Blog Post
What Causes Low Testosterone in Men
Low testosterone affects an estimated 4 to 5 million men in the United States, yet most never get a clear answer about why their levels dropped in the first place. Understanding what causes low testosterone in men is more than academic. The root cause determines whether lifestyle changes will move the needle, whether additional testing is needed, or whether medically supervised testosterone replacement therapy is the appropriate path forward.
Testosterone production can fail at two distinct points in the body: the testicles themselves, or the hormone-signaling centers in the brain. Identifying where the breakdown occurs changes both the diagnosis and the treatment plan. This guide covers both categories of causes, the signs and symptoms to watch for, how diagnosis works, and what your options look like once you have answers.
What Is Clinically Low Testosterone?
Testosterone levels below 300 nanograms per deciliter (ng/dL) on two separate morning blood draws are generally considered clinically low, according to guidelines from leading urology and endocrinology organizations. But that number alone does not tell the complete story.
What actually matters for how you feel is how much testosterone is available to your tissues, not just how much is circulating in total. A significant portion of testosterone in the bloodstream is bound to a protein called sex hormone-binding globulin (SHBG). Only the "free" or loosely bound fraction can act on receptors in muscle, brain, and other tissues.
This distinction matters because a man with high SHBG levels can have a total testosterone reading that looks normal on paper while his free testosterone is well below the range needed to feel and function well. A complete hormone panel includes both total and free testosterone, along with SHBG, to give an accurate picture of what is actually going on.
How Your Body Produces Testosterone
To understand what causes low testosterone, it helps to understand how the body produces it. The process runs through a chain of signals called the hypothalamic-pituitary-gonadal (HPG) axis.
It starts in the hypothalamus, a region of the brain that releases gonadotropin-releasing hormone (GnRH) in regular pulses. GnRH signals the pituitary gland to release two hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH travels through the bloodstream to the testicles, where it tells specialized cells called Leydig cells to produce testosterone. FSH, along with testosterone, supports spermatogenesis, the process of sperm production.
When testosterone levels are adequate, the brain reduces its GnRH output, keeping levels balanced in a feedback loop. When testosterone drops, the brain increases its signal to drive more production.
Low testosterone in men occurs when this system breaks down at one of two points: either the testicles fail to respond to the signal (primary hypogonadism), or the signal itself never arrives properly (secondary hypogonadism). Your LH and FSH levels on a lab panel reveal which type you have, which is why a full panel matters more than a single testosterone number.
Primary Causes of Low Testosterone in Men
Primary hypogonadism means the testicles themselves are the problem. LH and FSH levels are typically elevated because the brain is sending stronger signals to compensate, but the testicles are not responding adequately.
Aging and the Natural Decline of Testosterone
Starting around age 30, testosterone levels decline at a rate of roughly 1 to 2 percent per year. By the time a man reaches his 60s, approximately 20 percent have clinically low levels; by his 80s, the figure rises to about half. This gradual decline, sometimes called andropause, reflects a reduction in the number and responsiveness of Leydig cells over time.
Importantly, aging does not guarantee low testosterone. Research has shown that men in good metabolic and cardiovascular health often maintain healthy levels well into older age. This means that age-related decline is a contributor, not an inevitability, and that other factors covered in this article frequently accelerate it. Reviewing testosterone levels by age can give you useful context for where your numbers stand relative to your peer group.
Testicular Injury, Infection, and Orchitis
Physical trauma to the testicles can damage the Leydig cells responsible for testosterone production, sometimes permanently. Infections also pose a risk: mumps orchitis, an inflammation of the testicles caused by the mumps virus, is one of the more common infectious causes of primary hypogonadism in men who were not vaccinated. Testicular torsion, if not treated promptly, can also cause lasting damage to testosterone-producing tissue.
Klinefelter Syndrome and Genetic Conditions
Klinefelter syndrome, in which a man is born with an extra X chromosome (XXY instead of XY), is the most common genetic cause of low testosterone. Men with Klinefelter syndrome typically have smaller testicles, reduced Leydig cell function, and significantly lower testosterone output from adolescence onward. Other congenital conditions, including undescended testicles (cryptorchidism), can impair long-term testosterone production if not addressed in childhood.
Chemotherapy and Radiation Therapy
Cancer treatments, particularly chemotherapy agents and radiation directed toward the pelvic region, are gonadotoxic, meaning they can damage or destroy testosterone-producing tissue. The degree of impact depends on the type of treatment, the dose, and the individual. Testosterone monitoring is an important but frequently overlooked part of follow-up care for men who have undergone cancer treatment.
