What Is Testosterone? Function, Levels, Test and Treatment Explained

Testosterone might be the most talked-about hormone in men’s health, showing up everywhere from gym locker room conversation to primetime “Low T” commercials. But a lot of that conversation skips past what testosterone actually does, how it’s controlled, and what the real evidence says about treating low levels. This guide connects those dots, building directly on the hormone system covered throughout this series.

Key Takeaways

• Testosterone is a steroid hormone produced mainly in the testes, triggered directly by LH, and also produced in smaller amounts by the ovaries and adrenal glands in women.
• It supports far more than reproduction alone, including muscle mass, bone density, mood, and red blood cell production.
• Levels naturally decline with age, and diagnosis of low testosterone requires both confirmed low blood levels and actual symptoms, not age alone.
• The FDA offers several approved testosterone formulations, including injections, gels, patches, and pellets.
• A major 2023 trial and a 2026 FDA label update have provided real reassurance about cardiovascular safety, though specifically for men with properly diagnosed hypogonadism, not as a blanket justification for widespread use.
• Fertility suppression, increased red blood cell counts, and prostate monitoring remain important considerations during treatment.

Testosterone is the primary male sex hormone, though it’s produced in smaller amounts by women too. It’s triggered by luteinizing hormone (LH) acting on specialized cells in the testes, and it supports everything from muscle mass and bone density to sperm production and mood. Levels naturally decline with age, and while FDA-approved testosterone therapy exists for diagnosed low testosterone, a major 2023 safety trial and a 2026 FDA label update have reshaped how doctors think about its cardiovascular risk.

What Is Testosterone?

Testosterone is a steroid hormone, chemically built from cholesterol rather than from amino acids the way peptide hormones like GnRH or LH are. It belongs to a hormone class called androgens, the hormones responsible for male sexual development and characteristics.

While testosterone is most associated with men, it’s genuinely present and functionally important in women as well, just in much smaller amounts. We’ll come back to that distinction shortly, since it’s more significant than most people realize.

Where Testosterone Is Made?

In men, testosterone is produced mainly by specialized cells in the testes called Leydig cells. As covered in detail in our guide to luteinizing hormone, this production doesn’t happen automatically. It’s directly triggered by LH, which travels from the pituitary gland to the testes and signals Leydig cells to produce testosterone.

This is a direct continuation of the hormone chain covered throughout this series: the hypothalamus releases GnRH in pulses, which triggers the pituitary to release LH, which then triggers testosterone production in the testes. A smaller amount of testosterone is also produced by the adrenal glands, sitting atop the kidneys, in both men and women.

In women, testosterone comes from the ovaries and adrenal glands, in much smaller quantities than in men, but it still plays a real functional role rather than existing as a byproduct.

What Testosterone Actually Does in the Body?

Testosterone’s effects reach far beyond the reproductive system alone:

• Supports puberty development, including deepening of the voice, facial and body hair growth, and growth spurts
• Maintains muscle mass and strength
• Supports bone density, helping prevent bone thinning over time
• Drives sperm production, working alongside FSH in the testes
• Influences libido and sexual function
• Affects mood and energy levels
• Supports red blood cell production in bone marrow
• Contributes to cognitive function, including aspects of memory and spatial reasoning, according to ongoing research

This wide-reaching list is exactly why unusually low or high testosterone levels can produce such varied, sometimes seemingly unrelated symptoms.

Testosterone Isn’t Just a “Male” Hormone

This point deserves real emphasis, because it’s frequently overlooked in casual conversation. Women’s ovaries and adrenal glands produce testosterone too, and it plays a genuine role in female libido, mood, energy, and bone health. Testosterone levels in women are naturally much lower than in men, but a meaningful drop, sometimes seen after surgical removal of the ovaries or during the menopause transition, can produce real, recognized symptoms.

That said, testosterone therapy for women is a much more limited and specialized area of medicine than for men, with far less established dosing and safety data, and is generally reserved for specific diagnosed conditions rather than general use.

Normal Testosterone Levels

Reference ranges for testosterone vary somewhat between laboratories and testing methods, but a commonly cited general range for adult men is roughly 300 to 1,000 nanograms per deciliter (ng/dL) of total testosterone. Levels below 300 ng/dL, confirmed on more than one properly timed morning blood draw, are generally what doctors use to help define low testosterone, alongside actual symptoms.

It’s worth stressing that a single lab value alone rarely tells the full story. Testosterone naturally fluctuates throughout the day, generally peaking in the morning, which is exactly why guidelines recommend testing with an early morning blood draw, sometimes repeated on a separate day for confirmation.

Why Testosterone Naturally Declines With Age?

As mentioned in our reproductive hormones overview, this gradual, age-related decline is sometimes called andropause, though it’s a much slower and less universally defined process than menopause in women. Testosterone levels in men typically begin a slow decline starting around age 30, continuing gradually throughout life.

This decline alone doesn’t necessarily require treatment. Many men experience this natural drop without significant symptoms, which is part of why diagnosing “low testosterone” as a condition requiring treatment depends on both blood levels and actual symptoms together, not age or lab values in isolation.

What Causes Low Testosterone?

Low testosterone, medically called hypogonadism, generally falls into two categories:

Primary hypogonadism

originates in the testes themselves, meaning the testes aren’t producing enough testosterone even when properly stimulated by LH. Causes include genetic conditions, testicular injury, certain infections, and treatments like chemotherapy.

