Testosterone is the primary male sex hormone, produced mainly in the testicles, that drives physical development during puberty and continues regulating muscle mass, bone density, red blood cell production, mood, and sexual function throughout adulthood. While most associated with men, women also produce testosterone in smaller amounts, where it plays a role in metabolism, energy, and libido.
If you’ve heard testosterone mentioned constantly but never gotten a clear, complete picture of what it actually does, here’s a thorough breakdown.
Where Testosterone Comes From
In men, testosterone is primarily produced in the testicles, with small amounts also produced by the adrenal glands. The process is regulated by a feedback system called the hypothalamic-pituitary-gonadal (HPG) axis:
- The hypothalamus in the brain releases a hormone that signals the pituitary gland
- The pituitary gland responds by releasing luteinizing hormone (LH) and follicle-stimulating hormone (FSH)
- These hormones travel to the testes, where specialized cells called Leydig cells produce testosterone
In women, testosterone is produced in much smaller amounts by the ovaries and adrenal glands.
Testosterone’s Role During Puberty
Testosterone surges during puberty and drives many of the physical changes associated with male development:
- Deepening of the voice
- Growth of facial and body hair
- Increased muscle mass and strength
- Development of male reproductive organs
- Growth spurts and changes in bone structure
Testosterone’s Functions in Adulthood
Beyond puberty, testosterone continues to play several essential roles throughout a man’s life:
Muscle and strength
Testosterone is classified as an anabolic hormone, meaning it directly supports protein synthesis and muscle tissue development. This is why declining testosterone is closely linked to age-related muscle loss.
Bone density
Testosterone helps maintain bone mineral density. Low levels over time are associated with increased fracture risk, similar to the bone health concerns more commonly discussed in relation to menopause in women.
Red blood cell production
Testosterone stimulates the production of red blood cells in bone marrow, which is part of why low testosterone can sometimes contribute to anemia, and why TRT requires monitoring of red blood cell counts.
Fat distribution and metabolism
Testosterone influences how and where the body stores fat, with low levels linked to increased fat storage, particularly visceral fat around the organs.
Sexual function
Testosterone drives libido and plays a role in achieving and maintaining erections by supporting nitric oxide production, which helps regulate blood flow.
Mood and cognitive function
Testosterone affects mood regulation, motivation, and aspects of cognitive function, including focus and mental clarity.
Sperm production
Testosterone is essential for fertility, working alongside FSH to support healthy sperm production.
How Testosterone Changes With Age
Testosterone levels don’t remain constant throughout life:
- Levels peak during young adulthood, typically in the late teens to twenties
- A gradual decline of roughly 1% per year begins around age 30
- By their 40s and 50s, many men begin noticing symptoms related to this decline, even though the drop has been happening gradually for years
This natural decline is different from clinically low testosterone (hypogonadism), which involves levels low enough, combined with symptoms, to warrant medical evaluation and potential treatment.
What Counts as a Healthy Testosterone Level
Normal testosterone levels for adult men generally range from 300 to 1,000 nanograms per deciliter (ng/dL), though exact reference ranges can vary slightly between labs. The American Urology Association defines low testosterone as levels below 300 ng/dL, combined with relevant symptoms.
It’s worth noting that a “normal” total testosterone number doesn’t always tell the full story. Factors like SHBG (sex hormone-binding globulin) affect how much of your testosterone is actually available for your body to use, which is why understanding your full testosterone levels guide matters beyond a single number.
Signs Testosterone May Be Low
Common symptoms associated with declining testosterone include:
- Persistent fatigue
- Reduced muscle mass and strength
- Increased body fat
- Low libido or erectile difficulties
- Mood changes, including irritability or low mood
- Difficulty concentrating
Reviewing the complete list of symptoms of low testosterone can help you determine whether your experience aligns with a potential hormone issue.
What Influences Testosterone Levels
Beyond natural aging, several factors can affect testosterone production:
- Body weight. Excess body fat is linked to lower testosterone through increased conversion to estrogen
- Sleep quality. Testosterone production peaks during deep sleep, making chronic poor sleep a meaningful factor
- Chronic stress. Elevated cortisol directly competes with and can suppress testosterone production
- Underlying health conditions. Type 2 diabetes, metabolic syndrome, and certain other conditions are linked to lower testosterone
- Certain medications, including some pain medications and steroids
How Testosterone Is Tested
A simple blood test, ideally drawn in the morning when levels naturally peak, measures total testosterone. If results are borderline or don’t match symptoms, additional testing for free testosterone, SHBG, LH, and FSH can provide a more complete picture.
When to Consider Treatment
For men with confirmed low testosterone and relevant symptoms, testosterone replacement therapy is a well-established treatment option. Treatment is available through injections, gels, and pellet therapy, typically structured as monthly programs including labs and physician monitoring. Pricing commonly starts in the low hundreds of dollars per month, and your provider can confirm specifics after reviewing your labs.
Take the Next Step
Want to understand your own testosterone levels and what they mean? Book an appointment for bloodwork and expert guidance.
FAQs
What is testosterone’s main function in the body?
Testosterone drives male physical development and continues supporting muscle mass, bone density, sexual function, and mood throughout adulthood.
At what age does testosterone start declining?
Levels typically peak in young adulthood and begin a gradual decline of about 1% per year starting around age 30.
What is considered a normal testosterone level?
Generally 300-1,000 ng/dL for adult men, though exact ranges vary slightly by lab.
Do women need testosterone too?
Yes, women produce smaller amounts of testosterone that play a role in energy, metabolism, and libido.
Can lifestyle changes alone fix low testosterone?
Lifestyle changes can support healthy testosterone levels, but they may not fully correct clinically low testosterone caused by aging or medical conditions.
How is low testosterone officially diagnosed?
Diagnosis requires a blood test showing levels below 300 ng/dL, combined with relevant symptoms, not symptoms or a single low reading alone.
Sources
- Cleveland Clinic – Low Testosterone (Low T): Hypogonadism, Symptoms & Treatment: https://my.clevelandclinic.org/health/diseases/15603-low-testosterone-male-hypogonadism
- Urology Care Foundation – Low Testosterone: Symptoms, Diagnosis & Treatment: https://www.urologyhealth.org/urology-a-z/l/low-testosterone