Secondary Causes of Low Testosterone in Men
Secondary hypogonadism originates in the brain's signaling system rather than the testicles. LH and FSH levels are typically low or normal even though testosterone is also low, because the pituitary is not sending an adequate signal. The testicles themselves are capable of producing testosterone but are not being properly directed to do so.
Pituitary Gland Disorders and Tumors
The pituitary gland sits at the base of the brain and serves as the relay station between the hypothalamus and the testicles. Tumors called pituitary adenomas can compress or displace the hormone-producing cells, reducing LH and FSH output. Inflammation, prior injury, or surgery in the pituitary area can have similar effects. Pituitary dysfunction is one reason a full hormone panel should always include LH and FSH, not just testosterone.
Elevated Prolactin Levels
Prolactin is a hormone best known for stimulating breast milk production, but it is present in men as well. When prolactin levels are elevated, a condition called hyperprolactinemia, it suppresses GnRH release from the hypothalamus and disrupts the downstream LH and FSH signals, resulting in reduced testosterone production without obvious testicular damage.
Causes of elevated prolactin in men include prolactinomas (benign pituitary tumors that secrete prolactin), hypothyroidism, and certain medications including antipsychotics and some antiemetics. Hyperprolactinemia is a consistently missed cause of low testosterone in men because many standard panels do not include prolactin and the symptoms overlap with other conditions.
Medications That Suppress Testosterone Production
Several widely used medications disrupt the HPG axis at different points:
Opioids: Chronic opioid use suppresses GnRH release at the hypothalamic level, reducing LH output and ultimately testosterone. This effect is dose-dependent and often goes unrecognized in men managing long-term pain.
Corticosteroids: High-dose or prolonged use of medications like prednisone suppresses pituitary LH secretion.
Androgen deprivation therapy: Used in prostate cancer treatment, these medications intentionally suppress testosterone, but the effect can persist after treatment ends.
Anabolic steroids: Exogenous androgens signal the brain that testosterone is already sufficient, shutting down the HPG axis. With prolonged use, this suppression can become semi-permanent even after stopping.
Chronic Illness: Diabetes, Kidney Disease, and HIV
Type 2 diabetes has a well-documented bidirectional relationship with low testosterone in men. Men with diabetes are significantly more likely to have low testosterone, and low testosterone appears to worsen insulin sensitivity, creating a cycle that is difficult to break without addressing both conditions. Chronic kidney disease and advanced liver disease both impair hormone metabolism and can reduce testosterone through multiple mechanisms. HIV and its treatments can also suppress testosterone production, either through the disease process itself or as a medication side effect.
Lifestyle and Metabolic Causes of Low Testosterone
Many of the most common causes of low testosterone in men are driven by modifiable lifestyle and metabolic factors. These tend to produce secondary hypogonadism, and addressing them can produce meaningful improvements in levels, though not always to the clinical normal range on their own.
Obesity, Aromatase, and Estrogen Conversion
Excess body fat, particularly visceral fat around the abdomen, is one of the most significant reversible contributors to low testosterone in men. Adipose tissue expresses an enzyme called aromatase, which converts testosterone into estradiol, a form of estrogen. The more visceral fat present, the more active this conversion becomes, reducing circulating testosterone while simultaneously raising estrogen levels.
Elevated estrogen then feeds back to the hypothalamus and pituitary, suppressing GnRH and LH output and further reducing testosterone production. Insulin resistance, which commonly accompanies obesity and metabolic syndrome, also directly impairs testosterone synthesis within the Leydig cells. This creates a self-reinforcing cycle where low testosterone encourages fat gain, and fat gain drives testosterone lower.
Obstructive Sleep Apnea
Most testosterone is released during sleep, particularly during slow-wave and REM stages. Obstructive sleep apnea fragments this process by causing repeated nighttime oxygen drops and sleep disruptions that prevent the body from completing its normal hormonal restoration cycle. Studies show that men with moderate to severe untreated sleep apnea have meaningfully lower testosterone levels than matched controls, and that treating sleep apnea with a CPAP device can partially restore levels in some men.
Chronic Stress and Elevated Cortisol
Cortisol, the primary stress hormone, has a direct inhibitory effect on the HPG axis. Chronically elevated cortisol suppresses GnRH release from the hypothalamus, reducing downstream LH output and ultimately testosterone production. This is the body's way of deprioritizing reproductive function during prolonged stress.
For men in high-pressure careers or dealing with long-term physical or psychological stress, this mechanism is a commonly overlooked cause of low testosterone. It also represents one of the more practical arguments for treating sleep quality, recovery, and stress management as legitimate parts of any hormone health strategy, not lifestyle add-ons.
Alcohol Use and Anabolic Steroid History
Chronic heavy alcohol use has a direct toxic effect on Leydig cells and also disrupts liver metabolism of hormones, elevating estrogen and reducing testosterone. Even moderate alcohol intake has measurable effects on hormonal output in some men.
