Quick answer
What is low testosterone?
Low testosterone (hypogonadism) means the body produces insufficient testosterone — causing fatigue, low libido, mood changes, reduced muscle, and sometimes erectile dysfunction. Diagnosis requires symptoms plus two low morning blood tests — a single low result is not enough. Causes include obesity, diabetes, medicines, pituitary problems, and age-related decline. NHS testosterone replacement helps when properly diagnosed — not for anti-ageing use in healthy older men.
Low testosterone — male hypogonadism explained
Testosterone is the main male sex hormone — essential for libido, muscle and bone strength, mood, red blood cell production, and sperm development. Hypogonadism means the testes (or brain signalling to them) produce insufficient testosterone for the body’s needs.
Online searches for “low T” have exploded — but true pathological deficiency is less common than marketing suggests, and symptoms overlap with depression, poor sleep, obesity, and normal ageing.
Normal vs low — what changes with age
Testosterone peaks in late teens and twenties, then falls gradually — roughly 1% per year after 30. By 80, levels may be half young-adult values.
Age-related decline alone is not a disease — many older men have low-normal levels without symptoms. NHS treatment targets symptomatic men with confirmed biochemical deficiency, not every man seeking more energy.
Symptoms — when to suspect hypogonadism
Sexual:
- reduced libido — often the earliest and most specific symptom
- erectile dysfunction — especially loss of morning erections
- reduced ejaculate volume
Physical:
- fatigue and reduced stamina
- loss of muscle mass and strength
- increased body fat — especially abdominal
- gynaecomastia — breast tissue enlargement
- loss of body/facial hair (partial)
- hot flushes — more often associated with low T than commonly realised
- osteoporosis risk long term
Mood and cognition:
- low mood, irritability, poor concentration — can mimic depression
Important: These symptoms are non-specific. Depression, sleep apnoea, hypothyroidism, anaemia, and chronic illness cause similar pictures.
Types of hypogonadism
Primary — testicular failure
Problem in the testes themselves:
- Klinefelter syndrome (XXY) — most common congenital cause
- undescended testes history
- mumps orchitis — testicular inflammation
- testicular injury, surgery, radiation
- ageing (primary testicular senescence)
- chemotherapy
Blood tests show high LH and FSH — brain pumps harder but testes under-respond.
Secondary — pituitary/hypothalamic
Problem with brain signalling:
- pituitary adenoma — may raise prolactin
- head injury, surgery, radiotherapy
- obesity — suppresses GnRH signalling
- chronic opioid use
- anabolic steroid abuse — shuts down natural production
- severe illness — temporary suppression
Blood tests show low or inappropriately normal LH/FSH with low testosterone.
Functional/reversible causes (common and treatable)
- obesity — aromatase in fat converts testosterone to oestrogen
- type 2 diabetes and metabolic syndrome
- sleep apnoea
- excessive alcohol
- severe chronic illness
- glucocorticoid medicines
Treat the cause first — testosterone may normalise without lifelong TRT.
Diagnosis — not one blood test
NHS standard:
- Clinical assessment — symptoms, examination (testicular size, gynaecomastia, BMI)
- Two separate morning samples (before 11am — diurnal variation matters)
- Total testosterone — repeat if borderline (8 to 12 nmol/L)
- Free testosterone or SHBG-adjusted calculation if borderline
- LH, FSH, prolactin — distinguish primary vs secondary
- PSA (men over 40) before TRT
- FBC, lipids, HbA1c — assess comorbidity
Pituitary red flags — headache, visual field loss, galactorrhoea (milky nipple discharge), severe headaches — urgent MRI.
Who should not assume low testosterone
- Vague tiredness without sexual symptoms
- Single borderline blood test
- Normal morning erections with only situational ED
- Men wanting TRT for anti-ageing or bodybuilding without deficiency
Private clinics sometimes prescribe inappropriately — risks outweigh unproven benefits.
Treatment overview
See our testosterone replacement therapy guide for medicine details.
First-line for reversible causes:
- weight loss — often raises testosterone 2 to 3 nmol/L
- CPAP for sleep apnoea
- reduce alcohol
- optimise diabetes
- stop anabolic steroids — recovery takes months
Testosterone replacement (TRT) when:
- confirmed hypogonadism on two tests
- symptoms consistent
- no contraindications (prostate/breast cancer, untreated sleep apnoea, high haematocrit)
- fertility plans discussed — TRT suppresses sperm
TRT and fertility
Exogenous testosterone shuts down pituitary LH/FSH → testicular sperm production stops. Men wanting children need:
- gonadotropin injections (hCG, FSH) — fertility specialist
- clomifene — off-label to stimulate own production — specialist use
- ** sperm banking** before TRT if possible
Never start TRT without understanding fertility impact.
