Most people think of testosterone as a sex hormone — something that drives muscle mass, aggression, and libido. That framing captures only a fraction of what testosterone actually does in the human body. Testosterone is a whole-body regulatory hormone that governs energy production, mood, cognitive function, bone density, metabolic health, red blood cell production, and far more. Men often do not realize the full scope of what testosterone regulates until levels decline and systems they had taken for granted start to feel different.
For men with clinically confirmed low testosterone, testosterone replacement therapy is a medical treatment that restores deficient levels to the normal physiological range — allowing the many systems that testosterone regulates to function as they should. Understanding what testosterone does is the foundation for understanding why deficiency has such a broad clinical impact.
What Is Testosterone?
Testosterone is a steroid hormone belonging to the androgen class. It is produced primarily in the Leydig cells of the testes in men, with smaller amounts produced in the adrenal glands. Production is regulated by the hypothalamic-pituitary-gonadal (HPG) axis: the hypothalamus releases gonadotropin-releasing hormone (GnRH), which signals the pituitary to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which in turn signal the testes to produce testosterone. This is a feedback loop — when testosterone levels are adequate, the signal from the hypothalamus is reduced; when levels drop, the signal increases.
Once in circulation, testosterone can act directly on androgen receptors or be converted to two other active hormones: dihydrotestosterone (DHT) via the enzyme 5-alpha reductase, and estradiol (a form of estrogen) via the enzyme aromatase. Both DHT and estradiol have important physiological roles in men — estradiol, for instance, plays a documented role in bone density, cognitive function, and libido in the male body.
What Testosterone Does in the Male Body
The effects of testosterone span virtually every organ system. The most clinically relevant functions for men experiencing deficiency are:
Energy and Metabolism
Testosterone stimulates erythropoiesis — the production of red blood cells — by signaling the kidneys to release erythropoietin. Red blood cells carry oxygen to muscles and organs; reduced production results in a form of functional fatigue. Testosterone also influences mitochondrial function and cellular energy metabolism, and it plays a role in insulin sensitivity. Testosterone deficiency is associated with increased insulin resistance and metabolic dysfunction.
Mood and Mental Health
Testosterone interacts with dopamine and serotonin signaling pathways in the brain — neurotransmitter systems that regulate motivation, reward, mood, and emotional regulation. Low testosterone is associated in research with increased rates of depressive symptoms, reduced motivation, irritability, and a diminished sense of well-being. These are not simply psychological — they have a hormonal basis.
Sexual Health
Testosterone is the primary driver of sexual desire (libido) in men. It also plays a role in sperm production and in supporting erectile function — though erectile function involves additional vascular and neurological components. Normal testosterone levels are necessary for normal male sexual health across multiple dimensions.
Bone Density
Testosterone (and its aromatized form, estradiol) maintains bone mineral density throughout a man's life. Long-term testosterone deficiency is associated with reduced bone density and increased fracture risk over time — a consequence that develops gradually and is often underrecognized until deficiency has been sustained for years.
Muscle and Body Composition
Testosterone stimulates muscle protein synthesis and inhibits muscle protein breakdown — supporting the maintenance of lean body mass. It also influences fat distribution, with deficiency associated with increased visceral fat accumulation. These body composition effects of testosterone deficiency are among the most visible and frequently reported by patients.
How Testosterone Levels Change with Age
Testosterone levels peak in the late teens to early 20s. After age 30, research by Travison et al. (2007) documented a population-level decline of approximately 1–2% per year. This is a normal physiological process — not every man who experiences this decline will develop clinically significant hypogonadism.
The distinction between normal age-related decline and clinical hypogonadism is important. Clinical hypogonadism requires both lab-confirmed low testosterone AND symptoms attributable to the deficiency. Age is a contributing factor in many cases but not a sufficient explanation on its own — and not an acceptable reason to dismiss symptoms without evaluation.
Total vs. Free Testosterone — What the Numbers Mean
When your lab report comes back with a testosterone number, that number is typically your total testosterone — the sum of all testosterone circulating in the blood. But most of that testosterone is bound to proteins and unavailable to cells.
