectiveness.
- Injections (typically every one to two weeks) deliver the highest effective doses and produce the strongest results for muscle strength and bone density. A meta-analysis found that all six injection-based studies showed significant strength gains, while none of four transdermal studies did. Injections also showed an 8% increase in lumbar spine bone density, compared with no increase from transdermal methods. On the cardiovascular side, injectable TRT showed a non-significant trend toward protection, while oral forms showed elevated risk.
- Gels and patches are applied daily and provide steadier, more physiologic testosterone levels without the peaks and valleys of injections. The trade-off is lower absorption, which means the musculoskeletal benefits tend to be modest. Gels can also transfer testosterone to partners or children through skin contact if you’re not careful about application sites and hand-washing.
- Pellets are implanted under the skin every three to six months, offering the most hands-off approach. They’re less commonly used and require a minor in-office procedure for insertion.
What Improvement Looks Like
TRT doesn’t work overnight, and different symptoms respond on different timelines. Sexual interest typically improves within three weeks and plateaus around six weeks. Mood improvements, particularly for depressive symptoms, begin between three and six weeks but don’t reach their full effect until four to seven months in. Changes in body composition, including reduced fat mass, increased lean muscle, and greater strength, take 12 to 16 weeks to become noticeable, stabilize between 6 and 12 months, and can continue improving marginally beyond that.
These timelines matter for setting expectations. If you start therapy hoping for quick results with energy or body composition, you’ll need patience. If libido was your primary concern, you’ll likely notice a difference within the first month.
Ongoing Monitoring While on TRT
Starting TRT isn’t a one-time decision. It requires regular blood work to make sure your levels are in the target range and to catch side effects early. Hematocrit should be checked at least every six months, since testosterone stimulates red blood cell production and levels above 54% require pausing treatment. Your doctor will also track your testosterone levels to confirm they’re landing in the 400 to 700 ng/dL range (for injection-based therapy, measured about one week after an injection).
PSA levels are typically monitored as well, particularly in older men, to watch for any prostate changes. The commitment to regular follow-up is part of the therapy itself. If you’re not willing or able to get blood drawn on a consistent schedule, that’s worth factoring into your decision about whether to start.