Secondary hypogonadism

originates further up the chain, in the hypothalamus or pituitary gland, meaning the testes could function normally but aren’t receiving adequate GnRH or LH signals to begin with. Causes include pituitary tumors, certain medications, obesity, and chronic illness.

Symptoms of Low Testosterone

Common symptoms associated with low testosterone include:

• Reduced sex drive
• Erectile dysfunction
• Fatigue and reduced energy
• Depressed mood
• Reduced muscle mass and strength
• Increased body fat
• Reduced bone density over time

These symptoms overlap significantly with many other common conditions, including depression, thyroid disorders, and sleep apnea, which is exactly why proper diagnosis requires blood testing rather than symptoms alone.

FDA-Approved Testosterone Therapies

Each formulation has different absorption patterns, dosing frequency, and practical considerations, so the right choice often comes down to a conversation between patient and prescriber about lifestyle and preference, not just clinical need alone.

The Cardiovascular Safety Question: What the Evidence Actually Shows

This is a key, evolving part of the testosterone story. It deserves a clear explanation, not outdated fears.

In 2015, the FDA ordered a dedicated heart safety trial for testosterone products. This followed conflicting studies that raised cardiovascular concerns.

The resulting study, called TRAVERSE, followed over 5,200 men. All participants had confirmed hypogonadism along with existing heart disease or high risk. Published in the New England Journal of Medicine in 2023, the study found no meaningful difference in major adverse cardiovascular events. Rates were 7.0% for men on testosterone versus 7.3% on placebo.

Following these results and further evidence review, the FDA requested label updates in June 2026. They removed the previous statement claiming safety and efficacy were unestablished for age-related low testosterone.

However, researchers stress a vital caveat. These reassuring findings apply only to men with properly diagnosed hypogonadism. Cleveland Clinic trial investigators warned against using this data to justify widespread prescribing to aging men without a clear diagnosis—a practice common at commercial “low T” clinics.

Other Risks and Side Effects of Testosterone Therapy

Beyond the cardiovascular question, testosterone therapy carries several other considerations worth knowing about:

• Fertility suppression. This connects directly to the HPG axis feedback loop covered in our reproductive hormones guide. Externally supplied testosterone signals the brain to reduce its own GnRH and LH output, which can shrink the testes and suppress natural sperm production, sometimes significantly, while on therapy.
• Increased red blood cell count (polycythemia), which requires periodic blood monitoring since it can increase clotting risk if left unchecked.
• Prostate monitoring. While testosterone therapy hasn’t been shown to cause prostate cancer, it requires appropriate prostate monitoring, particularly in older men.
• Skin reactions, especially with topical gels and patches, including irritation at the application site.
• Sleep apnea worsening, in some individuals.

These are exactly the kinds of considerations a doctor weighs when deciding whether testosterone therapy makes sense for a specific patient, rather than treatment being a one-size-fits-all decision.

A Word on Testosterone Misuse

It is also worth noting how often testosterone comes up in casual fitness conversations. People frequently discuss using it for bodybuilding and athletic performance at doses far above standard medical levels.

This type of unsupervised, off-label use carries much higher and less predictable risks than monitored, medically prescribed therapy. Higher doses can disrupt natural hormone production, strain the cardiovascular system, and cause severe side effects.

Anyone considering testosterone outside of a clear medical need should speak directly with a licensed physician. Only a qualified doctor can evaluate your actual health status and explain the real risks, ensuring you do not rely on unsafe, unsupervised dosing.

How Testosterone Is Tested?

Diagnosing low testosterone typically involves:

• A morning blood draw, since testosterone peaks in the morning and testing later in the day can produce misleadingly low results
• Confirmation with a second test on a separate day, since a single low reading isn’t considered sufficient for diagnosis
• Additional hormone testing, sometimes including LH and FSH, to help distinguish primary from secondary hypogonadism
• Symptom evaluation, since a low number without corresponding symptoms doesn’t necessarily indicate a condition requiring treatment

Frequently Asked Questions

What is Testosterone Test?

A testosterone test also known as Testosterone Levels Test, is a diagnostic blood test that measures the concentration of testosterone in your bloodstream. It is primarily used to evaluate hormone imbalances, diagnose reproductive or endocrine disorders, and monitor treatments such as Testosterone Replacement Therapy (TRT).

Can women be prescribed testosterone therapy?

In limited, specific cases, yes, though it’s far less standardized than for men and generally reserved for particular diagnosed conditions rather than general use.

Does testosterone therapy cause infertility?

It can suppress natural sperm production while being used, due to the same negative feedback mechanism covered in our reproductive hormones guide. This is an important discussion to have with a doctor, especially for men who may want children in the future.

Is testosterone therapy safe for the heart?

The 2023 TRAVERSE trial and a subsequent 2026 FDA label update provided meaningful reassurance for men with confirmed hypogonadism, though researchers specifically caution against extending these findings to men without a proper diagnosis.

What’s the difference between low testosterone and normal age-related decline?

Low testosterone as a treatable condition requires both a confirmed low blood level and actual symptoms. A gradual age-related decline without significant symptoms doesn’t necessarily require treatment on its own.

Why do “Low T” clinics get specifically called out as a concern?

Some commercially oriented clinics have been criticized for prescribing testosterone without properly confirming a genuine diagnosis, which is different from the carefully monitored, medically indicated treatment reflected in trials like TRAVERSE.