Approximately 44–78% of circulating testosterone is tightly bound to SHBG (sex hormone-binding globulin) — this fraction is biologically inactive. Another 20–54% is loosely bound to albumin and can dissociate for cellular use. Only 1–3% circulates as free (unbound) testosterone. Free testosterone and “bioavailable” testosterone (free plus albumin-bound) are the clinically meaningful fractions that affect tissues.
A complete understanding of where a man stands hormonally requires measuring SHBG alongside total and free T. Our guide to normal testosterone levels by age covers reference ranges and the clinical significance of the different measurements in more detail.
What Happens When Testosterone Is Too Low
When testosterone falls below the physiological range required for normal bodily function, the many systems it regulates begin to underperform. The full range of symptoms of low testosterone spans energy, mood, cognition, sexual health, and body composition.
Clinical hypogonadism — the medical diagnosis for low testosterone combined with symptoms — is defined by both laboratory criteria (two morning draws below the appropriate threshold) and clinical criteria (symptoms attributable to the deficiency). The diagnosis requires both: lab values without symptoms, or symptoms without lab confirmation, are not sufficient for treatment.
How Testosterone Is Measured and Monitored
Accurate testosterone measurement requires attention to timing and technique:
- Morning draw — testosterone follows a diurnal pattern, peaking between 7 and 10 AM. Afternoon or evening draws can be 20–30% lower, producing misleadingly low results in men with normal testosterone.
- Two-draw guideline — clinical guidelines recommend two separate morning draws approximately four weeks apart before confirming a diagnosis of hypogonadism. Day-to-day variability in testosterone production means a single draw can be misleading.
- Comprehensive panel— Vitality's evaluation includes total testosterone, free testosterone, SHBG, estradiol, complete metabolic panel, CBC (to assess hematocrit), PSA (prostate baseline), and thyroid function where clinically relevant.
During testosterone replacement therapy, the same panel is monitored at regular intervals — typically every 3–6 months — to confirm that levels are within the appropriate physiological range, that hematocrit is not elevated, that PSA is stable, and that the treatment is producing the expected clinical response.
Frequently Asked Questions
At what age does testosterone start to decline?
Testosterone levels peak in the late teens to early 20s and begin a gradual decline of approximately 1–2% per year after age 30. This is a normal physiological process. However, not every man who experiences age-related decline will develop clinically significant hypogonadism — symptoms and lab values together determine whether a deficiency is clinically meaningful and whether treatment is warranted. Age alone does not determine treatment eligibility.
Can women have low testosterone?
Yes — women produce testosterone at lower levels than men, primarily in the ovaries and adrenal glands. Testosterone plays a role in female energy, libido, bone health, and mood. Low testosterone can be a clinical concern in women as well, particularly after menopause or surgical removal of the ovaries. Dr. Jaqua treats patients with hormone-related concerns including low testosterone. The physiology and treatment thresholds differ from those in men.
Is testosterone a steroid?
Technically yes — testosterone is classified as a steroid hormone because of its chemical structure (a four-ring steroid backbone). However, it is fundamentally different in purpose and use from the synthetic anabolic steroids sometimes misused for athletic performance enhancement. Testosterone replacement therapy prescribed by a physician replaces a hormone the body is not producing adequately — restoring physiological levels, not elevating them above the normal range. It is a medical treatment for a documented deficiency, not a performance-enhancing drug.
What is the difference between total testosterone and bioavailable testosterone?
Total testosterone measures all testosterone in the blood. However, most testosterone circulates bound to proteins — primarily SHBG (sex hormone-binding globulin) and albumin — and bound testosterone is unavailable to cells and tissues. Free testosterone (unbound) and bioavailable testosterone (free plus loosely albumin-bound) are the fractions that actually affect cells. In some men, total testosterone appears within the normal reference range while free testosterone is actually low due to elevated SHBG — which can still produce symptoms of deficiency. This is why Vitality measures SHBG alongside total and free testosterone.
References
- Bhasin S, et al. “Testosterone Therapy in Men with Androgen Deficiency Syndromes: An Endocrine Society Clinical Practice Guideline.” Journal of Clinical Endocrinology & Metabolism. 2010.
- Travison TG, et al. “A population-level decline in serum testosterone levels in American men.” Journal of Clinical Endocrinology & Metabolism. 2007.