Patient Education Guide
Some Things You Should Know About Men's Testosterone
What testosterone actually does in your body, how to know if yours is low, and what your treatment options really look like.
From Bull Testes to Modern Medicine
The story of testosterone starts nearly 200 years ago with a curious scientist and some roosters. It took decades of discovery, controversy, and landmark clinical trials to get to where we are today, a point where we finally have real answers about what testosterone therapy can do, who it helps, and whether it's safe.
The First Experiment
Arnold Berthold removed testes from roosters, watched them lose male traits (combs shrank, stopped crowing, lost interest in hens). Reimplanted testes, everything came back. First proof testes produce something controlling male characteristics.
Testosterone Is Found
Scientists in the Netherlands isolated the hormone from bull testes, named it "testosterone" (from "testes" and "sterol"). Two other teams figured out lab synthesis. Earned a Nobel Prize.
First Injectable Treatments
Testosterone cypionate and enanthate developed. Longer-lasting injectable forms, shot every 1–4 weeks. Still the most commonly used today, over 70 years later.
The Dose-Response Study
Dr. Shalender Bhasin published a landmark study in the NEJM showing testosterone increases muscle mass and strength dose-dependently, even without exercise. Foundation for understanding how TRT works.
The Gel Revolution
AndroGel became the first FDA-approved testosterone gel. Daily skin application instead of injections. Made treatment much more accessible.
Safety Concerns Emerge
Small TOM trial in frail elderly men stopped early due to heart events. European Male Ageing Study clarified most "low T" in aging men is functional (caused by obesity/illness, not true hormonal failure). Forced the field to get more rigorous about who truly needs treatment.
The Testosterone Trials (TTrials)
Largest coordinated clinical trials in older men with low T. Published in the NEJM. Showed testosterone reliably improves sexual desire, physical activity, corrects anemia, but did NOT significantly improve cognition or vitality.
A Pill That's Safe for the Liver
FDA approved Jatenzo (oral testosterone undecanoate). First oral testosterone NOT toxic to the liver. Absorbed through the lymphatic system, bypassing the liver entirely. Major breakthrough.
Testosterone Helps Prevent Diabetes
T4DM trial: testosterone + lifestyle changes reduced progression to type 2 diabetes by 41% in overweight men with low T. Established testosterone as a meaningful metabolic treatment.
Heart Safety Finally Answered
TRAVERSE trial, 5,246 men, published in the NEJM. Result: testosterone therapy does NOT increase risk of heart attacks, strokes, or cardiovascular death. Put to rest over a decade of fear.
What Does Testosterone Actually Do?
Most people think of testosterone as just the “sex hormone.” But it does far more than that. Testosterone has receptors (like locks waiting for a key) all over your body: in your brain, muscles, bones, heart, fat cells, and more. Here's what it does in each place.
Signs Your Testosterone May Be Low
Low testosterone can show up in many ways. Here are the symptoms organized by category. Having some of these doesn't automatically mean your testosterone is low, but if several resonate, it's worth getting tested.
Physical Symptoms
- Decreased muscle mass or strength
- Increased body fat, especially belly fat
- Fatigue or low energy
- Decreased bone density
- Hot flashes or increased sweating
- Loss of body hair
Mental & Emotional
- Brain fog or difficulty concentrating
- Decreased motivation or drive
- Irritability or mood swings
- Depressed mood
- Poor memory
- Decreased confidence or assertiveness
Sexual
- Decreased sex drive (libido)
- Erectile dysfunction
- Fewer morning erections
- Difficulty reaching orgasm
- Decreased ejaculate volume
- Reduced sensation
Important: Having some of these symptoms doesn't automatically mean you have low testosterone. Many of these can be caused by stress, poor sleep, depression, medications, or other medical conditions. That's why proper testing, not just a symptom checklist, is essential.
How I Like to Test and Diagnose Low Testosterone
Diagnosing low testosterone isn't as simple as one blood draw and a number. Here's what a proper evaluation looks like and what the numbers actually mean.
My Lab Panel
Testosterone, Total, MS
The main number. Measures all testosterone in your blood, both the active portion and the portion bound to proteins. I use mass spectrometry (MS) for accuracy.