Low testosterone and other conditions
Type 2 diabetes: Bidirectional link — low T associated with insulin resistance; treating either helps the other.
ED: Testosterone improves libido more than vascular erections — PDE5 inhibitors still needed for many.
Depression: Overlap common — treat depression; testosterone if biochemically deficient.
Osteoporosis: Long-standing hypogonadism weakens bones — DEXA scan and TRT considered.
Myths
Myth: “All tired men over 40 need testosterone.” Fact: Fatigue has many causes — test only when clinical picture fits.
Myth: “Testosterone supplements from health shops work.” Fact: OTC “boosters” rarely contain meaningful testosterone — may contain unlisted steroids or useless herbs.
Myth: “TRT is safe for everyone.” Fact: Risks include polycythaemia (thick blood), sleep apnoea worsening, acne, fertility loss, and uncertain long-term cardiovascular effects in older men.
When to see a GP — summary
Book appointment if you have persistent low libido, ED with lost morning erections, or fatigue plus risk factors (obesity, diabetes, opioid use).
Expect morning blood tests and examination — diagnosis takes weeks, not one visit.
Low testosterone is real, treatable when properly diagnosed — but the path starts with accurate testing and treating reversible causes, not online gels without monitoring.
Common questions
- What are the symptoms of low testosterone?
- Reduced sex drive, erectile dysfunction, fatigue, low mood, irritability, reduced muscle mass and strength, increased body fat, hot flushes, poor concentration, and loss of body hair. Symptoms overlap with depression, sleep apnoea, and thyroid disease — blood tests and examination exclude other causes.
- What testosterone level is considered low?
- NHS laboratories vary, but total testosterone below roughly 8 to 12 nmol/L on two morning samples with symptoms suggests hypogonadism. Borderline results (8 to 12 nmol/L) need repeat testing and sometimes free testosterone calculation. Levels fall naturally with age — treatment is for symptomatic biochemical deficiency, not every older man.
- Does low testosterone cause weight gain?
- Low testosterone and obesity interact both ways — obesity lowers testosterone through aromatisation and inflammation; low testosterone promotes fat gain and muscle loss. Weight loss often raises testosterone significantly. Treat underlying obesity before assuming lifelong TRT is needed.
- Can low testosterone cause erectile dysfunction?
- Yes — testosterone supports libido and nocturnal erections. Pure hypogonadism causes reduced desire more than vascular ED alone. Many men with ED have normal testosterone — PDE5 inhibitors work independently. Testosterone helps when both are low and symptoms match.
- How is low testosterone treated on the NHS?
- Testosterone replacement — gels (Testogel), patches, or injections — when symptomatic hypogonadism is confirmed and fertility is not immediately desired. Treat reversible causes first — obesity, sleep apnoea, excessive alcohol, opioid medicines. Endocrinology referral for pituitary or testicular disease.
- Does testosterone replacement cause prostate cancer?
- TRT does not appear to cause prostate cancer but may accelerate growth of existing undetected cancer — PSA is checked before and during treatment. Men with active prostate or breast cancer must not take testosterone. Discuss individual risk with your prescriber.
- Can I boost testosterone naturally?
- Evidence-based approaches — lose weight if overweight, treat sleep apnoea, exercise (especially resistance training), reduce alcohol, optimise diabetes control, and correct iron overload if present. Vitamin D correction if deficient. Most "testosterone booster" supplements lack robust evidence and are unregulated.
- Will testosterone replacement help me build muscle?
- TRT restores muscle in men with true deficiency — not a performance enhancer in men with normal levels. Abuse for bodybuilding carries serious cardiovascular, liver, and fertility risks. Sports organisations ban exogenous testosterone.
- How is low testosterone diagnosed in the UK?
- Symptoms suggestive of hypogonadism plus two separate morning fasting total testosterone blood tests (before 11am) below the laboratory reference range. SHBG and free testosterone may be calculated if borderline. Pituitary MRI if prolactin high or secondary causes suspected.
- Does TRT affect fertility?
- Yes — exogenous testosterone suppresses the pituitary signal to the testes and reduces sperm production, often to zero. Not suitable if you are actively trying to conceive. Clomifene or gonadotrophins may be used instead under fertility specialist care.