Prior use of anabolic steroids deserves specific mention. Even after stopping, non-medical steroid use can leave the HPG axis suppressed for months or years. In some men, the suppression becomes persistent, effectively converting a self-induced secondary hypogonadism into a long-term condition that requires clinical management.
Signs of Low Testosterone in a Man
The signs of low testosterone in a man develop gradually, which is part of why they are so frequently attributed to aging, stress, or other causes rather than a hormonal shift. Most men experience a combination of symptoms across several categories before it becomes obvious something is wrong.
Physical signs include:
Persistent fatigue that sleep does not fully resolve
Loss of muscle mass and strength despite maintaining activity levels
Increased body fat, particularly around the midsection
Reduced body and facial hair
Declining bone density over time, which becomes clinically significant in older men
Sexual health changes are among the more commonly recognized signs of low testosterone:
Reduced libido and interest in sex
Difficulty achieving or maintaining erections
Decreased semen volume and reduced frequency of morning erections
Cognitive and emotional symptoms are often the most disruptive to daily life, and the most frequently dismissed:
Brain fog and difficulty concentrating
Low motivation and reduced drive
Irritability and mood instability
A flat or depressed mood without an obvious psychological cause
None of these symptoms are exclusive to low testosterone, which is why lab confirmation is essential. They also tend to develop slowly, making them easy to normalize until the cumulative impact becomes significant enough to prompt a visit to a provider.
Reversible Causes vs. Causes That Require Clinical Management
Understanding what causes low testosterone in men also means understanding which causes can improve with targeted lifestyle changes and which require a more structured clinical approach. This distinction is one of the most practically useful things a diagnosis can reveal, and one of the most commonly skipped over in general health content.
Causes that are often at least partially reversible include obesity and metabolic syndrome, where meaningful weight loss reduces aromatase activity and improves insulin sensitivity; obstructive sleep apnea when treated; medication-induced suppression when the offending drug can be changed or discontinued; and chronic stress and poor sleep when properly addressed.
Causes that are structural, permanent, or progressive require a different conversation. Klinefelter syndrome, post-chemotherapy Leydig cell damage, long-term HPG axis suppression from anabolic steroid use, and age-related decline that has crossed a clinical threshold all fall into this category. For men in this group, medically supervised TRT is not a workaround. It is the clinically appropriate treatment for confirmed deficiency, managed with regular lab monitoring to maintain levels within a healthy physiologic range.
The distinction also matters in the other direction. Initiating TRT without first ruling out reversible causes misses an opportunity for the patient to improve their overall health, not just their testosterone number. Understanding how testosterone is prescribed and what evaluation should look like before any treatment starts is worth knowing before your first provider conversation.
How Low Testosterone Is Diagnosed
Diagnosing low testosterone properly requires more than a single blood test. Best practice involves two separate morning draws on different days, since testosterone peaks in the morning and can be significantly lower in the afternoon. Both draws should confirm levels below the clinical threshold before any treatment is considered.
A comprehensive diagnostic panel goes well beyond total testosterone:
Free testosterone: the biologically active portion available to act on tissues
SHBG: high levels reduce free testosterone independently of total testosterone
Luteinizing hormone (LH) and follicle-stimulating hormone (FSH): distinguish primary from secondary hypogonadism and guide the diagnostic workup
Prolactin: screens for hyperprolactinemia as an underlying secondary cause
Hematocrit and complete blood count (CBC): establishes baseline values important for monitoring during any subsequent treatment, as covered in the discussion of TRT safety monitoring
For men with secondary hypogonadism, additional pituitary imaging may be warranted depending on LH, FSH, and prolactin results. This is the level of evaluation that a provider specializing in hormone health should conduct before recommending any treatment.
The Bottom Line
Low testosterone in men has multiple distinct root causes, spanning structural, hormonal, metabolic, and lifestyle-related factors. The cause matters because it determines what treatment makes sense. Some men will see meaningful improvement by addressing sleep apnea, reducing body fat, or eliminating a medication that suppresses their HPG axis. Others have a permanent or progressive deficiency that warrants medically supervised testosterone replacement therapy under regular monitoring.
If you are experiencing symptoms consistent with low testosterone and have not had a comprehensive hormone evaluation, the right first step is a complete lab panel, not a guess. Getting clear answers on what is actually driving the decline is what makes it possible to address it effectively.
If you're experiencing signs of low testosterone and want a clear picture of what is actually behind them, PeakPerforMAX offers comprehensive hormone evaluations through secure telemedicine visits. Your provider reviews your labs, symptoms, and health history before making any recommendations.