Low Testosterone in Men: Symptoms, Causes & Diagnosis

Testosterone is an essential hormone for male sexual development, reproductive function, muscle maintenance, bone health, and overall well-being. Although testosterone levels naturally change throughout life, unusually low levels combined with compatible symptoms may indicate testosterone deficiency, also called hypogonadism.

Low testosterone can affect more than sexual function. Men may experience reduced libido, erectile difficulties, fatigue, decreased muscle mass, changes in mood, or problems with fertility. However, these symptoms can have many different causes, so a diagnosis should not be based on symptoms alone — poor sleep, chronic stress, obesity, depression, and certain medications can produce identical complaints. Establishing a diagnosis requires compatible symptoms together with consistently low morning testosterone levels on repeat blood testing.

Testosterone is the primary male sex hormone, produced mainly by the Leydig cells in the testicles, with a smaller amount secreted by the adrenal glands. Production is regulated by a complex feedback loop known as the hypothalamus–pituitary–testicular (HPT) axis: the hypothalamus releases Gonadotropin-Releasing Hormone (GnRH), which signals the pituitary gland to release Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH); LH stimulates the testicles to produce testosterone, while FSH, alongside testosterone, supports sperm production.

It is vital to distinguish between a low testosterone blood value and testosterone deficiency (hypogonadism): a low testosterone level simply indicates a numerical value below a laboratory reference range on a single lab draw, while hypogonadism is a clinical syndrome that requires both consistently low serum testosterone levels and the presence of specific clinical symptoms or signs. A single low blood test does not automatically mean a man has hypogonadism — testosterone levels fluctuate throughout the day and can temporarily drop due to acute illness, poor sleep, extreme physical stress, or recent dietary intake.

Symptoms of testosterone deficiency can develop gradually and vary significantly between individuals. Sexual symptoms include reduced sexual desire, decreased frequency of spontaneous or morning erections, erectile difficulties, reduced sexual activity and difficulty achieving orgasm, and possible fertility problems. Physical symptoms include persistent fatigue or decreased energy levels, loss of muscle mass and physical strength, an increase in body fat particularly around the abdomen, reduced exercise performance and endurance, decreased bone mineral density (osteopenia or osteoporosis), and mild, unexplained anemia.

Psychological and cognitive symptoms include depressed mood or persistent sadness, increased irritability, difficulty concentrating and memory fatigue, and reduced motivation, drive, and self-confidence. Important clinical note: none of these symptoms are unique to low testosterone. Conditions such as thyroid disease, obstructive sleep apnea, major clinical depression, diabetes, and metabolic syndrome can present with identical clinical signs.

Physicians categorize testosterone deficiency based on where the disruption occurs along the HPT axis. Primary hypogonadism (testicular origin) occurs when the testicles fail to produce adequate testosterone despite receiving sufficient hormonal signals from the brain, with LH and FSH levels typically elevated. Causes include testicular trauma, torsion, or surgical removal, testicular infections, undescended testicles (cryptorchidism) during childhood, genetic conditions such as Klinefelter syndrome, and cancer treatments such as chemotherapy or testicular radiation therapy.

Secondary hypogonadism (hypothalamic/pituitary origin) occurs when the testicles are capable of working, but the hypothalamus or pituitary gland fails to send adequate hormonal signals; causes include pituitary tumors, long-term use of opioids, glucocorticoids, or anabolic steroids, severe systemic illness, elevated prolactin, and head trauma affecting the pituitary stalk. Both primary and secondary regulation can be suppressed by obesity (through several mechanisms, including increased aromatase activity), obstructive sleep apnea, chronic alcohol or substance abuse, and severe metabolic or physical stress.

Testosterone levels may gradually decline with age, but the rate varies considerably between individuals and is also influenced by body weight, metabolic health, medications, sleep, and other medical conditions. While age-related hormonal shifts occur, symptomatic testosterone deficiency is not something doctors should automatically attribute to normal aging.

In many older men, declining testosterone is driven primarily by accumulating co-morbidities — such as weight gain, cardiovascular disease, type 2 diabetes, and medication side effects — rather than aging alone. Identifying and managing these underlying metabolic conditions can often help restore hormonal balance without lifelong hormone replacement.

Diagnosing testosterone deficiency requires a systematic medical evaluation to confirm low levels safely and accurately while identifying any underlying root cause. The clinician evaluates medical and sexual history (libido, morning erections, energy, mood, sleep quality), performs a physical examination (body composition, body hair distribution, testicular volume and consistency, breast tissue enlargement, blood pressure), and orders a morning total testosterone blood test — ideally before 10 AM, repeated on a separate morning when the initial result is low.

If repeat morning total testosterone is low, the physician orders Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH) to determine whether the issue stems from the testicles or the pituitary gland; prolactin and iron studies may also be checked. If LH/FSH are abnormally low alongside severe testosterone deficiency, or if prolactin is elevated, a pituitary MRI may be ordered to rule out structural lesions.

When evaluating blood work, it is important to distinguish total testosterone (all testosterone in the blood, including both protein-bound and unbound hormone) from free testosterone (the fraction not bound to proteins and biologically available to tissues). Sex Hormone-Binding Globulin (SHBG) levels can make total testosterone readings misleading: high SHBG (caused by aging, hyperthyroidism, liver disease, or extreme fasting) can cause free testosterone to be low even if total testosterone appears normal, while low SHBG (caused by obesity, hypothyroidism, or type 2 diabetes) can cause total testosterone to look abnormally low even when free testosterone is adequate.

Low testosterone and male infertility are related, but they are not the same condition — a man can have normal fertility with borderline testosterone, or normal testosterone with severely impaired sperm production. Exogenous testosterone can markedly suppress sperm production, so men who want to preserve fertility should discuss options with a specialist before starting therapy. It is also important to understand that testosterone primarily regulates sexual desire, while erections depend heavily on vascular health, nerve function, and nitric oxide release — treating low testosterone alone does not always fix erectile dysfunction when underlying vascular or nerve issues are present.

A medical evaluation by an andrologist or urologist is recommended if you experience persistent loss of sexual desire or lack of morning erections, unexplained chronic fatigue that does not improve with rest, progressive muscle loss or difficulty maintaining strength despite training, inability to achieve pregnancy with a partner after 12 months of unassisted trying, or mood changes and severe motivation loss combined with physical symptoms. Routine screening is not recommended for men without symptoms — testing is only indicated when clinical signs match potential hormone deficiency.

What blood level is considered low testosterone? Guidelines generally define low total testosterone as consistently below 300 ng/dL (10.4 nmol/L) on morning blood samples, although reference ranges vary slightly between laboratories and numbers must always be interpreted alongside clinical symptoms.

Can stress cause low testosterone? Yes — chronic psychological or physical stress elevates cortisol, and sustained high cortisol can suppress the hypothalamus–pituitary axis, leading to temporary drops in testosterone production.

Can testosterone therapy affect future fertility? Yes. Exogenous testosterone can markedly suppress sperm production and may cause severe oligospermia or azoospermia. While sperm counts often recover after stopping therapy, recovery times vary, and preservation strategies should be evaluated before starting treatment.