Free Testosterone (Calculated)
Only about 2-3% of your testosterone is “free” (not attached to proteins) and available for your body to use. I calculate this from your total testosterone and SHBG rather than measuring it directly, because calculated free T is more reliable than direct assay methods.
Sex Hormone Binding Globulin (SHBG)
A protein that grabs onto testosterone and makes it unavailable. If SHBG is high, your total T might look fine while your free T is actually low. If SHBG is low (common in obesity), your total T might look low but your free T is normal. This is essential for interpreting your total T number.
Estradiol
Men make estrogen too, from testosterone. Checking this helps me see the full hormonal picture and guide treatment decisions.
FSH and LH
The signals from your brain telling your testes to make testosterone and sperm. This helps me figure out WHERE the problem is: is it the brain’s signal (secondary) or the testes’ response (primary)?
Prolactin
A pituitary hormone. If elevated, it could point to a pituitary problem suppressing testosterone production.
CBC (includes Differential and Platelets)
Checks red blood cells, white blood cells, and platelets. Important because testosterone can increase red blood cell production (polycythemia), and I need a baseline before starting therapy.
Comprehensive Metabolic Panel
Covers liver function, kidney function, electrolytes, and blood sugar. Gives me a complete picture of your overall health and helps rule out other causes of your symptoms.
TSH
Thyroid-stimulating hormone. Low thyroid function can mimic many symptoms of low testosterone (fatigue, weight gain, brain fog, low mood). I need to rule this out.
T4 Free (FT4)
Free thyroxine, the active thyroid hormone. Together with TSH, this gives me a complete thyroid picture.
Cortisol, Total
Your primary stress hormone. Chronically elevated cortisol suppresses testosterone production. This helps me understand if stress or adrenal issues are contributing to your symptoms.
DHEA Sulfate, Immunoassay
A precursor hormone made by your adrenal glands. Low DHEA-S can indicate adrenal insufficiency, which can contribute to fatigue and low energy independent of testosterone.
Ferritin
Your iron storage protein. Low ferritin causes fatigue that looks exactly like low testosterone. I need to rule this out before attributing fatigue to low T.
PSA, Total
Prostate-specific antigen. A prostate marker checked as a baseline before starting testosterone therapy and monitored during treatment.
What the Numbers Mean
The Endocrine Society (2018): Uses 300 ng/dL as the threshold for diagnosing low testosterone.
American Urological Association (AUA, 2018; revalidated 2024): Also uses 300 ng/dL as the diagnostic threshold, requiring two separate early morning measurements on different occasions. The AUA recommends targeting the 450-600 ng/dL range (middle tertile of normal) when dosing testosterone therapy. If testosterone normalizes but symptoms remain unchanged after 3-6 months, they recommend discussing cessation.
Both major guidelines agree on the 300 ng/dL cutoff, but the AUA provides a more specific treatment target (450-600 ng/dL). In practice, I prioritize symptoms over rigid cutoffs and use the AUA treatment target as a dosing guide.
Many men with levels between 350-450 ng/dL still have significant symptoms and respond well to treatment. Research shows younger men especially may feel symptoms in this range.
Symptoms matter more than a single number. Almost all top testosterone experts would consider a trial of therapy for a symptomatic man with levels in the 350-400+ range.
Young men should have higher levels. A study of men aged 20-44 found that the middle 50% (25th-75th percentile) of healthy men in their 20s had testosterone between 409-575 ng/dL. A level of 310 in a 28-year-old, while technically above the 300 cutoff, is well below average for his age.
Treatment Options Compared
There are several ways to take testosterone, and they are NOT all the same. Each has real trade-offs in safety, convenience, and how your body responds. Click any option to see the pros and cons.
Oral Testosterone (Testosterone Undecanoate)
Kyzatrex, Jatenzo, Tlando
A modern oral form of testosterone designed to be safe for the liver. Unlike older oral steroids (which were toxic), this version is absorbed through the lymphatic system in your gut, completely bypassing the liver. Taken as capsules twice daily with meals.
Injections (Cypionate or Enanthate)
Testosterone Cypionate, Testosterone Enanthate
The most traditional form. Oil-based injection into muscle every 1-2 weeks. Weekly injections produce more stable levels than every-2-week dosing.
Topical Gel
AndroGel, Testim, Vogelxo
A clear gel applied daily to shoulders, upper arms, or abdomen. Testosterone absorbs through skin over several hours.
