- 01Two halves have to agree
Low testosterone is a two-part diagnosis. Symptoms of deficiency and a genuinely low morning testosterone, confirmed on a second morning, both have to be present; either half on its own is not a diagnosis, because levels swing by time of day and with illness.
- 02The symptoms that count are the sexual ones
Fatigue and low mood are what most men blame on testosterone, but in a large study of ageing men only the sexual symptoms, low libido, weak morning erections, and erectile dysfunction, tracked a low number.
- 03SHBG can make the total lie
Because most testosterone rides on the carrier protein SHBG, a high SHBG can make a normal man look low and a low SHBG can hide a real deficiency; free testosterone settles it when the total is borderline.
Low testosterone, or male hypogonadism, is diagnosed when two things line up: symptoms of testosterone deficiency (low sex drive, fewer morning erections, erectile trouble, often fatigue or low mood) and a genuinely low testosterone measured the right way, on a morning fasting blood sample and confirmed on a second morning. One low result on its own is unreliable, because testosterone peaks in the early morning, drifts down through the day, and falls when you are ill or run down, so a single low reading often comes back normal when it is repeated.
What are the symptoms of low testosterone?
The symptoms of low testosterone fall into two groups, and they are not equally useful. The ones that track the number are sexual: low libido (reduced sex drive), fewer early-morning erections, and erectile dysfunction (trouble getting or keeping an erection). The broader complaints, fatigue, low mood, poor focus, loss of muscle and strength, more body fat, come up often but have many other causes, from poor sleep to depression to thyroid problems.
| Category | What it looks like | How closely it tracks the testosterone number |
|---|---|---|
| Sexual | Low libido, fewer morning erections, erectile dysfunction | Closely; these are the three symptoms that flagged low testosterone in the European Male Ageing Study 3 |
| Physical | Fatigue, low energy, loss of muscle and strength, more body fat | Loosely; many other causes |
| Mood and thinking | Low mood, irritability, trouble concentrating | Loosely; overlaps with sleep, stress, and depression |
| Later or severe | Hot flashes, reduced body hair, thinning bones over time | Mainly with markedly low levels |
In the European Male Ageing Study, the label of late-onset low testosterone fit best when a man had at least three sexual symptoms (reduced morning erections, low desire, erectile dysfunction) together with a total testosterone below about 317 ng/dL (11 nmol/L) and a low free testosterone. The non-sexual symptoms, the fatigue and low mood people most often worry about, tracked the number poorly. 3
How is low testosterone diagnosed?
A diagnosis of low testosterone needs both halves to agree: symptoms of testosterone deficiency and a testosterone level that is genuinely and repeatedly low. Neither the symptoms nor the number carries the diagnosis alone. The measurement has to be done right, a morning fasting total testosterone, and a single low value has to be confirmed on a second morning before it counts. 1
Diagnose hypogonadism only in men with symptoms or signs consistent with testosterone deficiency and unequivocally and consistently low testosterone on blood testing, judged first on total testosterone and on free testosterone when the total is borderline.Paraphrasing the Endocrine Society Clinical Practice Guideline (Bhasin et al., JCEM 2018) [[1]]
The time of day the blood is drawn changes the number. Testosterone peaks in the early morning and falls through the day, and it drops further during illness, poor sleep, or a stressful stretch. An afternoon sample, or a reading taken while you are run down, can look low in a man whose morning level is fine, which is why one low result calls for a repeat rather than treatment. 1
The agreed-across-studies normal range for a healthy young man is 264 to 916 ng/dL, and the number only means something on a morning, fasting draw.
Where an example total testosterone fallsng/dL
When the total misleads: free testosterone and SHBG
Total testosterone counts the hormone bound to proteins plus the small free fraction. Most of it rides on SHBG (sex hormone binding globulin, the protein that carries testosterone in your blood), and SHBG itself moves with other conditions. Obesity, insulin resistance, type 2 diabetes, and an underactive thyroid lower SHBG, which drags the total down; aging, an overactive thyroid, and estrogen raise SHBG, which pushes the total up. When SHBG is off or the total sits near the lower limit, free testosterone, measured by equilibrium dialysis (the reference lab method) or a validated calculation, reflects the active hormone more accurately. 14
SHBG moves your total testosterone without touching the free hormone, so a low total on high SHBG can be a normal man, and a normal total on low SHBG can hide a real deficiency.
What causes low testosterone?
Two pituitary hormones sort out the cause. LH (luteinizing hormone) and FSH (follicle-stimulating hormone) are the signals the brain sends to the testes. When the testes themselves are failing, the pituitary pushes harder and LH and FSH run high (primary hypogonadism). When the problem is the signal itself, from the pituitary or hypothalamus, LH and FSH sit low or only normal despite the low testosterone (secondary hypogonadism). 1
Primary vs secondary low testosterone: where the problem sits
Many causes are worth finding because they are reversible. Obesity and poorly controlled type 2 diabetes lower testosterone; so do opioid painkillers, long-term glucocorticoids, heavy alcohol use, poor sleep, and the temporary dip that comes with any acute illness. As LifeFrom's LH explainer notes, it is the low or normal LH and FSH pattern, the kind that travels with obesity, opioids, or poorly controlled diabetes, that is worth chasing first, because treating the driver can lift testosterone without therapy. 1
How the test is done, and how often to repeat it
- 1The sample is taken in the morning, fastingTestosterone is highest in the early morning and food lowers it, so guidelines ask for a sample before roughly 10am after an overnight fast.
- 2A low result is confirmed on a second morningBecause levels swing day to day and drop with illness or poor sleep, one low value is confirmed with a second morning total testosterone before any label, typically on a separate day a few weeks later and not while you are unwell, since illness lowers the reading.
- 3Add free testosterone and SHBG if the total is borderlineWhen the total sits near the lower limit or SHBG may be off, free testosterone is the better measure of the active hormone.
- 4Add LH and FSH to find the causeThese pituitary signals separate a testicular cause (high LH and FSH) from a central one (low or normal).
Total testosterone is an inexpensive, widely available blood test, though coverage varies by plan and indication; free testosterone by equilibrium dialysis (the reference method) or a calculated estimate is a separate order. Screening men without symptoms is not recommended. Testing is for men with symptoms, or with conditions that lower testosterone, such as pituitary disease, long-term opioid or steroid use, poorly controlled diabetes, or HIV-related weight loss. 1
What about testosterone therapy?
Testosterone therapy is a decision you make with a clinician after a confirmed diagnosis, not a fix to start on your own. It carries real trade-offs, including effects on fertility, red blood cell counts, and the prostate, and it can paper over a reversible cause. Because obesity, poorly controlled diabetes, thyroid disease, opioids, glucocorticoids, and even a short-term illness can all lower testosterone, the useful first move is finding and fixing what is driving the number down. 1
To step back to the whole cluster, see all hormone markers.
Citations
- Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. JCEM 2018;103(5):1715-1744.
- Travison TG, et al. Harmonized Reference Ranges for Circulating Testosterone Levels in Men of Four Cohort Studies in the United States and Europe. JCEM 2017;102(4):1161-1173.
- Wu FCW, et al. Identification of Late-Onset Hypogonadism in Middle-Aged and Elderly Men (EMAS). N Engl J Med 2010;363:123-135.
- Rosner W, et al. Utility, Limitations, and Pitfalls in Measuring Testosterone: An Endocrine Society Position Statement. JCEM 2007;92(2):405-413.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.