Testosterone

What it is

Testosterone is the primary androgen (sex hormone) produced mainly in the testes and, in smaller amounts, the ovaries and adrenal glands, regulating sexual function, muscle mass, bone density, and mood. It belongs to a class of hormones called steroids, meaning it is built from cholesterol. Both men and women produce testosterone, though typical male levels run roughly 10–20 times higher than typical female levels. A small fraction of circulating testosterone is converted by an enzyme called aromatase into estradiol (the main form of estrogen), which means some effects once attributed solely to testosterone are partly driven by estrogen.

Causes and mechanism

The hypothalamus releases gonadotropin-releasing hormone (GnRH), which signals the pituitary gland to secrete luteinizing hormone (LH). LH then travels to the testes (or ovaries) and triggers testosterone production. This is called the hypothalamic-pituitary-gonadal (HPG) axis. Testosterone levels peak in early adulthood and decline roughly 1–2% per year after age 30–40. When the testes or ovaries cannot produce adequate testosterone — due to disease, injury, or genetic conditions — the condition is called hypogonadism. Research from Massachusetts General Hospital (Finkelstein et al., NEJM 2013) showed that some effects attributed to low testosterone, such as increased body fat, are actually driven by the accompanying drop in estradiol.

Symptoms and diagnosis

Low testosterone (hypogonadism) can cause:

• Reduced sex drive and fewer spontaneous erections
• Loss of muscle mass and strength
• Increased body fat, sometimes with breast tissue growth (gynecomastia)
• Decreased bone density, raising fracture risk
• Fatigue, low mood, and difficulty concentrating
• In women: low libido, fatigue, and reduced bone density

Diagnosis requires a morning fasting blood test measuring total testosterone, because levels fluctuate throughout the day and are highest in the morning. The Endocrine Society defines male hypogonadism as a consistently low total testosterone (generally below 300 ng/dL) combined with symptoms. A single low reading is not sufficient — at least two separate measurements are recommended before treatment is considered.

Treatment options

Treatment depends on the cause and severity:

• Testosterone replacement therapy (TRT): Available as injections, transdermal gels or patches, subcutaneous pellets, and oral or buccal formulations. Indicated for confirmed hypogonadism.
• Clomiphene citrate: An off-label oral option that stimulates the pituitary to increase LH, prompting the body's own testosterone production. Preserves fertility, unlike exogenous TRT.
• Lifestyle modification: Resistance exercise, weight loss, improved sleep, and reduced alcohol intake can raise testosterone levels modestly in men with obesity or metabolic syndrome.

Debate on "optimization" in older men: The American College of Physicians notes that TRT may modestly improve sexual function in some men, but evidence for broader benefits — energy, cognition, cardiovascular health — remains limited and mixed. The 2023 TRAVERSE trial found no significant increase in major cardiovascular events with TRT in men with hypogonadism and high cardiovascular risk, but long-term safety data beyond a few years are still limited.

When to see a clinician

See a clinician if you have persistent symptoms of low testosterone lasting more than a few weeks — especially reduced libido, unexplained fatigue, or significant muscle loss. Men under 40 with symptoms warrant evaluation for an underlying cause (e.g., pituitary tumor, genetic condition). Women experiencing unexplained low libido, fatigue, or bone loss should also ask about testosterone testing. Do not self-treat with over-the-counter supplements or unregulated products; many have not been tested for safety or efficacy.