Transdermal Patch
Androderm
Adhesive patch applied every night. Less commonly used today due to high rates of skin reactions.
Nasal Gel
Natesto
Gel applied inside nostrils three times daily. Designed to produce short testosterone pulses.
Pellets
Testopel
Small rice-grain-sized pellets surgically implanted under the skin. They slowly dissolve over months. While convenient in theory, the inability to adjust or stop quickly and the consistently too-high initial testosterone levels are significant limitations.
A note about fertility
All forms of testosterone therapy can reduce sperm production and affect fertility. If you're planning to have children, tell your provider before starting. I use a fertility-sparing protocol combining Kyzatrex with clomiphene citrate, and in a small cohort in my clinic, I've seen maintained fertility with this approach. Alternative standalone approaches like clomiphene or hCG can raise testosterone while preserving sperm production.
Why I Recommend Kyzatrex for Most Men Who Need TRT
After reviewing the clinical evidence, I believe oral testosterone undecanoate (Kyzatrex) is the best option for the majority of men starting testosterone replacement therapy. Not because it's the newest, but because it has the best overall balance of safety, efficacy, and quality of life. Here's why.
Bottom line
For most men starting TRT, Kyzatrex offers the best combination of safety (lower polycythemia risk, no liver toxicity, less estrogen conversion), efficacy (higher free testosterone through SHBG lowering), and quality of life (no needles, no transfer risk, stable levels). The main trade-offs are cost and twice-daily dosing. I believe the safety advantages are worth it.
What Can You Expect from Treatment?
Testosterone therapy doesn't fix everything, and it works better for some symptoms than others. Here's what clinical studies actually show, with the strength of evidence clearly labeled.
How to read this section
- The bars represent the consistency and magnitude of improvement seen in clinical studies, not a guarantee of your personal result.
- Evidence quality badges tell you how confident I am in the data behind each claim.
- Individual results vary based on your starting levels, age, overall health, and other factors.
Sexual desire and libido
StrongTTrials showed significant gains in sexual desire across multiple validated measures.
Morning erections
StrongErectile function
ModerateMost ED in men over 50 is vascular in origin. Testosterone works best when combined with PDE5 inhibitors.
Lean muscle mass
StrongBhasin dose-response studies demonstrated clear, dose-dependent increases in lean mass.
Fat loss (especially belly fat)
Moderate–StrongBone density
StrongTTrials demonstrated a 7.5% increase in spinal bone density, a clinically meaningful improvement.
Anemia correction
StrongEnergy and vitality
ModestTTrials did NOT show significant vitality improvement on their primary outcome measure. Some men feel better, others notice little change.
Mood and depressive symptoms
MixedCognitive function
WeakTTrials and TEAAM found no cognitive benefit from testosterone therapy.
Diabetes prevention
ModerateT4DM trial showed this benefit required BOTH testosterone and lifestyle changes working together.
Bottom line
The strongest, most consistent improvements are in sexual desire, lean body mass, bone density, and anemia. The most variable responses are in mood, energy, and erectile function, because those symptoms have many causes beyond testosterone. Cognitive function is the one area where current evidence shows no benefit from therapy.
Safety, Side Effects, and Monitoring
Testosterone therapy is generally safe when properly monitored, but like any medical treatment, it comes with things to watch for. The key is regular blood work and open communication with your provider.
Who should NOT start testosterone therapy?
- Men with known prostate cancer or breast cancer
- Men actively trying to conceive (alternative treatments are available)
- Men with untreated severe sleep apnea
- Men with uncontrolled heart failure
- Men with a hematocrit already above 50%
- Men with an elevated PSA that hasn't been evaluated
These are standard contraindications from the Endocrine Society guidelines.
The TRAVERSE trial (5,246 men, New England Journal of Medicine, 2023) is the most important safety study ever done on testosterone therapy. Over about 3 years of follow-up, it showed that testosterone does NOT increase the risk of heart attacks, strokes, or cardiovascular death compared to placebo (7.0% vs 7.3%, essentially identical). This put to rest over a decade of fear based on smaller, flawed studies.
Myths vs. Facts
Testosterone therapy is surrounded by more myths than almost any other medical treatment. Some keep men from getting help they need. Others push men toward treatments they don't need. Let's clear them up.