Testosterone

Testosterone is an androgen hormone produced by both men and women, although men typically have much higher circulating levels. It plays an important role in sexual development, libido, fertility, muscle mass, bone strength, and red blood cell production.

Testosterone levels often change with age, health, sleep, body composition, medications, and underlying medical conditions. A low result on one blood test does not automatically mean someone has testosterone deficiency or needs testosterone replacement therapy.

What Is Testosterone?

Testosterone is often described as the primary male sex hormone, but it is present and biologically active in everyone. In men, most testosterone is produced by the testes. In women, the ovaries produce testosterone, while the adrenal glands produce other androgens that can be converted into testosterone.

Production is controlled by a signaling system involving the hypothalamus, pituitary gland, and reproductive organs. Testosterone contributes to several functions, including:

• Sexual development and puberty
• Libido and sexual function
• Sperm production and fertility
• Muscle mass and strength
• Bone density
• Red blood cell production
• Body-hair growth and fat distribution

Most testosterone in the bloodstream is attached to proteins, particularly sex hormone-binding globulin, or SHBG. A smaller amount circulates as “free testosterone,” which is not bound to these proteins.

How Do Testosterone Levels Change With Age?

Testosterone levels tend to decline gradually as men grow older, but the pattern varies considerably between individuals. Unlike menopause, this is not normally a sudden loss of hormone production. The gradual change is sometimes discussed under the term andropause, although “male menopause” can be misleading.

Age is only one factor. Obesity, type 2 diabetes, sleep disorders, chronic illness, certain medications, pituitary conditions, testicular injury, and previous anabolic steroid use can also reduce testosterone production.

A lower level in an older adult is therefore not automatically caused by aging, and aging alone does not establish a medical need for treatment.

What Does Low Testosterone Mean?

Clinically low testosterone in men is called male hypogonadism. According to the Endocrine Society’s clinical guideline, it should be diagnosed only when a man has both compatible symptoms and consistently low testosterone concentrations.

Possible symptoms include:

• Reduced libido
• Fewer spontaneous erections or erectile dysfunction
• Reduced sperm production or infertility
• Loss of muscle mass
• Lower bone density
• Unexplained anemia
• Reduced energy
• Depressed mood or difficulty concentrating
• Hot flashes when testosterone is severely low

Many of these symptoms are nonspecific. Poor sleep, depression, medication effects, chronic disease, calorie restriction, and excessive training can produce similar problems. This is why symptoms and test results need to be evaluated together.

High testosterone can also require investigation. In women, elevated levels may be associated with acne, excess facial or body hair, scalp hair thinning, or irregular menstrual periods. Abnormal levels in any sex should be interpreted by a qualified healthcare professional.

How Is Testosterone Measured?

Testosterone is measured with a blood test and can be tracked as a biomarker. Total testosterone is normally the starting measurement.

Because levels fluctuate throughout the day and can temporarily fall during illness, testing in men is generally performed in the early morning. A low result should usually be confirmed with a second morning test on a different day. The National Library of Medicine notes that reference ranges vary between laboratories, so results should be interpreted using the testing laboratory’s range and the patient’s clinical circumstances.

Free testosterone or SHBG may also be considered when total testosterone does not fit the symptoms or when a condition may be altering hormone-binding proteins. Additional tests can help determine whether the cause involves the testes, pituitary gland, hypothalamus, medication use, or another health condition.

What Is Testosterone Replacement Therapy?

Testosterone replacement therapy, commonly shortened to TRT, provides prescription testosterone through injections, gels, patches, oral formulations, or other delivery methods.

In men with confirmed hypogonadism, TRT may improve sexual symptoms and help support muscle, bone, and red blood cell production. Treatment should be individualized and regularly monitored rather than based on age, fatigue, athletic goals, or one isolated test result.

Potential risks and side effects include:

• Reduced sperm production and fertility
• An elevated red blood cell count
• Acne or oily skin
• Increased blood pressure
• Prostate enlargement
• Worsening of untreated sleep apnea
• Skin-to-skin transfer from topical products

TRT is generally not appropriate for men who are trying to conceive because external testosterone can suppress sperm production. Certain prostate or breast cancers, an elevated red blood cell count, untreated severe sleep apnea, recent cardiovascular events, and other medical conditions may also affect whether treatment is appropriate.

In 2025, the FDA updated testosterone product labeling. It removed boxed-warning language concerning increased major cardiovascular events after reviewing the TRAVERSE trial, but required warnings about increased blood pressure and retained limitations on using testosterone for age-related low levels. FDA-approved testosterone products remain intended for men with low testosterone linked to an associated medical condition.

Is Testosterone a Longevity Treatment?

Healthy testosterone function matters for muscle, bone, sexual health, and overall quality of life. However, testosterone should not be treated as a score that must be maximized. Low levels may sometimes be a sign of poor health, but that does not prove that raising testosterone above an individual’s healthy range will improve healthspan or extend life.

NIH-supported trials found that testosterone treatment improved sexual function in some older men with low levels, while effects on walking ability, fatigue, and vitality were smaller or inconsistent. The trials did not establish testosterone as a life-extension therapy. National Institute on Aging

For longevity-focused care, testosterone is best understood as one biomarker within a broader clinical picture. Sleep, resistance training, adequate nutrition, healthy body composition, medication review, and treatment of underlying conditions may all influence hormone health. TRT is a treatment for appropriately diagnosed deficiency, not a general anti-aging shortcut.